Craters are telltale evidence of massive meteorite impacts. But on an eroding planet like Earth, they disappear over time. Scientists now have found a much more subtle calling card of an impact—tiny nodules of glass, forged at high temperatures and pressures—strewn over hundreds of square kilometers in Chile’s Atacama Desert. These centimeter-sized objects, which researchers have dubbed “atacamaites,” were likely formed when an iron-rich meteorite struck Earth roughly 8 million years ago, the team concluded.

Space rocks that enter Earth’s atmosphere—meteors—typically move at several kilometers per second, and they deliver a ferocious blow if they strike the planet. All of that energy can melt terrestrial quartz-containing rocks and launch the resulting molten material high into the atmosphere, where it can resolidify midflight. The resulting nodules of glass often have characteristic aerodynamic shapes reflective of their airborne journey.

Such “impact glasses” are relatively rare, however: Prior to this discovery in Chile, only five geographically distinct groupings of impact glasses were known. “There are shockingly few of them,” said Aaron Cavosie, a planetary scientist at the Space Science and Technology Centre at Curtin University in Perth, Australia, not involved in the research. “They’re special.”

Finding another site of impact glasses is always exciting, added Marc Fries, a curation scientist at NASA Johnson Space Center in Houston also not involved in the research. “It adds to the record of impacts on the planet.”

Michael Warner, an electrical engineer at the National Optical-Infrared Astronomy Research Laboratory in La Serena, Chile, has been searching the Atacama Desert for meteorites since 2002. It’s an ideal place to look for space rocks, he said, because they tend to just sit there rather than being washed away or buried by erosion. “The surface hasn’t been altered for about 20 million years.”

“I thought they looked like rat poop.”In 2007, Warner found his first meteorite, a roughly half-kilogram specimen. Five years later, on another trip to the Atacama Desert, Warner and his son found a plethora of small, black objects. They weren’t much to look at, the elder Warner remembered. “I thought they looked like rat poop.” But he picked up some of the centimeter-sized objects nonetheless and mailed six to Jérôme Gattacceca, a geologist at the National Centre for Scientific Research in Aix-en-Provence, France, who had previously helped Warner classify his meteorite finds.

Gattacceca was immediately intrigued. “They looked unusual,” he said. Gattacceca ruled out common basaltic rock, and he concluded that the samples were impact glasses. Their smooth, rounded shapes—including rods, teardrops, and dumbbells—were one giveaway. “You can see that they’ve flown in the atmosphere,” said Gattacceca.

Gattacceca and several of his colleagues have since traveled to the Atacama Desert to collect more. Their fieldwork, which began in 2014, has since revealed more than 23,000 of these atacamaites.

“Atacamaites have no equivalent among the few known terrestrial ejected impact glasses.”Gattacceca and his collaborators analyzed several atacamaites in the laboratory and showed that they’re made largely of terrestrial rock, as expected. But meteoritic material—most notably, iron, nickel, and cobalt—accounts for about 5% by weight of atacamaites, the researchers noted. That’s a significantly larger extraterrestrial contribution than what’s typically found in other impact glasses. “Atacamaites have no equivalent among the few known terrestrial ejected impact glasses,” the team reported in June in Earth and Planetary Science Letters.

On the basis of extraterrestrial material found in atacamaites, Gattacceca and his colleagues surmised that the meteorite that produced these impact glasses was most likely dominated by iron. The impact that created atacamaites occurred roughly 8 million years ago, fission-track dating suggested, but there’s mysteriously no evidence of a crater within the roughly 25-kilometer × 25-kilometer strewn field. It might have eroded away, the researchers suggested, but they’re not giving up the search yet. They’re scouring satellite imagery and are planning future fieldwork in the region. “We’ll go back,” said Gattacceca.

—Katherine Kornei (@KatherineKornei), Science Writer

In 2019, a team of astronomers caught the first hints of a young exoplanet surrounded by the right stuff to form satellites. Those hints now have been confirmed by high-resolution images that capture light from a potentially moon-forming swirl of dust that surrounds that planet.

The young planetary system, PDS 70, “is the first system where two growing planets, at least one with a circumplanetary disk, have been observed directly,” said Stefano Facchini, an astronomer at the European Southern Observatory and a coauthor of the recent discovery. “The circumplanetary disk around PDS 70 c today is the perfect environment to study the conditions of satellite formation.”

It takes millions of years to form a planetary system from an interstellar cloud of gas and dust. Gravitational instabilities in a cloud will cause it to slowly collapse onto itself until the temperature at the center is hot enough to ignite a protostar. Most of the remaining material falls onto the star, and the remainder flattens out into a disk (called a circumstellar disk) that might, after millions more years, form planets.

“Satellite formation is possible precisely when the accretion rate onto the planet is low, of the order of what we are observing today.”This same process is thought to repeat itself on a smaller scale when planets try to form their own moons (instead of capturing them): After a young planet accumulates enough mass to carve out gaps in the circumstellar disk, dust and gas still surround the growing planet and can flatten out into a smaller disk around it. That circumplanetary disk can then coalesce into one or more satellites. The four Galilean moons of Jupiter are thought to have formed in this way, but the only way to prove that this mechanism forms moons is to catch it in the act.

Enter PDS 70, a star merely 5.4 million years old that is still surrounded by a circumstellar disk. Two gas giant planets that are still accumulating mass have so far been detected as they carve gaps and shape rings within the circumstellar disk.

Upon a closer look at the outer planet PDS 70 c, which is a few times Jupiter’s mass and orbits its protostar slightly farther than Neptune does from the Sun, astronomers detected a faint, fuzzy emission haloing it. They tentatively identified that fuzz as a circumplanetary disk. “The planet has already acquired most of its mass during its past evolution,” Facchini explained. “As for the moons, theoretical models show that satellite formation is possible precisely when the accretion rate onto the planet is low, of the order of what we are observing today.”

After that first identification 2 years ago, the team pushed to observe this still-forming planet and the satellites it could be growing. With the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the team was able to capture high-resolution images of the entire PDS 70 system at wavelengths favored by planet-forming dust. When combined with previous observations, the images revealed that the dust surrounding PDS 70 c extends one Earth-to-Sun separation (1 astronomical unit) from the planet, about 4 times wider than Saturn’s rings. “Today the circumplanetary disk has a dust mass that is at least three Moon masses,” Facchini said, “but during the remaining lifetime much more dust mass can be acquired by the system,” maybe as much as an Earth mass of material. The team published this discovery in Astrophysical Journal Letters on 22 July.

“Our work presents a clear detection of a disc in which satellites could be forming,” lead author Myriam Benisty of the University of Grenoble in France and the University of Chile said in a statement. “Our ALMA observations were obtained at such exquisite resolution that we could clearly identify that the disc is associated with the planet and we are able to constrain its size for the first time.”

Long predicted, this seems like the first really robust observation of a circumplanetary disk busy (perhaps) making exomoons…simply fabulous data from ESO https://t.co/TGtPTNjMNl

— Caleb Scharf (@caleb_scharf) July 22, 2021

So far, the team has been able to measure the dust component of the circumstellar and circumplanetary disks. However, there might be 100 or 1,000 times more gas than dust in the disk that hasn’t yet been mapped. The team is currently using ALMA to study how that gas moves throughout the system, Facchini said. With ALMA and also future observatories, the researchers hope to determine the chemical composition of the material that is forming the atmospheres of PDS 70 c, the inner planet PDS 70 b, and any moons that may be growing around them.

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Each year, millions of tourists visit geysers around the world, marveling at the jets of water spouting high into the air from subterranean reservoirs. Fascination with these rare features is nothing new, of course: Written records of their occurrence date back to the 13th century at least, and for more than 2 centuries, scientists have been improving our understanding of Earth’s geysers.

The English word geyser originates from geysir, a name given by Icelanders in the 17th century to intermittently discharging hot springs. The name descends from the verb gjósa, which means to gush or erupt. Natural geysers are rare—fewer than a thousand exist today worldwide, and only a handful of fossil examples are known from the geological record. About half of Earth’s geysers are located in Yellowstone National Park in the United States. Other large geyser fields include the Valley of Geysers in the Kamchatka Peninsula of Russia, El Tatio in Chile, and Geyser Flat at Te Puia, Rotorua, in New Zealand.

In 1846, French mineralogist Alfred Des Cloizeaux and German chemist Robert Wilhelm Bunsen formulated an early model to explain geyser eruptions based on field measurements of temperature, chemistry, and circulation and eruption patterns at Geysir in Iceland. Since then, scientific knowledge of geysers has advanced significantly [Hurwitz and Manga, 2017], providing valuable insights into volcanic processes, the origin and environmental limits of life on Earth (and potentially elsewhere, including on Mars), and similar geysers on icy outer solar system satellites. Demonstrating these connections, geologist and planetary scientist Susan Kieffer wrote the following in a perspective on her research career:

“[M]y initial idea of studying Old Faithful geyser as a volcanic analog [sic] led me to work not only on the dynamics of eruption of Mount St. Helens in 1980 but also on geysers erupting on Io (a fiery satellite of Jupiter), Triton (a frigid satellite of Neptune), and Enceladus (an active satellite of Saturn).”

Continuing research into the inner workings of geysers will help us further understand and protect these natural wonders and will reveal additional insights about volcanism on and off Earth.

Similar to volcanoes, geysers are transient features with periods of activity and dormancy. Geyser eruption patterns can change following large earthquakes, shifts in climate, and variations in the geometry of their conduits and subsurface reservoirs. Eruption processes of geysers, which can be driven by geothermal heating and the formation of vapor bubbles, are also akin to those operating in volcanoes.

Eruption processes of geysers, which can be driven by geothermal heating and the formation of vapor bubbles, are akin to those operating in volcanoes.The model developed by Des Cloizeaux and Bunsen showed that as water rises toward the surface and pressure decreases, boiling forms bubbles. The liquid water containing the bubbles further lowers the density and pressure of the mixture. Decreasing pressure similarly causes changes in magma that underpin key volcanic processes, such as melt generation in the mantle and the formation of bubbles in magma that drive eruptions.

Because geysers have smaller eruptions and erupt more frequently than volcanoes, they provide useful natural laboratories to study eruption processes and test new monitoring technologies. Volcanic eruptions are sometimes preceded by magma movement that is difficult to monitor because of the large spatial scales and long timescales involved. In contrast, measurements of fluid movement, for example, can be made relatively easily through many geyser eruption cycles, providing data that can be used to improve the interpretation of volcanic phenomena. Measurements and video observations can also be collected within the conduits of active geysers—a feat that is impossible at active volcanoes.

Signals such as seismic tremor—sustained ground vibrations that are common prior to and during volcanic and geyser eruptions—can be very informative for monitoring subsurface processes at active volcanoes and geysers. Tremor in volcanoes can last for days, weeks, or even longer leading up to volcanic eruptions [Chouet and Matoza, 2013]. Tremor may be caused by degassing of magma and by the movement of fluids within a volcanic edifice. However, identifying fluid types (gas, liquid water, magma) and the processes responsible for episodes of tremor is challenging because of the geometric complexities and sizes of volcanic systems.

Seismometers deployed around the iconic Old Faithful and Lone Star geysers in Yellowstone have detected tremor caused by continuous bursts of rising steam bubbles, analogous to bubbles forming and bursting in a teakettle. Thus, by analogy, such measurements of tremor in geyser systems can help elucidate processes that generate volcanic tremor.

Tracking tremor signals in time and space using dense arrays of seismometers also has illuminated the subsurface structure of volcanoes and geysers [Eibl et al., 2021; Wu et al., 2019]. The locations of tremor sources around Strokkur Geyser in Iceland, and Old Faithful, Lone Star, and Steamboat in Yellowstone, for example, indicate that these geysers’ reservoirs are not located directly beneath their vents. Tilting of the ground surface around Lone Star Geyser and a geyser at El Tatio, as well as video observations in the conduits of geysers in Kamchatka, also indicate reservoirs that are not aligned below the geysers’ vents. This type of reservoir, in which liquid and steam bubbles accumulate and pressure builds prior to an eruption, is called a bubble trap and might be a common feature of many geysers [Eibl et al., 2021].

Laboratory experiments of geysers have shown how heat and mass transfer between laterally offset reservoirs and conduits control eruption patterns [Rudolph et al., 2018]. Geophysical imaging has similarly revealed that although most volcanic vents are located directly above their magma reservoirs, many reservoirs are laterally offset from their associated volcanic edifices [Lerner et al., 2020].

A striking example of an offset magma reservoir was highlighted in a 1968 study of the Great Eruption of 1912 in Alaska [Curtis, 1968], in which magma erupted from Novarupta volcano, but collapse occurred some 10 kilometers away at Mount Katmai, where most of the magma that erupted at Novarupta had been stored. Mapping of such laterally offset magma storage systems, as well as detailed physical knowledge of how they work as gleaned from studies of and experiments with geysers, may help scientists design better volcano monitoring networks.

Eruptions at geysers and volcanoes are controlled by delicate balances in heat supply and gas and fluid flows within their systems, and by the tortuous pathways that liquid water, steam, and magma take to the surface—balances that can be affected by external forces. Documenting whether geysers and volcanoes respond to tides and earthquakes provides opportunities to quantify their sensitivity to changes in physical stress in the subsurface and to help evaluate whether they are poised to erupt [Seropian et al., 2021].

Past studies have suggested, on the basis of statistical correlations, that small forces exerted by Earth tides can trigger volcanic eruptions. However, statistical tests of tidal influence on volcanic eruptions are limited because of the rarity of eruptions from a single volcano. In contrast, the thousands of geyser eruptions that occur annually form a much broader sample pool on which to base statistical tests. One such evaluation uncovered a lack of correlation between Earth tides and the intervals between geyser eruptions, a finding that suggests that a correlation between Earth tides and volcanic eruptions is also unlikely.

In Yellowstone, some geysers stopped erupting whereas others started erupting, after the magnitude 7.3 Hebgen Lake earthquake in Montana in 1959.Although tides might not affect geyser eruptions, regional and even very distant large earthquakes can. Written accounts document renewed activity of Geysir following large earthquakes in southern Iceland in 1294. In Yellowstone, some geysers stopped erupting whereas others started erupting, after the magnitude 7.3 Hebgen Lake earthquake in Montana in 1959. The magnitude 7.9 Denali earthquake in Alaska in 2002 affected eruptions of some Yellowstone geysers 3,000 kilometers away.

Earthquakes can also promote volcanic unrest and eruptions. Establishing causal relations between earthquakes and eruptions is challenging because few active volcanoes occur in any given area, and changes in the subsurface can take longer to manifest as an eruption. However, geysers erupt more frequently than volcanoes, which again points to the utility of studying geysers as volcanic analogues.

