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      Phosphine gas found in Venus’ atmosphere may be ‘a possible sign of life’

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      Venus’ clouds appear to contain a smelly, toxic gas that could be produced by bacteria, a new study suggests.
      Chemical signs of the gas phosphine have been spotted in observations of the Venusian atmosphere, researchers report September 14 in Nature Astronomy. Examining the atmosphere in millimeter wavelengths of light showed that the planet’s clouds appear to contain up to 20 parts per billion of phosphine — enough that something must be actively producing it, the researchers say. 
      If the discovery holds up, and if no other explanations for the gas are found, then the hellish planet next door could be the first to yield signs of extraterrestrial life — though those are very big ifs.
      “We’re not saying it’s life,” says astronomer Jane Greaves of Cardiff University in Wales. “We’re saying it’s a possible sign of life.”

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      Venus has roughly the same mass and size as Earth, so, from far away