Precipitation trends and climate changes can affect geysers as well. Eruption intervals at Old Faithful Geyser have changed in the past, and it even ceased erupting in the 13th and 14th centuries because of a severe drought. How often geysers erupt may also change in response to seasonal and decadal changes in precipitation, which affect the supply of groundwater that feeds the eruptions.

Volcanoes also display slight seasonal patterns in their eruptions, and they respond to changing climate. As air temperatures warm, for example, glaciers covering volcanoes melt, which in turn reduces pressure on underlying magma. Pressure reduction causes gas bubbles to form, and the buoyant mixture of magma and bubbles is then more primed for eruption.

On longer timescales, rates of volcanism vary over glacial cycles, with more eruptions and larger volumes of magma erupted as glaciers retreat. In line with this observation, we know from dating sinter deposits and from geologic mapping that most geyser fields were inactive during Earth’s last glacial period (which ended between ~20,000 and 12,000 years ago) when they were covered by ice [Hurwitz and Manga, 2017].

Sinter deposits form when hot water erupting from geysers cools and evaporates rapidly at the surface, causing dissolved silica to precipitate as opaline or amorphous (noncrystalline) solids. High-temperature, vent-related sinter that forms in surge and splash zones around or near erupting geysers is termed geyserite. Around geysers and in downslope pools and discharge channels, the complex sedimentary structures preserved in sinter reflect physical, chemical, and biological processes occurring in hot spring subenvironments. For example, sinter textures produced in hot spring fluid outflows record temperature and pH gradients across a given geothermal field, from vents to discharge channels to pools, and from terraces to marsh settings.

Sinter typically entombs both biotic (e.g., microbes, plants, animals) and abiotic (e.g., weathered sinter fragments, volcanic ash, detritus) materials. Geyserite, in particular, serves as an archive of conditions in Earth’s hottest environment on land (up to about 100°C) and of extreme thermophilic (high temperature–adapted) life therein [Campbell et al., 2015].

Research on modern hot springs suggests that extended hydration and dehydration cycles in geyser outflow channels can give rise to prebiotic molecular systems, which hints at a possible role for geysers in the origin of life on Earth.Research on modern hot springs suggests not only that they can host extant life, but also that extended hydration and dehydration cycles in geyser outflow channels can give rise to prebiotic molecular systems that display fundamental properties of biology, such as enclosed, cell-like structures composed of lipids and polymers [Damer and Deamer, 2020]. This observation hints at a possible role for geysers in the origin of life on Earth billions of years ago. Indeed, inferred geyserite deposits associated with rocks containing microbial biosignatures have recently been reported in approximately 3.5-billion-year-old hydrothermal sedimentary deposits in Western Australia [Djokic et al., 2017].

On Mars, silica-rich deposits detected by the Spirit rover amid Columbia Hills in Gusev Crater closely resemble fingerlike sinter textures on Earth. This site was proposed as a landing site for the NASA Mars 2020 mission, which will cache samples for eventual return to Earth. Although the Perseverance rover was instead sent to explore deltaic deposits in Jezero Crater, the digitate silica structures at Columbia Hills remain as biosignature candidates that may one day be collected and brought to Earth for in-depth verification of their origin. Therefore, sinters remain a key target in the search for ancient life on Mars, particularly from the time in its history when volcanoes and liquid water were active at the surface—about the same time that life was taking hold in hot water here on Earth.

In addition to benefiting our understanding of what constitutes life and where it can thrive, advanced biotechnology has also benefited from geyser studies. In 1967, microbiologist Thomas Brock and his student Hudson Freeze isolated the bacterium Thermus aquaticus from the hot waters of Yellowstone’s geyser basins. Later, biochemist Kary Mullis identified an enzyme, named Taq polymerase, in a sample of T. aquaticus that was found to replicate strands of DNA in the high temperatures at which most enzymes do not survive. This discovery formed the basis for developing the revolutionary polymerase chain reaction (PCR) technique in the 1980s (for which Mullis shared the 1993 Nobel Prize in Chemistry). PCR is now the workhorse method used in biology and medical research to make millions of copies of DNA for various applications, such as genetic and forensic testing. Recently, PCR also became widely used for COVID-19 testing.

Sinter deposits can also inform exploration for geothermal energy, helping locate resources, as well as for mineral deposits. Whereas currently active hydrothermal systems provide energy for electricity generation, industry, and agriculture, giant fossil hydrothermal systems host many of the world’s most productive precious metal mining operations [Garden et al., 2020]. Such epithermal ore deposits form in the shallow subsurface beneath geothermal fields as high-temperature fluids—both magmatic and meteoric in origin—gradually deposit valuable metals including gold, silver, copper, and lithium.

Geyserites form at the surface emission points of rising hot fluids tapped from deep reservoirs and can point to completely concealed subsurface ore deposits [Leary et al., 2016], thus informing exploration for mineral resources; they may also contain traces of precious metals themselves.

Studies of physical processes in easily observable geysers on Earth can also guide and constrain models proposed to explain eruptions elsewhere in our solar system. The geysers of the icy outer solar system satellites Enceladus (Saturn), Triton (Neptune), and Europa (Jupiter) are similar to Earth’s geysers in that changes of state of materials (e.g., melting and vaporization) drive mixtures of solids and gases to erupt episodically.

At the south pole of the ice-covered ocean world Enceladus, some 100 geysers erupt from four prominent fractures, delivering water from a habitable ocean into space and supplying ice particles to Saturn’s E ring. At Triton, the largest of Neptune’s 13 moons, NASA’s Voyager 2 spacecraft detected surface temperatures of −235°C and geysers that erupt sublimated nitrogen gas. Whether eruptions currently occur on Europa remains debated.

As on Earth, studying physical controls on geyser location, longevity, and eruption intervals on these other worlds can improve our understanding of interactions between their interiors and their surface environments.

New sound and visual approaches developed to convey complex patterns in geyser systems may help identify relationships between volcanic signals that might otherwise be overlooked.Tourists and amateur enthusiasts are captivated by the views and sounds of geyser eruptions. These spectacular events also provide public showcases for curiosity-driven scientific research. For example, new sound and visual approaches developed to convey complex patterns in geyser systems could provide valuable educational tools and may also help identify relationships between volcanic signals—such as surface deformation and seismicity indicating preeruptive activity—that might otherwise be overlooked.

Characterizing the sources of thermal water feeding geyser eruptions and mapping the subsurface hydraulic connections between geyser fields and adjacent areas are needed to protect and preserve these natural wonders from human impacts. Geothermal energy production and hydroelectric dam siting have drowned or driven more than 100 geysers to extinction in New Zealand and in Iceland, for example, and geyser eruptions completely ceased in Steamboat Springs and Beowawe in Nevada owing to exploitation of geothermal resources. In contrast, some dormant geysers in Rotorua, New Zealand, resumed erupting a few decades after geothermal extraction boreholes were shut down.

Geysers are curious and awe-inspiring natural phenomena, and they provide windows into a broad range of science questions. They deserve both our wonder and our protection.

We thank the communities and agencies that enabled research on land they own or manage (Amayras Communities of Caspana and Toconce in El Tatio, Chile; Environment Agency of Iceland for research near Strokkur; the Department of Conservation, Wai-O-Tapu Thermal Wonderland, the Ngati Tahu–Ngati Whaoa Runanga Trust, and Orakei Korako Geothermal Park and Cave in New Zealand; and the National Park Service in the United States for research in Yellowstone). We thank Wendy Stovall, Lauren Harrison, and Mara Reed for constructive reviews. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government.

Just like the planets of our solar system, most exoplanets tend to orbit their star in the same direction that the star spins. But when they don’t, exoplanet orbits overwhelmingly prefer to be perpendicular. This new understanding of planetary orbits, published in Astrophysical Journal Letters, raises questions about which planets can become misaligned from the direction that their star spins and how the orbits get that way in the first place.

When seeking to explain strange exoplanet phenomena, the most useful point of comparison is our own solar system. We know more about it than any other of the thousands of planetary systems discovered to date. The dynamics of the solar system are relatively neat and tidy: The orbits of the eight planets all sit very neatly in the same plane, that plane lines up almost exactly with the Sun’s equator, and the whole system rotates in the same direction.

Within the solar system, the largest angle of misalignment between a planet’s orbit and the Sun’s equator—which defines the plane of the Sun’s spin—is Earth’s at just over 7°. Exoplanet scientists have been able to make similar measurements of the spin-orbit alignment within other planetary systems. “Is 7° a small value or a large value?” asked Simon Albrecht, an astronomer at Aarhus University in Denmark and lead author on the recent study. “The jury on that is still out.”

“That alignment in our solar system is part of what led us to believe that planets form out of a disk that’s around the star,” added astrophysicist and coauthor Rebekah Dawson of Pennsylvania State University in University Park. The prevailing theory of planet formation posits that a large cloud of dust and gas collapses under its own gravity to create a star in the center. The leftover material flattens out into a disk that coalesces into one or more planets (see video at right). In that simplified model, all of the star- and planet-forming material swirls in the same direction, which should make the resulting star and planets all spin in a common direction.

However, “we have known for over a decade that there are planets that are not orbiting in the same plane as their star,” Dawson explained. Although most exoplanets orbit in the same direction as the star’s spin (prograde) and with a very small angle between spin and orbit (0°), there are plenty whose orbits don’t follow suit, including some that orbit opposite to the direction of the star’s spin (retrograde) and others that travel completely backward (180°). “The angle between the planet’s orbit and the star’s spin was some of the first three-dimensional information that we started to get about other planetary systems.…We have to imagine something that’s different or more complicated than the history that we’ve naively invoked for our solar system.”

Astronomers can calculate the angle of inclination between the exoplanet’s orbit and the star’s spin by measuring the transit of the planet in different wavelengths and comparing the different transit profiles, a method called the Rossiter-McLaughlin effect (Figure 1).

Usually, however, astronomers can measure only one dimension of a star’s 3D spin—the component of the spin that’s pointed at Earth. “That can tell you that something is misaligned but not by how much,” Dawson said. How much of the star’s total spin we can see and measure depends on the geometry of our vantage point: If a star’s spin axis points directly at Earth, we would measure no spin at all and see no planetary misalignment. To understand the physical reasons why planetary systems are misaligned, it’s not the perceived angle of misalignment that matters, but the true one.

A recent mathematical advancement helped Albrecht and his team calculate our viewing angle for 57 stars that host misaligned planets. With that additional information, the researchers determined that the planets’ misalignments weren’t as random as previously thought. In fact, they found that a significant number of the true misalignment angles were close to 90°, meaning that the planets orbit their stars from pole to pole rather than across the star’s equator.

For now, the data on perpendicular planets are outpacing the theories that explain them. There’s no obvious commonality that groups these stars and planets together that might explain why misaligned planets end up on polar orbits: The stars range from hot to cold, the planets range from Neptune mass to more massive than Jupiter, and the planetary orbits range from very close in to quite far away.

“The biggest thing these planets have in common is that we can measure this [viewing angle] for them,” Albrecht said. There are no models of planetary dynamics that predict a preference for perpendicular planets, he explained, because, quite simply, no one knew that their models needed to explain it.

No one theory can yet explain all of the perpendicular planetary systems.Regardless, Albrecht and his team offered a few potential ideas to start with, although they acknowledged that no one theory can yet explain all of the perpendicular planetary systems they analyzed. Three of the proposed explanations rely on the gravity of another object—the star, an unseen planet, or the planet-forming disk—tugging a planet’s orbit into a 90° misalignment; the fourth theory invokes a magnetic interaction during planet formation.

J. J. Zanazzi, a postdoctoral researcher at the Canadian Institute for Theoretical Astrophysics in Toronto, said that the team “did a great job summarizing the primary theories which can lead to their very exciting result that spin-orbit misalignments come in two flavors,” well aligned or perpendicular. “All the mechanisms have different strengths and weaknesses, and each mechanism fails to explain some part of [the] observation.” Zanazzi was not involved with this research.

The good news, Zanazzi said, is that “all of the astrophysical mechanisms which have been proposed make specific predictions when the mechanism does not work.…For me, a big thing observers can do in the near future is look for companion planets or stars which can cause the required tilts.” If they fail to find any that fit the bill, such a pattern would narrow down the potential explanations.

Moreover, Albrecht said, as theorists begin to refine their models to explain a cluster of polar orbits, those models can help guide the observers toward the right planetary systems to take a closer look at. Will polar orbits be more prevalent around cool stars or hot stars? Will perpendicular planets be found mostly in multiplanet systems or as loners? More observations, new theories, and time will tell.

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

This is an authorized translation of an Eos article. Esta es una traducción al español autorizada de un artículo de Eos.

Dune de Frank Herbert cuenta la historia de Paul Atreides, un hijo de una familia noble enviada al hostil planeta desértico Arrakis para supervisar el comercio de una misteriosa droga llamada melange (apodada “especia”), que otorga a quien la consume habilidades sobrenaturales y longevidad. Sobreviene la traición, el caos y las luchas políticas internas.

Imagina que estás en Arrakis, rodeado por un océano de arena. El aire es irrespirable, el cielo brumoso, el paisaje misterioso. Arena por millas, hasta donde alcanza la vista. Sabes que a varios cientos de kilómetros de distancia hay una vasta red de cañones que, desde arriba, parecen haber sido tallados por enormes gusanos.

Antes de emocionarse demasiado, es importante saber que este no es el famoso planeta desértico que aparece en las novelas de Dune.

No, este Arrakis está más cerca de nuestro propio mundo.

Este Arrakis está a tan solo mil millones de kilómetros de la Tierra, en un mundo que orbita a Saturno.

Incluso hemos aterrizado una nave espacial cerca de allí.

Si aún no lo has adivinado, este Arrakis, oficialmente llamado Arrakis Planitia, pertenece a la segunda luna más grande de nuestro sistema solar, Titán. Arrakis es una vasta llanura de arena indiferenciada, pero no arena como la conocemos. La arena de Titán está hecha de grandes moléculas orgánicas, lo que la haría más suave y pegajosa, dijo Mike Malaska, científico planetario del Laboratorio de Propulsión a Chorro (JPL, por sus siglas en inglés) de la NASA en Pasadena, Calif.

A Malaska le gusta imaginar que la arena de hidrocarburos de Titán, que en realidad se conoce como tholin, o suciedad orgánica compleja, podría duplicarse como la especia infame en el centro del extenso arco narrativo de Dune.

En los libros de Dune, la especia huele a canela, mientras que el tholin en Titán “probablemente huele a almendras amargas…y a muerte”, dijo Malaska.

Arrakis no es el único nombre de las novelas de Dune que adorna las características geológicas de Titán. Todas las llanuras y laberintos (rasgos en forma de cañón tallados en la superficie) indiferenciados de Titán que tienen nombre llevan el nombre de planetas de la serie Dune. Está Buzzell Planitia, que lleva el nombre del “planeta del castigo” utilizado por una antigua orden de mujeres con habilidades sobrenaturales. Está Caladan Planitia, que lleva el nombre del planeta natal del héroe principal de Dune, Paul Atreides. Está Salusa Labyrinthus, que lleva el nombre de un planeta prisión. Y más.

“Estoy asombrado [de] cuánto se parece Titán a la descripción de Arrakis”, dijo Malaska. Además de las vastas llanuras de arenas de hidrocarburos que se extienden a lo largo de la superficie de Titán, el complejo clima de tormentas y lluvia de metano de la luna se siente como de Dune. “Titán es Dune”.

Y, por supuesto, están las dunas. Los campos de dunas de Titán rodean el ecuador de la luna de 16.000 kilómetros de largo. La luna tiene más dunas que la Tierra tiene desiertos.

Rosaly Lopes, otra científica planetaria del JPL, fue una de las primeras personas en ver las dunas de Titán. Ella y otros miembros del equipo Cassini estaban analizando imágenes de uno de los primeros sobrevuelos de Titán de la nave espacial, allá por 2005, y vieron extraños rasgos curvados en la superficie.

“Cuando vimos las dunas por primera vez, no sabíamos que eran dunas”, dijo Lopes. No fue hasta un sobrevuelo posterior de Cassini que confirmaron que Titán tenía dunas en todo alrededor de su ecuador.

Aunque Herbert se inspiró originalmente en las dunas de arena de la costa de Oregón, también podría haber estado imaginando Marte.De hecho, Lopes fue la primera en sugerir nombrar las llanuras y laberintos de Titán en honor a los planetas del universo Dune en 2009, aunque no recuerda exactamente cómo surgió la idea. Ella dijo que tenía sentido, considerando las dunas de Titán.

Los científicos planetarios no nombran los rasgos hasta que existe una necesidad científica para ellos, dijo Lopes. Primero se debe elegir un tema, ya sean aves míticas para áreas interesantes en el asteroide Bennu, o dioses del fuego para volcanes en la luna de Júpiter Io (Lopes nombró a dos de ellos, Tupan y Monan, en honor a deidades de culturas indígenas en su país de origen de Brasil). Hay otros rasgos literarios en el sistema solar, como los cráteres de Mercurio que llevan el nombre de artistas y escritores famosos.

Aunque Herbert se inspiró originalmente en las dunas de arena de la costa de Oregón, Malaska imagina que Herbert, y sus muchos lectores, también podrían haber estado imaginando Marte, el único planeta desértico que conocíamos en la época en que se publicó Dune, en 1965. De hecho, ese mismo año, la NASA hizo su primer sobrevuelo exitoso de Marte con su nave espacial Mariner 4 y la humanidad pudo ver de cerca el Planeta Rojo.

Pero los campos de dunas de Titán son únicos en el sistema solar, y es lógico que esta misteriosa luna lleve el nombre de un revolucionario universo de ciencia ficción.

—JoAnna Wendel (@JoAnnaScience), Escritora de ciencia

This translation by Daniela Navarro-Pérez (@DanJoNavarro) of @GeoLatinas and @Anthnyy was made possible by a partnership with Planeteando. Esta traducción fue posible gracias a una asociación con Planeteando.

New Telescope, New Worlds • Exoplanets in the Shadows   • Overture to Exoplanets   • The Forecast for Exoplanets Is Cloudy but Bright   • “Earth Cousins” Are New Targets for Planetary Materials Research   • Oddballs of the Exoplanet Realm   • Thousands of Stars View Earth as a Transiting Exoplanet   • Taking Stock of Cosmic Rays in the Solar System   • Gap in Exoplanet Size Shifts with Age   • Unveiling the Next Exoplanet Act  

If exoplanets were comic book characters, the first few ever confirmed would have been greeted with cries of “Zounds!” or “Zowie!” or even “Gadzooks!” Not only were these worlds unlike anything in our own solar system, but they were unlike anything scientists had even pondered. The first two were chunks of rock orbiting a pulsar, the remnant of an exploded star. The next one was a gas giant orbiting at just a fraction of the distance between the Sun and Mercury—so close that the planet’s outer atmosphere was heated to more than 1,500°C.

Astronomers have since added more than 4,000 confirmed exoplanets to the list (although the exact number depends on which list you check). Thousands more await verification.

Most of those worlds fit into a few major categories, some of which are alien to our own neighborhood. According to NASA’s exoplanet catalog, for example, there are more than 1,300 super-Earths, which are pretty much what they sound like—rocky planets a few times the size of Earth. Hundreds more are mini-Neptunes, which are bigger than super-Earths but smaller than Neptune, the Sun’s most distant major planet.

Some exoplanets don’t fit into the major categories, though. They are the oddballs. And like many oddballs, they can be more interesting than the conformists.

The first two confirmed exoplanets, discovered 3 decades ago, remain among the oddest and rarest of all: “zombie” planets that probably were born after their star died. Both orbit the pulsar PSR B1257+12. A pulsar is a rapidly spinning neutron star, the corpse of a massive star that exploded as a supernova. As the neutron star spins, it emits pulses of energy that form an extremely accurate clock—and provide clues for exoplanet hunters. The tug of an orbiting object alters the timing of the pulses a tiny bit, revealing a planet’s presence.

Astronomers have discovered a handful of other pulsar planets (including a third for PSR B1257+12). Pulsar timing is so precise that it can reveal orbiting objects as small as asteroids, so the dearth of discoveries suggests that pulsar planets are rare.

It’s unlikely that planets could survive a supernova, so astronomers say these must be “second-chance” planets. They may have formed from debris from a pulsar’s destroyed companion star, such as a white dwarf. “If the star is in a binary with a low-mass star or a compact companion, the pulsar irradiates the companion and the companion evaporates,” said Rebecca Martin of the University of Nevada, Las Vegas. “This can lead to a runaway effect where the companion is dynamically disrupted and forms a disk around the neutron star. Planets may form from this disk.”

The first exoplanet found orbiting a star in the prime of life, similar to the Sun, was just as shocking as the pulsar planets (and earned its discoverers a share of the 2019 Nobel Prize in Physics). Exoplanet 51 Pegasi b is roughly half the mass of Jupiter, the giant of our solar system, yet is close enough to its star that it orbits in just 4 days (compared to 12 years for Jupiter). That makes the planet extremely hot.

And 51 Pegasi b is not even the most extreme “hot Jupiter.” Of the few hundred known examples, some are many times Jupiter’s mass, one orbits its planet in just 18 hours, and some are being blasted by so much stellar radiation that their atmospheres are eroding into space. And although 51 Pegasi b was a true oddball when it was discovered, the roster of hot Jupiters has grown so large that these worlds form a category all their own. (A swelter of hot Jupiters, perhaps?)

Such worlds are hard to explain. Close to a star, temperatures should be too high, and stellar winds should be too strong to allow a planetary core to sweep up enough hydrogen and helium to grow that big.

Most astronomers have hypothesized that hot Jupiters formed farther out in their solar systems and migrated inward. As often happens in comics, though, one character can disrupt the entire narrative. HIP 67522 b, which orbits once every 7 days, belongs to a star that’s only about 17 million years old—hundreds of millions of years younger than most hot-Jupiter hosts. It seems unlikely that the planet could have formed far from the star and then migrated so close in such a short period of time. So scientists may have to go back to the drawing board to explain at least some hot Jupiters.Kepler-51 hosts three planets, all of which are oddballs. They are a few times the mass of Earth but roughly as big as Jupiter. That makes them not much denser than cotton candy.

The star Kepler-51 hosts three planets, all of which are oddballs. They are a few times the mass of Earth but roughly as big as Jupiter. That makes them not much denser than cotton candy. The Kepler-51 worlds are among a dozen or so confirmed “super-puff” planets.

Although some hot Jupiters have been puffed up by the heat from their nearby stars, super-puffs are much cooler, noted Jessica Libby-Roberts, a graduate student completing her Ph.D. at the University of Colorado Boulder. That temperature difference means the super-puffs must be inflated by some other mechanism.

Kepler-51 is a relatively young star, so its planets could be puffed up by the internal heat left over from their formation, Roberts said. Other super-puffs could have formed in “a really weird” region of the disk around the star where they could grab a lot of gas in a hurry. However, super-puffs might not be especially puffy at all. Instead, high haze layers or wide bands of rings might make them appear much larger than they really are.

Except for the planets of Kepler-51, most known super-puffs are the most distant members of multiplanet systems, Roberts said. If they really are puffy, then “either super-puffs need to form really far from their stars before migrating inwards, or they need to end up at a distance far enough from their stars to hold on to all that hydrogen-helium atmosphere, or a combination of both,” Roberts said. “There is still a lot to be done in this area.”

Some exoplanets fit into more than one “oddball” category. WASP-17 b, for example, is a super-puff. It’s half as massive as Jupiter but twice as wide, making it one of the largest and cotton-candiest planets yet discovered. It’s also a “wrong-way” exoplanet, orbiting in the opposite direction from its star’s rotation on its axis—one of only a handful of such planets yet seen.

Scientists suggest that WASP-17 b (and other retrograde planets) could have performed an about-face as the result of the gravitational influence of another planet­, through either a single especially close encounter or a more gradual long-range nudge.

If planet hunters could visit any fictional world of their choosing, there might be a mad dash for Tatooine, the home world of Luke Skywalker. The first Star Wars movie featured an iconic view of Luke watching twin suns set over the desert. Today, any planet found to orbit both members of a binary star is instantly compared to that famous world.

Although quite a few planets are known to orbit one member of a binary, circumbinary planets are about as common as stormtroopers who can shoot straight—astronomers have cataloged roughly a score of them. (One of them, Kepler-64 b, orbits one binary in a two-binary system, giving it four stars).

The known circumbinaries should remain in stable orbits for “at least 100 million years,” according to Jerome Orosz of San Diego State University. Some of the planets even lie within their host stars’ habitable zone, where conditions are most comfortable for life. “It’s obviously more complicated than the habitable zone for a single star,” Orosz said. “In particular, the habitable zone around a binary star moves as the two stars orbit….Keep in mind that the known circumbinary planets are gaseous, with diameters in the range of Neptune’s to Jupiter’s. Those planets probably won’t be habitable. There are no Earth-like planets known to be in circumbinary systems.”

The search for Tatooines in other systems continues, however—perhaps leading to more zowies or zounds in the years ahead.

—Damond Benningfield (damond5916@att.net), Science Writer

New Telescope, New Worlds • Exoplanets in the Shadows   • Overture to Exoplanets   • The Forecast for Exoplanets Is Cloudy but Bright   • “Earth Cousins” Are New Targets for Planetary Materials Research   • Oddballs of the Exoplanet Realm   • Thousands of Stars View Earth as a Transiting Exoplanet   • Taking Stock of Cosmic Rays in the Solar System   • Gap in Exoplanet Size Shifts with Age   • Unveiling the Next Exoplanet Act  

The whole field of exoplanet study is frustratingly tantalizing. We now know for sure there are alien worlds. We can see them! Kinda. We see their shadows; we can see their fuzzy outlines. We are so close to the tipping point of having enough knowledge to truly shake our understanding—in the best way, says this space geek—of Earth’s place in the universe.

The first light of the James Webb Space Telescope (JWST) may be what sends us over that exciting edge. In just a few months, the much-delayed launch will, knock on wood, proceed from French Guiana and take around a month to travel to its destination at the second Lagrange point (L2). “This is certainly an exciting time for exoplanet science, with current missions like Hubble and TESS [Transiting Exoplanet Survey Satellite] providing us with new discoveries and future missions like JWST, which promises to provide incredible new data that will answer some of our current questions and also create many new ones,” said Sarah Hörst of Johns Hopkins University, Eos‘s Science Adviser representing AGU’s Planetary Sciences section who consulted on this issue. “The field is moving very quickly right now.”

That’s why our August issue is all about exoplanets—what we know and what awaits us over the launch horizon. Who gets the first peek through JWST? In March, the proposals selected for the first observing cycle were announced. Meet the slate of scientists who will be pointing the telescope at other worlds, and read what they hope to learn in “Overture to Exoplanets.”

As with all new instruments, the data collected from JWST will be pieced together with observations from ongoing missions and other facilities around the world. “Over the last decade, we’ve gotten gorgeous images from the ALMA interferometer in Chile and have seen loads of fine-scale structure, tracing pebbles in planet-forming disks,” says astronomer Ilse Cleeves in our feature article. Hörst found this synergy with ALMA (Atacama Large Millimeter/submillimeter Array) especially intriguing: “Although I’ve thought a lot about what we’ll learn about individual planets, I hadn’t really thought much about what we’ll be able to learn about planet formation process by studying the disks themselves.”

“I’m excited for all the ‘well, that’s weird’ moments. Those are my favorite things in science because that’s when you know that new discoveries are going to be made.”In “The Forecast for Exoplanets Is Cloudy but Bright,” we learn the immense challenge posed by exoplanet atmospheres, when researchers are still struggling to understand the complex dynamics of clouds on our own planet. And in “Exoplanets in the Shadows,” we look at the rogues, the extremes, and a new field being coined as necroplanetology.

What awaits us when the first science results start coming in from JWST and all the coordinated missions next year? “I’m really excited for the unexpected,” says Hörst. “I’m excited for all the ‘well, that’s weird’ moments. Those are my favorite things in science because that’s when you know that new discoveries are going to be made. I’m also really happy for all of my colleagues who have worked so tirelessly for so many years to make JWST happen.”

We’re pretty happy, too, for the scientists long awaiting this day and for the rest of us who eagerly await a wide new window on our mysterious universe.

—Heather Goss (@heathermg), Editor in Chief, Eos

New Telescope, New Worlds • Exoplanets in the Shadows   • Overture to Exoplanets   • The Forecast for Exoplanets Is Cloudy but Bright   • “Earth Cousins” Are New Targets for Planetary Materials Research   • Oddballs of the Exoplanet Realm   • Thousands of Stars View Earth as a Transiting Exoplanet   • Taking Stock of Cosmic Rays in the Solar System   • Gap in Exoplanet Size Shifts with Age   • Unveiling the Next Exoplanet Act  

When astronomers gathered to reveal “new planets” at a press conference in January 1996, the world paid attention. Hundreds of journalists and fellow astronomers packed the meeting room, where presenters confirmed the identity of one exoplanet and reported the discovery of two others—the first planets known to orbit other Sun-like stars. The story made the front pages of major newspapers (“Life in Space? 2 New Planets Raise Thoughts,” wrote the New York Times), appeared in magazines (including a Time cover story), and aired on television news (including CNN) soon after.

A quarter of a century later, exoplanets still generate headlines—sometimes. With the number of confirmed planets well beyond 4,000 and more being added to the list almost weekly, however, a sort of exoplanet fatigue has set in. Only the most spectacular discoveries show up in our daily newsfeeds: potentially habitable planets, for example, or “extreme” worlds—those that are especially hot or young or blue or close to our solar system.

Yet some of the topics in the penumbra of exoplanet discussions are just as fascinating as those in the spotlight. They remain in the shadows in part because they involve objects that are rare or that are difficult to find and study with current technology. The recently named field of necroplanetology, for example, studies planets orbiting dead or dying stars, providing the only direct look at the innards of exoplanets. Gravitational microlensing allows astronomers to detect planets at greater distances than once thought possible. Several groups of researchers are developing instruments or small spacecraft to look at Earth as an exoplanet analogue, showing us what our planet would look like to an astronomer many light-years away. And the International Astronomical Union (IAU) has begun the long process of bestowing proper names on exoplanets—a process that simply may not have had enough time to filter into the consciousness of either professional astronomers or the public.

“We’ve discovered a lot of weird things,” said Laura Mayorga, an exoplanet researcher and postdoctoral fellow at the Johns Hopkins University Applied Physics Laboratory (APL). “When we first started studying exoplanets, we found that they got stranger and stranger. They put all of our understanding to the test.… Finding something new throws everything up in the air, and it has to resettle. That makes this a really exciting time.”

Although it sounds like something from a Syfy channel original movie, necroplanetology is the newest branch of exoplanet studies—a novelty that involves intrinsically rare targets. The term was coined by Girish Duvvuri, then a student working with Seth Redfield at Wesleyan University in Connecticut, in a 2020 paper. “We’re proud of the name,” said Redfield. “It’s a great way to describe the systems we’re studying. It has a small number of practitioners, but the larger community is just starting to look into this topic.”

The name was originally applied to the study of dead or dying planets around white dwarfs, which are the hot but dead cores of once normal stars. A typical white dwarf is at least 60% as massive as the Sun but only about as big as Earth. The size of white dwarfs makes it easier to detect the remains of pulverized planets as they transit, passing across the face of the star and causing its brightness to dip a tiny bit.

Starlight filtering through an exoplanet’s atmosphere during a transit would reveal its composition. (Astronomers have used the same technique to measure the atmospheres of planets transiting much larger main sequence stars, which are in the prime of life.) “What we started finding first was not whole planets but planetary debris,” Redfield said.

“All those clues made it clear that planets can exist around white dwarfs. They can be destroyed by white dwarfs as well.”In particular, using early observations from the K2 mission of the planet-hunting Kepler space telescope, they found WD 1145+017, a white dwarf about 570 light-years from Earth. The star’s light dipped several times in a pattern that repeated itself every few hours. The researchers concluded that they were seeing the debris of a planet that had been shredded by its star’s gravity—probably chunks or piles of rock surrounded by clouds of dust.

Observations with large ground-based telescopes revealed calcium, magnesium, iron, and other heavy elements in the white dwarf’s spectrum. Such heavy elements should quickly sink toward the core of a white dwarf, where they wouldn’t be detected. Their discovery suggested that the elements had been deposited quite recently, as rubble from a disrupted planet (or planets) spiraled onto the white dwarf’s surface.

“All those clues made it clear that planets can exist around white dwarfs,” said Redfield. “They can be destroyed by white dwarfs as well. The tidal forces are quite extreme, so they can break apart and grind up a planet.… As that material accretes onto the white dwarf, we’re actually learning about the innards of the planets.”

Such a planet may have been born far from its host star and migrated close enough to be destroyed. Astronomers know that such migrations are possible because they have discovered a few hundred “hot Jupiters”: worlds as massive as the largest planet in the solar system but so close to their stars that their upper atmospheres are heated to hundreds or thousands of degrees. Some of these planets are being eroded by stellar radiation and winds, perhaps marking the beginning of the end for worlds that could be subjects for future necroplanetologists.

Despite expectations of a bounty of such white dwarf systems, Redfield said, they seem to be rare. (A recent study found evidence of one intact giant planet around one white dwarf.) Astronomers have found evidence of similar processes at work around main sequence stars, though.

The best-known example is KIC 8462852 (also known as Boyajian’s Star), about 1,470 light-years from Earth. Large, but irregular, dips were discovered in the brightness of the star, which is bigger, hotter, and brighter than the Sun. Possible explanations for the decrease included the panels of a “megastructure” built by an advanced civilization orbiting the star—an idea (since abandoned) that generated plenty of headlines.

Astronomers have discovered other examples of “dipper” stars as well. Edward Schmidt, a professor emeritus at the University of Nebraska–Lincoln, reported 15 slow dippers, whose light varies over long timescales, in study released in 2019. He said he plans to publish details on 17 more in an upcoming paper.

One or more moons could be snatched away as a planet falls into its star. The planet essentially hands its moons to the star—they’re orphaned exomoons.The stars all have similar masses and temperatures, which suggests that their dipping patterns share a common explanation, Schmidt said. “It could be caused by disintegrating planets—that looks promising so far.” He’s looking through published spectra of the stars to see whether their surfaces are polluted by the residue of planets, which could solidify the idea.

A couple of systems discovered by Kepler seem to add credence to the hypothesis. Kepler-1520b, for example, shows dips in luminosity of up to 1.3%. A ground-based study found that the dimming is caused in part by clouds of dust grains, providing “direct evidence in favor of this object being a low-mass disrupting planet,” according to 2015 paper. And K2-22, discovered in Kepler’s K2 mission, appears to be a disintegrating planet more massive than Jupiter but only 2.5 times the diameter of Earth.

Another study suggested a slightly altered explanation for Boyajian’s Star and other dippers: disintegrating exomoons. Researchers suggested that one or more moons could be snatched away as a planet falls into its star. “The planet essentially hands its moons to the star—they’re orphaned exomoons,” said Brian Metzger, one of the study’s authors and a physicist at Columbia University and senior research scientist at the Flatiron Institute.

Stellar radiation could be eroding the surviving moons, releasing solid grains of material that then form a clumpy disk around the star. So the young field of necroplanetology may need a new subfield: necrolunarology.

For some planets, though, the death of a star isn’t necessarily the end—it may be the beginning. The first confirmed exoplanets, discovered 3 decades ago, orbit a pulsar, a dead star whose composition is more exotic than a white dwarf. A pulsar is a rapidly spinning neutron star, the collapsed core of a massive star that exploded as a supernova. As the neutron star spins, it emits pulses of energy that form an extremely accurate clock. The gravitational tug of a companion alters the timing of the pulses a tiny bit, revealing the presence of an orbiting planet.

The first identified pulsar planets orbit PSR B1257+12. Astronomers have since discovered a handful of others, but most searches have come up empty. An examination of more than a decade of observations made by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), a project that is using pulsar timing to hunt for gravitational waves, for example, found no evidence of planets around a set of 45 fast rotating pulsars. The search could have revealed planets as light as the Moon in orbital periods of 1 week to almost 5 years, said Erica Behrens, a graduate student at the University of Virginia who conducted the study during an internship at the National Radio Astronomy Observatory.

“Since we’ve seen so few, it seems like they’re pretty rare,” Behrens said, which may explain why they’ve received so little attention since the early discoveries. “They must have formed after the star has blown up. No planet that existed while the star was still living would be able to survive the supernova.”

Theoretical work hints that instead of supernova survivors, pulsar planets may be “zombies,” born from the debris of companion stars.

Metzger and Ben Margalit, also of Columbia, have suggested, for example, that the companion could be a white dwarf. The extreme gravity of the neutron star tears the white dwarf apart—perhaps in a matter of seconds—and the debris forms a disk around the pulsar. Some of the material in the disk falls onto the neutron star while the outer edge of the disk expands and cools. Solid material in those precincts may condense to form solid bodies, which then merge to make planets.

The scenario would explain the frequency of pulsar planets, which is roughly equal to the frequency of neutron star–white dwarf binaries, Metzger said. It would not, however, explain the birth of a pulsar planet that’s been discovered in a globular cluster, where the density of stars is extremely high. “You’d have to invoke more exotic interactions,” which scientists are still trying to model, he said.

Although most exoplanets have been discovered through transits or radial velocity measurements, which detect a back-and-forth shift in the wavelengths of starlight caused by the pull of orbiting planets, a few stragglers have been found through other methods. Such methods are difficult to apply, or they’re looking for objects or phenomena that are rare, so they’ve yielded far fewer discoveries than the most favored methods.

Astrometry, for example, precisely measures a star’s position to detect tiny wobbles caused by the gravitational tug of orbiting planets. Such measurements are hard to make and have yielded only one or two discoveries. However, astronomers expect observations by the Gaia spacecraft, which is plotting the positions and motions of more than 1 billion stars, to yield thousands of new Jupiter-sized exoplanets in relatively wide orbits, which would create a whole new population for study.

The most successful of the lesser known techniques, however, has been gravitational microlensing which has revealed more than 100 planets. “It’s very complementary to other techniques,” said Matthew Penny, an astronomer at Louisiana State University. “You get an instant detection of some very distant planets that would take decades to find with other techniques.”

Gravitational microlensing relies on general relativity, which posits that if a star or planet passes in front of a more distant star, the intervening object’s gravity bends and magnifies the background star’s light, creating a double image. If the alignment is perfect, it creates a bright circle of light known as an Einstein ring. (The same technique is used on a larger scale to study galaxies and quasars billions of light-years away.)

The length and magnification of a lensing event allow astronomers to calculate the intervening object’s mass and, in the case of a planet, its distance from its star. Astronomers have measured planet-star separations of up to more than 10 astronomical units (AU), which is far wider than with other techniques.

Microlensing can reveal planets that are thousands of light-years away (the current record holder, according to the NASA Exoplanet Archive, is at 36,500 light-years, many times farther than planets discovered with other techniques). Microlensing allows astronomers to study planets in regions of the Milky Way well beyond our own stellar neighborhood, including the central galactic bulge.

Perhaps most important, microlensing is the only technique that can reveal rogue planets, which travel through the galaxy alone, unmoored to any star.

Rogues might form as stars do, from the gravitational collapse of a cloud of gas and dust. That process would form only massive planets—a minimum of 5 times the mass of Jupiter, Penny said. “So far,” however, he explained, “the main results are that there are not a lot of free-floating giant planets out there,” with only a handful of confirmed discoveries to date.

Most rogues probably form from the disk of material around a star, then escape. “It could be an interaction between planets,” Penny said. “If you form a lot of planets in a disk, the disk keeps order until it dissipates. But once the damping effect of the disk is gone, all hell breaks loose,” and gravitational battles can sling planets into interstellar space. There may be billions of these smaller castaway worlds.

Although three searches are dedicated to finding planetary microlensing events, they’re restricted by daylight, clouds, and the other disadvantages of looking at stars from the ground.

As with astrometry discoveries and the Gaia mission, though, a space telescope may greatly expand the numbers of confirmed exoplanets. The Nancy Grace Roman Space Telescope, which is scheduled for launch later in the decade, could find 1,400 bound exoplanets and 300 rogues during its lifetime, Penny said. The telescope’s mirror will be the same size as that of Hubble Space Telescope, but with a field of view 100 times wider. That field of view will allow Roman to see a large area toward the galactic bulge—the preferred target for microlensing planet searches. Current plans call for it to scan the region six times for 72 days per session.

“It’s the ideal platform for doing microlensing because you can never predict when a lensing event will occur, and planetary events are very short,” Penny said.

Roman is expected to help with other exoplanet studies as well. As a technology demonstration, it will carry a coronagraph, which blocks the light of a star, allowing astronomers to see the light of planets directly. “It’ll try to get down to Jupiter-like exoplanets that are closer than Jupiter is now,” said Mayorga. “It might get as close as 1 AU for a Sun-like star.”

Current images of exoplanets, whether from telescopes in space or on terra firma, generally cover a single pixel. To better understand those pictures, scientists use the planets of our solar system as exoplanet analogues. In essence, they take the beautiful pictures of Earth and the other worlds that fill Instagram pages and squish them down to a pixel. “That sets a ground truth for the weird things we find in the universe,” Mayorga said. “It allows us to connect that disk-integrated light to the underlying cloud bands or continents or oceans. It’s the only place we can make that connection.”

Mayorga and colleagues used Cassini images snapped during a flyby of Jupiter as one analogue. They saw how the planet’s brightness and color changed as viewed under different Sun angles or as the Great Red Spot rotated in and out of view.

Several teams are developing missions or instruments that would use Earth as an exoplanet analogue. Mayorga, for example, is involved with a concept known as Earth transit observer, a proposed CubeSat mission that would watch Earth from L2, a gravitationally stable point in space roughly 1.5 million kilometers beyond Earth. Transits of the Sun would reveal the composition of our planet’s atmosphere, including its many “biomarkers,” such as oxygen, ozone, and methane.

Another mission, LOUPE (Lunar Observatory for Unresolved Polarimetry of Earth), would monitor Earth in both optical and polarized light from an instrument that hitches a ride on a lunar orbiter or lander.

“Measuring the linear polarization of a planet over a range of time yields a wealth of information about atmospheric constituents and clouds, as well as surface features like vegetation, water, ice, snow, or deserts,” said Dora Klindžić, a member of the mission team and a graduate student at Delft University of Technology and Leiden Observatory. “By observing Earth from a distance where we can reasonably pretend we are an outsider looking at the Earth, such as from the Moon, we can learn how a planet richly inhabited with life and vegetation appears when observed from another faraway planet. In a way, we are looking at ourselves to know others.”

Interstellar Probe could provide that type of understanding from an even more distant perspective. The proposed spacecraft could travel up to 1,000 AU from the Sun to study interstellar space and would look back toward the planets of the solar system. “Ten, 20, 30 years into the mission, we would have observations of the solar system from outside looking in, as if we were flipping the telescope and taking a look at a planetary system we do know,” said Michael Paul, project manager for the mission study at APL. “Tying that with in situ data we have for Mercury, Venus, Mars, Earth, Jupiter, Saturn will better inform the models we have of other planetary systems.”

Give an object a good name, and people are likely to pay attention. “The fact that [Boyajian’s Star] has this special name means that there aren’t many other objects like it,” said Redfield. Perhaps with catchier names, the “unsung” planets and techniques, which can produce some of the most thought-provoking discoveries, will gain their share of the spotlight.

The three exoplanets discussed at the January 1996 press conference, for example, were designated 51 Pegasi b, 47 Ursae Majoris b, and 70 Gamma Virginis b—the names of the parent stars followed by the letter b. Astronomers have used that naming scheme ever since, with extra planets in a system assigned the letters c, d, e, and so on, on the basis of the order of discovery.

The system works well, although the names get a little confusing when the star has only a long catalog designation; no one’s going to be enchanted by 2MASS J21402931+1625183 A b, for example. And such “telephone book” designations are hardly going to appeal to the public, which regularly sees planets with names like Tatooine and Vulcan and Gallifrey in movies and TV shows.

So the IAU has conducted two international competitions that have produced proper names for more than 140 exoplanets. In the most recent project, 112 countries held individual contests, with each country proposing the name for one planet and its star.

“It was great to tap into the public imagination,” said Eric Mamajek, cochair of the naming campaign steering committee and deputy chief scientist for NASA’s Exoplanet Exploration Program. “I was blown away by the ones that made it through the campaign. The names all have stories.”

Astronomers have been slow to adopt the names, though. The names don’t show up in most of the major online catalogs, for example. “Those phone book names take on the intimacy of a proper name for most astronomers,” said Redfield. “I know that HD 189733 b [an exoplanet he’s studied] is just a bunch of numbers, but for me it has the power of a proper name. I call it ‘189.’ We’re on a nickname basis.”

“I think it will be a long process,” said Mamajek. “It may take a new generation—people who grew up reading these names in textbooks.”

Perhaps that new generation will recognize the first exoplanet confirmed around a Sun-like star not as 51 Pegasi b but as Dimidium or the first pulsar planets not as PSR B1257+12 b and c but as Draugr and Poltergeist.

Damond Benningfield (damond5916@att.net), Science Writer

New Telescope, New Worlds • Exoplanets in the Shadows   • Overture to Exoplanets   • The Forecast for Exoplanets Is Cloudy but Bright   • “Earth Cousins” Are New Targets for Planetary Materials Research   • Oddballs of the Exoplanet Realm   • Thousands of Stars View Earth as a Transiting Exoplanet   • Taking Stock of Cosmic Rays in the Solar System   • Gap in Exoplanet Size Shifts with Age   • Unveiling the Next Exoplanet Act  

The long-awaited launch of the James Webb Space Telescope ( JWST) is finally in sight. Astronomers around the world are anticipating the wealth of information the flagship will gather on everything from the oldest galaxies in the universe to the birthplaces of stars and planets.

“It really is a Swiss army knife telescope with a huge range of applications,” said Elisabeth Matthews, an astronomer at Observatoire de Genève in Switzerland.

JWST, built by a team of more than 1,200 people from 14 countries, will collect infrared (IR) light across a broad range of wavelengths. That makes it ideally suited to studying exoplanets, which bury most of their secrets deep in the infrared spectrum. In this way, among many others, JWST will build on the legacies of the Hubble and Spitzer space telescopes, both of which astronomers have used to make revolutionary leaps in our understanding of distant worlds, although neither telescope was designed to do so. JWST’s instruments, on the other hand, were designed with exoplanets in mind.

The observatory is scheduled to launch by the end of this year, and exoplanet scientists have long been planning what they want to look at first. In 2020 they submitted their proposals to the telescope’s science team, and the selections for JWST’s first observing cycle were announced in March. (An observing cycle is 1 year, or 8,760 hours, of observing time.) More than 20% of JWST’s time during its first observing cycle will be dedicated to understanding exoplanets.

The unifying theme across the exoplanet observing programs? “One word: diversity.”The unifying theme across the exoplanet observing programs? “One word: diversity,” said Stefan Pelletier, an astronomy doctoral student at Université de Montréal. “All bases are being covered in terms of science cases as well as instrument and observing configurations.”

The list of principal investigators (PIs) and coinvestigators on the accepted programs is also more diverse across many axes of identity than space telescope programs have been in the past. Compared with a recent round of Hubble proposals, a higher percentage of PIs who are women and also PIs who are graduate students will make the first JWST observations.

It’s a testament to the hard work by the team at the Space Telescope Science Institute, Matthews said, “both in making sure [members of] the exoplanet community are able to understand the telescope and design good science experiments for it and also in ensuring that the proposals for these science experiments have been carefully and equitably judged.”

Andrew Vanderburg, an astronomer at the University of Wisconsin–Madison, added, “It’s awesome that the [dual anonymous] peer review—where the reviewers don’t know who wrote the proposals, and vice versa—makes it possible for young scientists with good ideas to be awarded time on the world’s most powerful observatory from day one.”

JWST promises to be a game changer for understanding how and what types of planets form and what makes them habitable, but for this first cycle it’s unknown how the telescope’s performance will measure up to expectations. “The reviewers very much wanted a robust ‘shakedown cruise,’” said Peter Gao, an exoplanet scientist at the University of California, Santa Cruz. “Several proposals focused on new and interesting observing methods and science cases that are sure to be the testing grounds for similar, larger, and more elaborate proposals in the next cycles.”

The selected exoplanet observations tend to stay well within the telescope’s expected limitations. “JWST time is very precious, so for the first cycle it is understandable that emphasis was put on programs that are ‘safe’ in that they are almost guaranteed to generate good results,” said Alexis Brandeker, an astronomer at Stockholm University. Some observations might be “risky” in that the scientists aren’t sure what they’ll find, but if they do find something, they’ll get a good look at it.

On the science side, there’s variety both in the types of planets targeted for observations and in the types of observations being made. “These include the measurement of mineral cloud spectral features as a way to probe the composition of exoplanet clouds, exploring asymmetries in the dawn and dusk limbs of exoplanets during transits, eclipse mapping, and getting a sense of which rocky exoplanets host atmospheres,” Gao said.

And on the target side, “there is a nice balance between some of the first exoplanets to be characterized, like HD 189733 b, and weird exoplanets whose observations were difficult to interpret, like 55 Cancri e,” said Lisa Đặng, a physics graduate student at McGill University in Montreal. Instead of making limited observations of a wide range of planets, most of the selected exoplanet programs seek to observe one or a few planets in great detail.

In this first observing cycle, “we are going after a lot of known exoplanets that we have observed in the past, so there aren’t many unexplored targets,” Đặng said. “This makes absolute sense since it will be the first time we are going to use these instruments in space and we don’t really know what challenges we will have to deal with yet.”

“There are some really interesting planets…that we already have tantalizing glimpses of from Hubble and Spitzer data,” said Hannah Wakeford, an astrophysicist at the University of Bristol in the United Kingdom. Wakeford, for example, will be targeting a well-studied, but still mysterious, hot Jupiter, HD 209458 b. “The data we currently have from Hubble tell us there is something in this atmosphere, and my program aims to show that it is clouds made from magnesium silicates (glass),” she said.

Tiffany Kataria, a planetary scientist at NASA Jet Propulsion Laboratory in Pasadena, Calif., is part of one of the five programs studying HD 189733b, a hot Jupiter so normal that it’s called canonical. “This planet was one of the first exoplanets whose atmosphere was observed with the Spitzer and Hubble space telescopes, yet there is still much we don’t know about the properties of its atmosphere,” she said. Kataria will make a 3D map of the planet’s glowing dayside to study its wind and temperature patterns, “which tells us a great deal about the physical processes taking place in the atmosphere.”

Néstor Espinoza’s target is hot Jupiter WASP-63 b and, more specifically, its sunrise and sunset. The program “aims to try to detect, for the first time, the infrared atmospheric signatures of the morning and evening limbs of a hot gas giant exoplanet…. It goes in the direction of exploring atmospheric structure of these distant exoplanets in 3D.” Espinoza is an astronomer at the Space Telescope Science Institute in Baltimore, Md.

Plenty of smaller planets reside among the old favorites that JWST will study, including the Earth-sized lava world 55 Cancri e. Brandeker’s program will examine changes in light when the glowing, molten planet passes behind its star. “We hope to see if consecutive eclipses show the same or different faces of the planet,” he said. Planets that orbit close to their stars are assumed to be tidally locked, having the same hemisphere always facing the star. If 55 Cancri e rotates faster or slower than it orbits, “this assumption, often taken for granted, can be questioned also for other planets. This in turn has major implications for how planets are heated, i.e., one side versus all sides.”

Another old favorite is the sub-Neptune GJ 1214 b, the target of one of Eliza Kempton’s observing programs. “Through a combination of mid-IR transmission spectroscopy, plus thermal emission and secondary eclipse observations, we aim to get a clearer picture of the atmospheric composition and aerosol properties of this enigmatic world,” she said.

“The overlap with existing observations is not a main motivator because we expect JWST to perform so much better than existing facilities,” said Kempton, an exoplanet astronomer at the University of Maryland in College Park. “But it will certainly be reassuring to see that the JWST data do agree with prior observations, and the level of agreement will help us to contextualize all data taken previously with facilities like Hubble and Spitzer.”

Newest among the old favorites soon to be studied by JWST is the TRAPPIST-1 system, which excited astronomers and the public alike when it was discovered to have seven possibly rocky Earth-sized planets.

“JWST has a small chance of finding biosignatures on TRAPPIST-1 planets …but a very good chance of telling us which molecules dominate the atmosphere and whether there are clouds.”A grand total of eight different programs will look at these planets’ atmospheric properties. “With this program,” said Olivia Lim, an astronomy doctoral student at Université de Montreal and PI for the program, “we are hoping to determine whether the planets have an atmosphere or not, at the very least, and if they do host atmospheres, we wish to detect the presence of molecules like [carbon dioxide, water, and ozone] in those atmospheres. This would be an important step in the search for traces of life outside the solar system.”

“JWST has a small chance of finding biosignatures on TRAPPIST-1 planets,” said Michael Zhang, “but a very good chance of telling us which molecules dominate the atmosphere and whether there are clouds.” Zhang is an astronomy graduate student at the California Institute of Technology in Pasadena.

Some exoplanets just don’t fit inside the box as neatly as other exoplanets do, and astronomers are really hoping that JWST will help them understand why that is. Kataria leads the program to study one of these oddballs, HD 80606 b.

“HD 80606 b is an extreme hot Jupiter, and that’s saying something, given that hot Jupiters are pretty extreme to begin with!” Kataria said. “This Jupiter-sized exoplanet is on a highly eccentric, or elliptical, orbit and experiences a factor of greater than 800 variation in flux, or heating, throughout its 111-day orbit.”

“Most of the time it spends at relatively temperate distances,” Brandeker added, “but once every 111 days it swooshes very closely by the star in a few days [and] gets ‘flash heated.’”

Studying HD 80606 b’s atmosphere as it heats and cools “will really help us examine the pure physics behind atmospheric energy transport, which is important for all worlds,” Wakeford said.

Kataria is also a coinvestigator on a program to make a 3D atmospheric map of a different oddity, WASP-121 b, a gas giant so hot that it bleeds heavy metals into space and orbits so close to its star that it’s shaped like a football. WASP-121 b is one example of a “super-puff” planet: These planets are roughly the size of Jupiter but far less massive, which makes their density closer to that of cotton candy. Pelletier will be looking at another super-puff, WASP-127 b. “Our hope is to gain a better understanding of the carbon budget on a planet vastly different from anything we have in our solar system,” he said.

What’s the most important thing to learn about super-puff planets? “Basically anything!” according to Gao, whose program will target super-puff Kepler-51 b. “All previous attempts at characterizing super-puff atmospheres have yielded featureless spectra and therefore very little information. If our observation is anything but a flat line, then we will have learned so much more than what we now know about these mysterious objects. It really is a fact-finding mission.”

M dwarf stars are the smallest and most common stars in the universe, and astronomers have found that they host plenty of planets. Rocky habitable planets around these stars are easier to find using the two most prevalent methods—transits and radial velocity—but whether those planets can host atmospheres is still debated.

“I think the Cycle 1 observations will teach us a ton about whether rocky planets around M dwarfs can keep their atmospheres,” said Laura Kreidberg, director of research into the atmospheric physics of exoplanets at the Max Planck Institute for Astronomy in Heidelberg, Germany. “This is one of the most fundamental questions about where life is most likely to arise in the universe. There are tons of these small planets around small stars”— more than 1,500 are known so far—“but they experience more high-energy radiation over their lifetimes, so it’s not known whether they can keep their atmospheres. No atmosphere [is] bad news for life!”

Both of Kreidberg’s observing programs will target rocky planets around M dwarfs. “One of the planets [LHS 3844 b] is already known to not have an atmosphere, so the goal of this program is to study the planet’s surface composition—what type of rock it’s made of—and search for any hints of volcanic activity, which could produce trace amounts of sulfur dioxide.”

Kreidberg is also looking at TRAPPIST-1 c, “which is very close to Venus in temperature. For that planet, I’m searching for absorption from carbon dioxide, to test whether the planet has a thick, Venus-like atmosphere or whether the atmosphere has been lost.”

“While we have made many models of atmospheric loss for small planets,” Gao said, “this will be our first real test of these theories. Will we find out that most characterizable rocky planets don’t actually have atmospheres and that our modeling efforts for their climates and habitability are futile? Or will we see a much more diverse set of atmospheric states? The results of these studies will be interesting and informative for future cycles regardless of what we find.”

About half of JWST’s exoplanet-specific observing time will be dedicated to studying worlds smaller than Neptune. “This tells me without a doubt that the community is overwhelmingly interested in the little guys,” Gao said. These planets might be rocky (if they’re small enough) or could have a rock-ice core and a thick atmosphere.

“The large program on sub-Neptune and super-Earth atmospheres led by Natasha Batalha and Johanna Teske is especially exciting to me because it will provide us with a systematic survey of a class of planets that is not present in our solar system and was not readily observable with previous facilities,” Kempton said. “The potential for this program to unlock greater insight into the atmospheres of small planets is quite high.”

“These planets are so small that they’re beyond the reach of current technology, so anything JWST discovers will be a big improvement on what we know,” Zhang said. “For small planets like GJ 367 b, my target, and 55 Cancri e, we basically don’t know anything, so we’ll learn the first thing about them. Do they have atmospheres? If so, are they carbon dioxide, oxygen, or exotic metal atmospheres made of sodium and silicon oxide?”

One of Espinoza’s programs will focus on super-Earth K2-141 b, a planet only slightly larger and more massive than Earth but much, much hotter. “Depending on the properties of this exoplanet like the presence or not of an atmosphere, the flux change during its orbit around the star should give rise to very different signals, which will enable us to infer what this exoplanet’s exterior is made of,” said Espinoza.

If K2-141 b does have an atmosphere, it might not be the one it started with. Lisa Đặng aims to find out. Rocky planets as hot as that one “are thought to have lost any primordial atmosphere but, instead, could sustain a thin rock vapor atmosphere [that] outgasses from the mantle,” she said. Does the atmosphere stick around or rain back down? “With our observations we are hoping to detect molecular signatures of the atmospheric constituents and also obtain a map of the planet’s atmosphere and surface.”

JWST should build upon discoveries made not only by space telescopes like Hubble and Spitzer but also by ground-based observatories like the Atacama Large Millimeter/submillimeter Array (ALMA). These observations will probe the birthplaces of planets: the disks of dust and gas around young stars. “Over the last decade, we’ve gotten gorgeous images from the ALMA interferometer in Chile and have seen loads of fine-scale structure, tracing pebbles in planet-forming disks,” said Ilse Cleeves, an astronomer at the University of Virginia in Charlottesville. “Some of the structures likely trace planets in formation, and so it’ll be very exciting to see what JWST uncovers, both in terms of patterns in the disk and perhaps even the drivers—protoplanets—themselves!”

Matthews added that “if JWST is able to successfully detect planets in these disks, it will be an important confirmation of our understanding of how planets interact with disks.” If no planets appear in the disks, astronomers will have to rethink how, and whether, planets shape disks.

Cleeves will be studying planet-forming disks to understand how they give rise to habitable planets. “How common are habitable planets? Availability of water is a natural place to start, but we don’t have great observational constraints on how much water is present or the distribution of water in disks. We are looking forward to mapping out water ice in a nearby disk that happens to be posing in front of a host of background stars.” If a star’s light passes through a part of the disk that has ice, the ice will imprint a spectroscopic signal on the light. With so many background stars, Cleeves said, they’ll be able to say not just whether ice is present, but also where.

The makings of a world well suited for life go beyond the presence of water, however, and Melissa McClure’s three observing programs will look for them. We’ll “trace how the elemental building blocks of life—like carbon, hydrogen, oxygen, nitrogen, and sulfur—evolve between molecular clouds, where they freeze out on dust grains as ices, and protoplanetary disks, where these ices are incorporated into forming planetesimals and, ultimately, planets,” she said. “I think that within a few years we will have an understanding of how much water terrestrial planets typically have and whether they inherited that water from their birth locations in their disks or if cometary delivery was necessary.” McClure is an assistant professor and a Veni Laureate at Leiden Observatory in the Netherlands.

“This planet orbits close enough to the white dwarf that it could not have originally orbited there before the star’s death. So how did it get there?”A perhaps underrecognized component of JWST’s observing capabilities is the coronagraph that will allow direct imaging of exoplanet systems, meaning that the telescope will see light emitted by the planet itself. Coupled with JWST’s infrared capabilities, the telescope will be able to observe planets much older and colder than is currently possible. That’s Matthews’s aim. “Eps Indi Ab is similar in age to the solar system and is similarly far from its star as Jupiter is from the Sun. Because JWST is able to image much further into the infrared than Earth-based telescopes and because old planets are brighter at these very long wavelengths, our project provides a unique opportunity to study a truly Jupiter-like planet outside the solar system,” she said.

Sometimes planets survive their star’s demise, as is the case of WD 1856+534 b, a gas giant planet that orbits the slowly cooling corpse of a star, also known as a white dwarf. In this case, the planet’s survival presents a puzzle. “This planet orbits close enough to the white dwarf that it could not have originally orbited there before the star’s death. So how did it get there?” asked Andrew Vanderburg, whose program will target this system.

Once the “shakedown cruise” is complete, Hannah Wakeford would like to see JWST used to study more worlds the size of Jupiter, Saturn, and Neptune. “There is so much we can learn that we can’t even get from our own solar system giant planets,” she said, “so it is, in my opinion, a low-risk, high-reward scenario.”

“The very first exoplanetary observations to be made by JWST are going to be a big jump into the known unknown…. As the title of an album of one of my favorite rock bands would say, ‘Expect the unexpected.’”On Vanderburg’s wish list: “Disintegrating planets. These will be great probes of the interior compositions of planets, so I hope we will get observations of them in the future.”

Cleeves called the first cycle “a great place to start. I have a feeling, though, that the most interesting next projects are those that we haven’t anticipated yet, so I’m really looking forward to the first couple of years with JWST, grappling with the data and finding those unexpected puzzles.”

Espinoza agreed. “I’m almost convinced features will show up in the data that we will perhaps not be able to explain right away,” he said. “As such, the very first exoplanetary observations to be made by JWST are going to be a big jump into the known unknown…. As the title of an album of one of my favorite rock bands would say, ‘Expect the unexpected.’”

Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

New Telescope, New Worlds • Exoplanets in the Shadows   • Overture to Exoplanets   • The Forecast for Exoplanets Is Cloudy but Bright   • “Earth Cousins” Are New Targets for Planetary Materials Research   • Oddballs of the Exoplanet Realm   • Thousands of Stars View Earth as a Transiting Exoplanet   • Taking Stock of Cosmic Rays in the Solar System   • Gap in Exoplanet Size Shifts with Age   • Unveiling the Next Exoplanet Act  

The first time scientists measured the atmosphere of an exoplanet—a planet outside our solar system—they found something unexpected in the signal. It was 2001, and the Hubble Space Telescope was trained on HD 209458 b, a recently discovered gas giant roughly the size of Jupiter.

When astronomers looked for the presence of sodium in light waves shining through the planet’s atmosphere as it crossed in front of its star, there was a lot less of it than they thought there would be, said Hannah Wakeford, a lecturer in astrophysics at the University of Bristol in the United Kingdom. “From the very first measurement of an exoplanet atmosphere, there was evidence that something else was happening, something else was there blocking the light.”

The most compelling theory for what that something could be? Massive banks of dark, hot clouds. “Clouds are essentially liquid or solid droplets or particles that are suspended in a gaseous atmosphere,” said Wakeford. But because the planet is so hot—5 times hotter than Earth—those droplets couldn’t be made of water, as they are on Earth.

In the 2 decades since analyzing the atmosphere of HD 209458 b, astronomers have discovered more than 4,000 exoplanets. Using spectroscopy, they have measured the atmospheres of more than 100 of those objects, and it looks like many of them are cloudy. The way those extraterrestrial clouds behave and the exotic things they could be made of—liquid sand, iron, even rubies—are stretching scientists’ ideas of what terms like clouds, rain, and snow even mean in the context of the universe.

“Clouds are everywhere,” said Laura Kreidberg, an astronomer at the Max Planck Institute for Astronomy in Germany. “And to have any hope of understanding what’s going on in [exoplanet] atmospheres, we have to understand the clouds.”

The trouble is, clouds are complicated. Even on Earth, clouds are difficult to model (one reason weather forecasts can still lack accuracy.) Their complexity arises partly because they are simultaneously very small and very large: made up of microscopic water droplets yet so vast they can cover more than two thirds of Earth’s surface. Another reason is that there are so many kinds of clouds, and they behave in complex ways, explained atmospheric physicist David Crisp at the Jet Propulsion Laboratory, California Institute of Technology.

Clouds are “ubiquitous; they can form in many different kinds of environments, and there are many processes associated with their formation,” Crisp said.

And they’re not made only of water, either. Most cloud particles start growing on condensation nuclei—a speck of dust or a grain of salt. And although most earthly cloud droplets are spherical and liquid, those that make up cirrus clouds are hexagonal ice crystals.

Clouds can frustrate scientists’ ability to see clearly, whether they are gazing at the heavens from the ground or peering back at Earth from space. In the 1980s, Crisp helped build the camera in the Hubble Space Telescope and now leads a NASA team that uses orbiting satellites to measure the dangerous levels of carbon dioxide accumulating in Earth’s atmosphere. “I’ve learned to hate clouds from both sides now,” he joked.

Clouds mess with models predicting future climate change, he said, because they simultaneously warm and cool the planet, depending in part on whether their droplets are mainly liquid or mainly ice. In general, low-lying, mostly liquid clouds provide shade and reflect solar energy back into space, whereas high-altitude, frozen cirrus clouds trap infrared radiation emitted by Earth’s continents and oceans and intensify surface heating. This duality has long frustrated exoplanet cloud watchers, too—scientists scrutinize cloud signals to better understand how or whether clouds are heating the atmosphere below them.

Scientists are still trying to understand whether, at a global level, those cooling and warming effects cancel each other out and how that balance could change in the future. (One recent study even suggested that at carbon dioxide levels of around 1,200 parts per million, global cloud cover could become unstable and dissipate, dramatically accelerating warming.)

“We’ve dropped a few dozen probes into the atmosphere of Venus. But you know, if you measured Earth’s atmosphere with only a dozen instruments, how much would you know about the Earth?”Despite the uncertainties, we know a lot more about Earth’s clouds than we do about those on other planets and moons of our solar system. It was only in the 1970s, for instance, that scientists figured out that Venus is enveloped in clouds of sulfuric acid. “This stuff will strip paint—and just about anything else,” said Crisp. Space missions to Venus have dropped mass spectrometers into the planet’s atmosphere that, “even though sulfuric acid is not very nice to our mass spectrometers,” have managed to send back data about the chemical makeup and concentrations of several cloud layers.

Jupiter’s atmosphere has been sampled too, and has been found to contain swirling ammonia clouds. Recent flybys of the tops of these clouds by NASA’s Juno mission identified mushballs—Jovian hailstones formed out of water-ammonia slush enrobed in an ice crust—that fall through the planet’s atmosphere. On the way down, these mushballs collide with upward moving ice crystals and electrify the clouds, causing shallow, high-altitude lightning visible from space.

Thanks to the Cassini spacecraft, we know that the atmosphere on Titan, the largest of Saturn’s moons, is largely made up of nitrogen, like Earth’s. There are seasons, monsoons, and wild windstorms. But Titan’s mountains are made of solid ice, and instead of a water cycle, it has a hydrocarbon cycle: On Titan, the rain, rivers, and lakes are made of methane and ethane.

But many questions remain when it comes to solar system weather. For example, we don’t know how deep into Jupiter the mushballs fall before they evaporate and rise again, said Wakeford. There are mysterious long-chain hydrocarbons floating high in the atmosphere of Titan too. “We have absolutely no idea how they got there; it’s baffling.”

What knowledge we do have is drawn from the briefest of snapshots, added Crisp. “We’ve dropped a few dozen probes into the atmosphere of Venus. But you know, if you measured Earth’s atmosphere with only a dozen instruments, how much would you know about the Earth? These planets are big places, and they have complicated climates—quite as complicated as ours.”

The challenges of analyzing extraterrestrial clouds are magnified when it comes to exoplanets. We can’t send a probe laden with instruments to any of them or record detailed images of their surfaces.

All we have is light, said Heather Knutson—the light coming from a far-off star. “We know there’s a planet in orbit around it, and we can indirectly infer some basic things about that planet, but it’s really a sort of poor man’s camera,” said Knutson, an astronomer at the California Institute of Technology

“If we’re going with the X-ray analogy, clouds are sort of like a lead blanket over the planet.”When an exoplanet passes in front of its star—an event called a transit—astronomers can measure the way light passes through the planet’s atmosphere on its way to us. Measuring how opaque the atmosphere is at different wavelengths of light (transit spectroscopy) offers clues to its composition. Kreidberg used an X-ray analogy to explain how it works: “Our bodies are opaque in optical light. If you shine a flashlight at a person, you can’t see through them. But if you look in the X-rays, you can see through the skin, but not through the bones.”

In the same way that our skin differs from our bones, molecules in planetary atmospheres are opaque or transparent at different wavelengths. “Whether it’s water or methane or oxygen or carbon dioxide, they have distinct opacity at different wavelengths of light,” said Kreidberg. “So if the planet looks a little bit bigger at a particular wavelength, then we can work backward from that to try to infer what’s in its atmosphere.”

But clouds get in the way of that process, said Knutson. “If we’re going with the X-ray analogy, clouds are sort of like a lead blanket over the planet. You see something that looks very featureless.”

Still, on the basis of the planet’s average atmospheric temperature—something astronomers can estimate from the brightness of the star and the planet’s distance from it—it’s possible to infer what those clouds are likely to be made of because of the varying temperatures at which different molecules condense from gas into liquid.

And the vast range of possible temperatures is something that distinguishes exoplanets from those in our solar system, said Nikku Madhusudhan, an astrophysicist and exoplanet scientist at the University of Cambridge. “Because of that vast range, you allow for a much wider range of chemical compositions [than in the solar system]. A lot more chemistry can happen.”

Here on Earth, with an average temperature of 290 K, clouds are made mostly of water. The atmospheres of some exoplanets, between 400 K and 900 K, are warm enough to condense salts and sulfides into clouds. At around 1,400–2,000 K (a third as hot as the Sun), we would expect to see clouds of molten silicates—the material that makes up the volcanic sand on some of Earth’s beaches and is used in the production of glass. On an even hotter planet like WASP-76b, which is estimated to reach 2,400 K, clouds are likely made of liquid iron. And the atmospheres of the hottest known exoplanets—giant, 2,500+ K ultrahot Jupiters orbiting very close to their stars—are the right temperature for clouds made of corundum, a crystalline form of aluminum oxide that forms rubies and sapphires on Earth.

“These are quite literally the gems and jewels that we have here on Earth forming clouds and lofted high into the atmospheres of Jupiter-sized worlds that are lit glowing from their star,” said Wakeford. She remembered walking through the Hall of Gems in London’s Natural History Museum after learning this, trying to imagine the crystals molten and forming clouds. “It just blew my mind.”

WASP-76 b made headlines in 2020 when a team of European researchers published a paper suggesting it had not only clouds of iron but iron rain as well.

“We see the iron, and then we don’t see the iron. So it has to go somewhere, and the physical process that we expect is rain.”Like our own Moon and many planets that orbit very close to a star, WASP-76 b is tidally locked, meaning one side of the planet always faces the star (dayside) and the other always faces away (nightside). Researchers found evidence of iron atoms in the atmosphere of WASP-76 b’s hotter dayside but not on the cooler nightside, which they argued meant that the iron must be condensing into liquid droplets as wind carries the atoms around the planet. “We see the iron, and then we don’t see the iron. So it has to go somewhere, and the physical process that we expect is rain,” said Kreidberg, who was not involved in the study. “This is some of the most convincing evidence I have ever seen for exoplanet weather.”

But Caroline Morley, an astrophysicist at the University of Texas at Austin, cautioned that the phenomenon could be more complex. Recent studies, including one co-authored by Kreidberg, have examined the microphysics of how iron droplets form, finding that the substance’s high surface tension means that it doesn’t easily condense from a gas to a liquid. There might be some other processes involved in WASP-76 b’s iron phenomenon, Morley said—perhaps the iron interacts with some other chemicals in the planet’s atmosphere, which helps it form a cloud.

“Statistically, I believe that there are exoplanets where it is raining right now,” she said. “But I think that we have not seen smoking gun evidence for rain on other planets yet.”

Crisp agreed. “Clouds we’ve detected. Rain and snow have not yet been detected—but I’d be surprised if they weren’t there. Those are logical outcomes of the systems we see.”

So when astronomers talk about possible rain on exoplanets, is it really what we would think of as rain? What do the concepts of rain and clouds even mean in the context of distant space? To some extent, it’s all a metaphor, said Wakeford.

“We have to be open to the fact that the complexity in nature may greatly surpass our imagination at the present time.”On Earth, the terms rain, clouds, and snow all apply almost exclusively to one substance: water. “Water is one of the most amazing materials in the universe,” Wakeford said, but not all substances behave like water when experiencing differences in pressure or temperature. “So when we frame these very alien clouds and rain and snow in that [water-based] context, it puts things in our minds that aren’t exactly what the physics is.”

For instance, words like snow and hail can be a bit misleading when you talk about solid particles in an atmosphere that’s hotter than a lava flow. “I tend to use rain instead of snow,” Wakeford said, “because snow to us evokes a temperature, a coldness. Rain is something that can define many different types of conditions, whereas snow for us is very much a cold thing. And this is not what’s happening here on some of these planets that are so incredibly hot.”

Still, Wakeford thinks a smattering of poetic license is justified to bring the public along on the journey and capture people’s imaginations. “If you start by saying, ‘It’s raining drops of glass on these planets’—that’s a starting point. I can use that; I’ve got [your attention] now. Then we can build on that and get a deeper understanding.”

When it comes to actually doing the research, though, scientists should be both circumspect and open-minded, said Madhusudhan. Although it can sometimes help to extrapolate from what we’re discovering about Earth’s clouds to these faraway planets, for instance, it’s important to remember that these worlds are so exotic that it’s possible there are processes going on in their atmospheres that we haven’t even considered. “The biggest mistake we could make is to try to simplify the complexity of exoplanetary systems just to fit a narrative.”

We may go on to discover kinds of weather we don’t even have words for, said Madhusudhan. “We have to be open to the fact that the complexity in nature may greatly surpass our imagination at the present time.”

So far, everything we know about clouds on exoplanets has been based on what Madhusudhan calls indirect inference: “It’s a bit more real than philosophical but a bit less real than an actual observation.” But the launch of the international James Webb Space Telescope ( JWST) near the end of this year promises to give astronomers the chance to make direct observations of exoplanet clouds for the first time.

JWST will keep Earth between it and the Sun and is designed to look at the longer wavelengths of infrared light. “Planets are easier to study in the infrared,” said Knutson. The telescope will make faraway objects look brighter than they do in visible light and will be better able to detect molecules in exoplanet atmospheres. It should also advance our understanding of alien weather.

“When you go to midinfrared, the composition of a cloud droplet starts to matter—the way that it scatters light is different for different cloud species,” said Knutson. “So we might, for the first time, directly measure what the clouds are made of.”

Morley is leading a team that will use JWST to examine a cold exoplanet called WISE J085510.83−071442.5 to test for the presence of water ice clouds and see whether they are changing as the planet rotates, implying that there are storm systems and weather. “That would give us real evidence, for the first time, that there’s water ice forming in a planet outside of the solar system,” Morley said.

Wakeford, meanwhile, will have a chance to train the telescope on HD 209458 b, the very first planet that 20 years ago was assumed to have clouds of magnesium silicate. JWST will give her a chance to prove (or disprove) that assumption with direct measurements.

Overall, “I think we think about clouds more broadly than anybody has thought about clouds in human history,” said Morley. “And we’re just on the cusp of being able to get a huge amount of really detailed information about those clouds. It’s a really exciting time to be in this field.”

Kate Evans (@kate_g_evans), Science Writer

New Telescope, New Worlds • Exoplanets in the Shadows   • Overture to Exoplanets   • The Forecast for Exoplanets Is Cloudy but Bright   • “Earth Cousins” Are New Targets for Planetary Materials Research   • Oddballs of the Exoplanet Realm   • Thousands of Stars View Earth as a Transiting Exoplanet   • Taking Stock of Cosmic Rays in the Solar System   • Gap in Exoplanet Size Shifts with Age   • Unveiling the Next Exoplanet Act  

The Milky Way is home to billions of stars and probably an even larger number of planets. To search for these planets, scientists watch for minute, but periodic, dips in a star’s light, the telltale signature of a planet passing in front of its host star. And now a team of astronomers has turned that search homeward.

Lisa Kaltenegger and Jackie Faherty calculated the number of stars whose past, present, or future vantage points in space afford a look at Earth passing in front of the Sun. The planets that orbit these roughly 2,000 stars, some of which have additionally been bathed in human-made radio waves, are poised to spot Earth, the researchers suggest.

Astronomers have confirmed the existence of more than 4,000 extrasolar planets, and we’re now turning the tables, said Kaltenegger, an astronomer at Cornell University in Ithaca, N.Y. “It’d be interesting to know if someone could have seen us.”

Kaltenegger and Faherty, an astrophysicist at the American Museum of Natural History in New York City, mined data collected by the European Space Agency’s Gaia mission. The Gaia spacecraft, launched in 2013, is conducting the most precise survey to date of the motions and positions of more than a billion stars in the Milky Way. “It’s the greatest kinematic and astrometric catalog of our time,” said Faherty.

The researchers honed in on stars within the so-called Earth transit zone, a ring-shaped region of space created by projecting Earth’s orbit around the Sun outward into the cosmos. Any stars—and, by extension, the planets orbiting them—located within this zone see Earth passing in front of the Sun. Astronomers on Earth have exploited this geometry, known as a transit, to detect thousands of far-away planets.

Recent studies of Earth’s transit zone have typically focused on the stars within it right now, said René Heller, an astrophysicist at the Max Planck Institute for Solar System Research in Germany not involved in the research. In 2016, Heller and a colleague published a study defining Earth’s transit zone and showing that roughly 80 stars could currently see Earth transiting the Sun. “We didn’t care too much about the temporal aspect,” he said.

But Kaltenegger and Faherty have now considered the changing vantage points of stars over time, into and out of Earth’s transit zone. That’s an important distinction, said Heller, because stars are in constant motion around the center of the galaxy. A solar system that observes Earth as transiting now might not have the same view in the future, he said.

Kaltenegger and Faherty restricted their analysis to the roughly 331,000 stars in the Gaia Catalog of Nearby Stars. These stars, all within roughly 300 light-years of Earth, are the best candidates for follow-up study given their relative proximity, said Faherty.

The researchers then propagated the motions of these stars backward and forward in time. At each annual time step over a 10,000-year window, they determined whether a star fell within Earth’s transit zone. The researchers found that 313 stars were in this zone in the past, 1,402 are in it currently, and 319 will enter it in the future. (Seventy-five of the closest stars in the sample receive yet another tip-off to Earth’s presence: These worlds are close enough to us to have already been bathed in human-produced radio waves.)

Astronomers already know that 7 of these 2,034 stars host planets—17 are known thus far, including 7 in the TRAPPIST-1 system. Many more, if not most, of the remaining stars likely also have their own planets, all of which could see Earth passing in front of the Sun. To get a handle on the total number of rocky, habitable planets potentially orbiting these stars, Kaltenegger and Faherty conservatively estimated that 25% of stars host such a planet. That means that more than 500 planets could witness Earth transiting the Sun, the team concluded.

These results were published in Nature in June.

Future observations in the search for extraterrestrial life ought to target these worlds, said Faherty. They’re the ones closest to us and most likely to have spotted Earth, she said. “These are our best shots.”

—Katherine Kornei (@KatherineKornei), Science Writer

Sometime this decade, humans will probably stand on the Moon for the first time since 1972. U.S. president Joe Biden recently committed to NASA’s Artemis program, which aims to land the first woman and the first person of color on the lunar surface by 2024. Other countries and private companies want to send people, too.

This time, they might take more than photographs and a few rocks.

Mining on the Moon is becoming increasingly likely, as growing numbers of countries and corporations hope to exploit its minerals and molecules to enable further exploration and commercial gain. The discovery of water on the lunar surface has raised the possibility of permanent human settlement, as well as making the Moon a potential pit stop on the way to Mars: Water can be split into hydrogen and oxygen and used to make rocket fuel.

In 2015, the U.S. Congress and President Barack Obama passed legislation that unilaterally gave American companies the right to own and sell natural resources they mine from celestial bodies, including the Moon. In 2020, President Donald Trump issued an executive order proclaiming that “Americans should have the right to engage in commercial exploration, recovery, and use of resources in outer space…and the United States does not view it as a global commons.”

Other countries are also interested in exploring our nearest celestial neighbor. In 2019, China landed a probe on the farside of the Moon. Russia is resurrecting its Moon program, planning a series of missions starting in 2021 to drill into the surface of the lunar south pole and prospect for water ice, helium-3, carbon, nitrogen, and precious metals.

Corporations have been plotting out their own ways to claim resources on the Moon, including U.S.-based SpaceX and Blue Origin, and the Japanese lunar exploration company ispace—which, according to its website, aims to mine water and “spearhead a space-based economy.” The company also anticipates that by 2040 “the Moon will support a [permanent] population of 1,000 people with 10,000 visiting every year.”

But what effects might these activities have on Earth’s only natural satellite? Who gets to decide what happens on the Moon?

In a bid to get more people thinking about these questions, and to start a conversation about the ethics of exploiting the lunar landscape for profit, a group of mainly Australian academics have come up with a draft Declaration of the Rights of the Moon, which they hope members of the global public will sign and discuss.

“We the people of Earth,” the declaration begins, and goes on to assert that the Moon is “a sovereign natural entity in its own right and…possesses fundamental rights, which arise from its existence in the universe.” These rights include “the right to exist, persist and continue its vital cycles unaltered, unharmed and unpolluted by human beings; the right to maintain ecological integrity…and the right to remain a forever peaceful celestial entity, unmarred by human conflict or warfare.”

Given the acceleration of planned missions and ongoing legal uncertainty over what private companies are allowed to do in space, the authors said, “it is timely to question the instrumental approach which subordinates this ancient celestial body to human interests.” Now is the time, they said, to have a clear-eyed global debate about the consequences of human activity in a landscape that has remained largely unchanged for billions of years.

The aim of the declaration is to give the Moon a voice of its own, as a celestial body with an ancient existence separate from human perceptions.The declaration was penned after a series of public fora organized by Thomas Gooch, a Melbourne-based landscape architect. The discipline of landscape architecture is well suited to having a voice in Moon exploration, he said: “We walk the line of science, art, creativity, nature, and human habitation.”

Existing international space agreements address safety, conflict reduction, heritage preservation, sharing knowledge, and offering assistance in emergencies. These are all people-centric concerns; the aim of the declaration is to give the Moon a voice of its own, as a celestial body with an ancient existence separate from human perceptions, Gooch said.

The Moon might not have inhabitants or biological ecosystems—or, at least, we haven’t found any yet—but that doesn’t mean it is a “dead rock,” as it is sometimes described. “Once you see something as dead, then it limits the way you engage with it,” said Gooch.

The declaration, as coauthor Alice Gorman sees it, is a position statement to which companies and countries operating on the Moon could be held accountable. Gorman is a space archaeologist studying the heritage of space exploration (and the junk humans leave behind) at Flinders University in Adelaide, Australia.

“Have they respected the Moon’s own processes?” she asked. “Have they respected the Moon’s environment? Some of the time, the answer to that is going to be no, because you can’t dig up huge chunks of a landscape and expect there to be no impact.

“But if that’s the guiding principle, if that’s something that they’re attempting to achieve from the beginning, then that’s surely got to give us a better outcome than if we turn around in 10 years’ time and realize that if you look at the Moon with the naked eye you can see the scars of mining activities.”

Recent discoveries suggest the Moon is a much more complex and dynamic place than was previously thought, said Gorman.

Mining will require extraction machinery, processing facilities, transportation infrastructure, storage, and power sources. “It’s not just, ‘Let’s dig a hole on the Moon.’”It has seismic activity, including moonquakes and fault lines. Ancient water ice was directly observed at both lunar poles in 2018, hiding in shadowy areas that haven’t seen sunlight in 2 billion years. “Surely that’s environmentally significant,” said Gorman. “Even in completely human terms, 2-billion-year-old shadows are aesthetically significant.”

Individual water molecules have also recently been identified on the Moon’s sunlit surface, and there may even be a water cycle happening, with the molecules bouncing around over the course of a lunar day.

Gorman is vice chair of an expert group affiliated with the Moon Village Association, an international organization that hopes to establish a permanent human presence on the Moon. “I’m as motivated by the excitement of space science as the most hardcore space nut,” she said.

As such, she recognizes it’s inevitable that human activities—building a village, conducting scientific experiments, or extracting minerals—will have some kind of environmental impact on the Moon. Mining will require extraction machinery, processing facilities, transportation infrastructure, storage, and power sources, Gorman said. “It’s not just, ‘Let’s dig a hole on the Moon.’”

Lunar dust, for instance, is an important concern. Sticky, abrasive, and full of sharp fragments of obsidian, it eroded the seals on Apollo astronauts’ spacesuits and coated their instruments, making data hard to read. It smelled of “spent gunpowder,” gave Apollo 17’s Harrison Schmitt a kind of hay fever, and turned out to be extremely hazardous to respiratory health—the grains are so sharp they can slice holes in astronauts’ lungs and cause damage to their DNA.

Machinery designed to operate on the Moon will need to be resistant to abrasion by the lunar dust. And some research suggests that too many rockets landing on and taking off from the Moon could lift significant quantities of dust into the exosphere. “There’s the potential to create a little dust cloud around the Moon,” said Gorman, “and we don’t yet know enough about how the Moon operates in order to properly assess those impacts.”

In theory, existing space law should already protect the Moon from commercial exploitation, said Gbenga Oduntan, a reader in international commercial law at the University of Kent in the United Kingdom. Originally from Nigeria, Oduntan was inspired to study law by the fact that nations got together to agree on and create the Outer Space Treaty—a “beautiful” idea that made him “proud of mankind.”

In the treaty, which came into effect in 1967, nations agreed that space (including the Moon) “is not subject to national appropriation by claim of sovereignty” and that “exploration and use of outer space shall be carried out for the benefit and in the interests of all countries and shall be the province of all mankind.” For Oduntan, the meaning is clear: Mining on the Moon would be legal if the resources were used for further exploration and scientific research on behalf of all humanity, “but appropriation for sale is a vastly new territory which we cannot allow countries, not to mention companies, to run along with on their own,” he said.

Successive U.S. administrations have had a different interpretation: that outer space is a space for capitalism. In 1979, the United States refused to sign the Moon Agreement, another United Nations treaty that specifically declared that lunar resources were the “common heritage of mankind” and committed signatories to establishing an international regime of oversight when resource extraction was “about to become feasible.” (Lack of support from the major space powers led to only 18 countries signing it, and it remains one of the most unpopular multilateral treaties.)

“Just because an area is beyond sovereignty doesn’t make it a global commons.”Instead, in 2015, once extraction actually was about to become feasible, the Space Act explicitly gave U.S. companies the right to own and sell resources they mine from space, as well as 8 more years mostly free of government oversight. (In a 2015 article, Oduntan called it “the most significant salvo that has been fired in the ideological battle over ownership of the cosmos.”)

Scott Pace, a professor of international affairs at George Washington University and director of the U.S. Space Policy Institute, said that legally speaking, space is not a global commons. (In his former role as head of the National Space Council, Pace worked on the 2020 Trump executive order—which also explicitly repudiated the Moon Agreement.)

“Just because an area is beyond sovereignty doesn’t make it a global commons,” he said. “Commons implies common ownership and common responsibility, which means…[other countries get] a say in what the United States does out there.”

Instead, the official American view is that “rules on frontiers and shared domains are made by those who show up, not by those who stay behind,” as Pace put it. To that end, the United States has signed nonbinding bilateral agreements—the Artemis Accords—with, so far, 11 other countries that hope to work with the United States on upcoming lunar missions. The accords aim to set norms of behavior for activity on the Moon, Pace said, although some experts have pointed out that they might also be designed to reinforce the U.S. interpretation of the Outer Space Treaty on resource exploitation.

Oduntan believes that all countries should get a say in what happens in space and on the Moon, even countries that are not yet capable of or interested in going there. Such a perspective is not about “exporting communism into outer space,” he said. Instead, the point is to recognize that conflict over resources is inevitable. “Commercialization of outer space in a Wild West mode is going to lead faster to disputes. There will be turf wars. And experience shows us that lack of regulation leads to tears.”

So could giving the Moon its own rights be one way to provide that kind of oversight and help ensure that countries and companies act in ways that minimize harm to its environment?

The Declaration of the Rights of the Moon was inspired by the growing Rights for Nature movement and uses some of its language. In the past 5 years, some natural entities—like New Zealand’s Whanganui River and Urewera forest, India’s Ganges River, and Colombia’s Atrato River—have been granted legal rights as part of efforts to protect and restore them. (Similarly, some astronomers have been investigating legal action to stop constellations of satellites, like Space X’s Starlink, from ruining their observations and altering the night sky.)

Pace was skeptical of the concept and said the Declaration of the Rights of the Moon has no legal standing.

“This [declaration] is saying that there should be something called rock rights—that a lunar rock has a right. It’s an interesting metaphor, but it doesn’t have any legal foundation, and it’s politically meaningless.”“The idea that the Moon as an inanimate object possesses fundamental rights as a result of its existence in the universe doesn’t make any sense. Rights are something which attach to human persons. We can have an argument about animal rights, but this is saying that there should be something called rock rights—that a lunar rock has a right. It’s an interesting metaphor, but it doesn’t have any legal foundation, and it’s politically meaningless.”

New Zealand’s Whanganui River might now have legal rights, Pace explained, but that’s because those rights were granted by the sovereign government of New Zealand. Countries agreed in the Outer Space Treaty that the Moon was beyond any nation’s sovereignty. That means there is no sovereign power that could legally grant the Moon rights, Pace reasoned—and efforts to have the Moon declared a national park or a World Heritage Site have failed for the same reason.

Erin O’Donnell, an expert on water law and the Rights for Nature movement at the University of Melbourne, foresees a different problem. Her research has shown that granting rights to rivers has frequently had unintended consequences for environmental protection.

Depending on the exact legal instrument used, some rivers now have the right to sue, enter into contracts, or own property. “But,” she said, “none of them have rights to water.”

“This is the real tension at the heart of the rights of nature advocacy movement: If something’s not legally enforceable, then it may not necessarily lead to a lot of change, because you can’t rely on it then in situations of conflict.”

Emphasizing legal rights can set up an adversarial atmosphere that can actually make conflict more likely, she said, and even weaken community support for protecting an environment, because people assume that if something has rights, it can look after itself. “If you emphasize the legal rights to the exclusion of all else, you can end up fracturing the relationship between people and nature, and that can be very hard to recover from.”

Where rights of nature movements have had success, she said, is in “reframing and resetting the human relationship with nature,” often by elevating Indigenous worldviews.

For Pace, the declaration is premature. Norms of behavior will evolve over time, he said, once we actually get to the Moon and figure out what we can possibly achieve there.

“What you don’t do is have a group of lawyers, no matter how smart, sit down in a room and try to draft up rules for things that are totally hypothetical. Environmental ethics considerations are rather speculative and not really necessary right now.”

“It sounds blunt, but the rules are made by the people who show up. Find a way to get in the game, and then you have a say.”If people really want to have an influence on space policy, Pace said, they should lobby their governments to get involved in the new space race. “Make sure you’re at the table. It sounds blunt, but the rules are made by the people who show up. Find a way to get in the game, and then you have a say.”

But Oduntan, O’Donnell, and Gorman disagreed. “By the time there’s a problem, it’s massively too late,” said O’Donnell. “We see that in the case of the rivers every day. All of the rivers around the world that have received legal rights are beloved, but heavily impacted.” The Moon is beloved, too, she said, but is as yet undamaged. “It would be nice if in this case we could act preventatively.”

The Declaration of the Rights of the Moon may not result in any legal outcomes, O’Donnell said, but it’s “a really important conversation starter.”

Most of us will never walk on its surface, but all human cultures tell stories about the Moon. It lights our nights, is a presence in our myths and legends, powers the tides, triggers animal (and, in limited ways, human) behavior, and marks the passing of time.

“The more of us who talk about these kinds of things,” said O’Donnell, “the more we’re likely to normalize seeing the Moon as something other than a piece of territory to be fought over by nation states and corporate investors.”

Supporters of the declaration want to democratize that conversation and give everyone a chance to take part.

“Every single person on Earth has a right to have a say in what happens to the Moon,” said Gorman. “It’s important for the environments in which we live, and for our cultural and scientific worldviews. It really does not belong to anyone.”

Kate Evans (@kate_g_evans), Science Writer

There is no escaping the reality of the climate crisis: There is no Planet B. A group of astronomers, united under the name Astronomers for Planet Earth (A4E), is ready to use their unique astronomical perspective to reinforce that important message.

A good number of exoplanets may potentially be habitable, but humans cannot simply cross the vast distances required to get there. And other planets of the solar system, although accessible, are all inhospitable. “Like it or not, for the moment the Earth is where we make our stand,” Carl Sagan famously wrote in his 1994 book Pale Blue Dot. The book’s title is based on the eponymous image showing Earth as small, fragile, and isolated. Sagan’s reflection on the image shows that astronomers can have a powerful voice in the climate debate.

The beginnings of A4E go back to 2019 when two groups of astronomers, one from the United States and the other from Europe, decided to join forces. Today the network numbers over a thousand astronomers, students, and astronomy educators from 78 countries. “We’re still trying to get ourselves together,” said Adrienne Cool, a professor at San Francisco State University. “It’s a volunteer organization that’s grown rapidly.”

Astronomy, its practitioners note, has a surprisingly wide earthly reach. “We teach astronomy courses that are taken by, just in the U.S., a quarter million students every year,” said Cool. “That’s a lot of students that we reach.”

And their influence goes way beyond students. About 150 million people visit planetariums around the world each year. Astronomers also organize countless stargazing nights and public lectures. Perhaps more than any other discipline, some researchers think, astronomy has the opportunity to address masses of people of all ages and occupations.

There is no guide for how best to incorporate climate science into an astronomy lecture. A4E works as a hub of knowledge and experience where astronomers can exchange teaching and outreach material. Members also learn about climate science and sustainability from regularly organized webinars.

However, although astronomers are spreading their message, they also acknowledge the need to address the elephant in the room: Astronomy can leave a significant carbon footprint. “I don’t feel comfortable telling the public, ‘Look, we really need to make a change,’ and the next moment I’m jumping on a plane for Chile [to use the telescopes],” said Leonard Burtscher, a staff astronomer at Leiden University in the Netherlands. “That’s a recipe for disaster in terms of communication.”

On average, an astronomer’s work-related greenhouse gas emissions are about twice as high as those of an average citizen in a developed country. The emissions per person are many times above the goal set by the Paris Agreement to limit the global increase in average temperature to less than 1.5°C relative to preindustrial levels.

“Let’s get real, and let’s figure out how to make sustainability the key part of what our institutions do in addition to astronomy.”At a recent virtual conference of the European Astronomical Society, hosted by Leiden University, A4E organized a session in which astronomers and climate crisis experts discussed the measures that would help reduce the carbon footprint of astronomy. Observatories and institutes are moving toward a greater reliance on renewable energy, and plans for future facilities take carbon assessment into account.

Perhaps the most contentious topic of discussion in academia is air travel. One solution is to hold fewer in-person conferences, as studies have shown that moving conferences to a virtual setting dramatically reduces the carbon footprint. “Good things [come] out of virtual meetings,” said Burtscher. “Better inclusivity, lower costs, often a higher legacy value, recordings of talks and discussions.” On the other hand, proponents of face-to-face meetings argue that a virtual setting impedes fruitful collaborations and networking that are especially important for young scientists. In the end, the community will likely have to make a compromise.

The impetus for change is strong. More than 2,700 astronomers signed an open letter released on Earth Day 2021 in which they recognized the urgency of the climate crisis and called for all astronomical institutions to adopt sustainability as a primary goal. But this is just the beginning, and the time for action is ticking away. “So let’s get real, and let’s figure out how to make sustainability the key part of what our institutions do in addition to astronomy,” said Cool.

—Jure Japelj (@JureJapelj), Science Writer

Thousands of tons of extraterrestrial material pummel Earth’s surface each year. The vast majority of it is too small to see with the naked eye, but even bits of cosmic dust have secrets to reveal.

By poring over more than 2,800 grains from micrometeorites, researchers have found that the amount of extraterrestrial material falling to Earth has remained remarkably stable over millions of years. That’s a surprise, the team suggested, because it’s long been believed that random collisions of asteroids in the asteroid belt periodically send showers of meteoroids toward Earth.

Birger Schmitz, a geologist at Lund University in Sweden, remembers the first time he looked at sediments to trace something that had come from space. It was the 1980s, and he was studying the Chicxulub impact crater. “It was the first insight that we could get astronomical information by looking down instead of looking up,” said Schmitz.

Inspired by that experience, Schmitz and his Lund University colleague Fredrik Terfelt, a research engineer, have spent the past 8 years collecting over 8,000 kilograms of sedimentary limestone. They’re not interested in the rock itself, which was once part of the ancient seafloor, but rather in what it contains: micrometeorites that fell to Earth over the past 500 million years.

“Ordinary chondritic asteroids don’t even appear to be common in the asteroid belt.”Schmitz and Terfelt used a series of strong chemicals in a specially designed laboratory to isolate the extraterrestrial material. They immersed their samples of limestone—representing 15 different time windows spanning from the Late Cambrian to the early Paleogene—in successive baths of hydrochloric acid, hydrofluoric acid, sulfuric acid, and nitric acid to dissolve the rock. Some of the reactions that ensued were impressive, said Terfelt, who recalled black smoke filling their laboratory’s fume hood. “The reaction between pyrite and nitric acid is quite spectacular.”

The chemical barrage left behind grains of chromite, an extremely hardy mineral that composes about 0.25% of some meteorites by weight. These grains are like a corpse’s gold tooth, said Schmitz. “They survive.”

Schmitz and Terfelt found that over 99% of the chromite grains they recovered came from a stony meteorite known as an ordinary chondrite. That’s perplexing, the researchers suggested, because asteroids of this type are rare in the asteroid belt, the source of most meteorites. “Ordinary chondritic asteroids don’t even appear to be common in the asteroid belt,” Schmitz told Eos.

“Everyone was telling us [we would] find several peaks.”An implication of this finding is that most of Earth’s roughly 200 known impact structures were likely formed from ordinary chondrites striking the planet. “The general view has been that comets and all types of asteroids were responsible,” said Schmitz.

When Schmitz and Terfelt sorted the 2,828 chromite grains they recovered by age, the mystery deepened. The distribution they found was remarkably flat except for one peak roughly 460 million years ago. We were surprised, said Schmitz. “Everyone was telling us [we would] find several peaks.”

Sporadic collisions between asteroids in the asteroid belt produce a plethora of debris, and it’s logical to assume that some of that cosmic shrapnel will reach Earth in the form of meteorites. But of the 15 of these titanic tussles involving chromite-bearing asteroids that occurred over the past 500 million years, that was the case only once, Schmitz and Terfelt showed. “Only one appears to have led to an increase in the flux of meteorites to Earth.”

Perhaps asteroid collisions need to occur in a specific place for their refuse to actually make it to our planet, the researchers proposed in the Proceedings of the National Academy of Sciences of the United States of America. So-called “Kirkwood gaps”—areas within the asteroid belt where the orbital periods of an asteroid and the planet Jupiter constitute a ratio of integers (e.g., 3:1 or 5:2)—are conspicuously empty. Thanks to gravitational interactions that asteroids experience in these regions of space, they tend to get flung out of those orbits, said Philipp Heck, a meteorist at the Field Museum of Natural History in Chicago not involved in the research. “Those objects tend to become Earth-crossing relatively quickly.”

We’re gaining a better understanding of the solar system by studying the relics of asteroids, its oldest constituents, said Heck. But this analysis should be extended to other types of meteorites that don’t contain chromite grains, he said. “This method only looks at certain types of meteorites. It’s far from a complete picture.”

—Katherine Kornei (@KatherineKornei), Science Writer

The Sun constantly emits a stream of energetic particles, some of which reach Earth. The density and energy of this stream form the basis of space weather, which can interfere with the operation of satellites and other spacecraft. A key unresolved question in the field is the frequency with which the Sun emits bursts of energetic particles strong enough to disable or destroy space-based electronics.

One promising avenue for determining the rate of such events is the dendrochronological record. This approach relies on the process by which a solar energetic particle (SEP) strikes the atmosphere, causing a chain reaction that results in the production of an atom of carbon-14. This atom subsequently can be incorporated into the structure of a tree; thus, the concentration of carbon-14 atoms in a tree ring can indicate the impact rate of SEPs in a given year.

To date, three events of extreme SEP production are well described in literature, occurring approximately in the years 660 BCE, 774–775 CE, and 992–993 CE. Each event was roughly an order of magnitude stronger than any measured in the space exploration era. Miyake et al. describe such an event, which occurred between 5411 BCE and 5410 BCE. Because of this burst, atmospheric carbon-14 increased 0.6% year over year in the Northern Hemisphere and was sustained for several years before dropping to typical levels.

The authors deduced the presence of this event by using samples collected from trees in three widely dispersed locales: a bristlecone pine in California, a Scotch pine in Finland, and a European larch in Switzerland. Each sample had its individual tree rings separated, and material from each ring underwent accelerator mass spectrometry to determine its carbon-14 content.

Using statistical methods, the researchers identified a pattern of small carbon-14 fluctuations consistent with the Sun’s 11-year solar cycle; the event recorded in the tree ring occurred during a time of solar maximum. Notably, other evidence suggests that the Sun was also undergoing a decades-long period of increasing activity.

If an extreme SEP burst is indeed the cause of the additional carbon-14, then these observations could aid in forecasting future events. However, tree ring measurements cannot rule out other extraterrestrial causes, such as a nearby supernova explosion. Confirmation will require isotopic measurements of beryllium and chlorine taken from ice cores, according to the authors. (Geophysical Research Letters, https://doi.org/10.1029/2021GL093419, 2021)

—Morgan Rehnberg, Science Writer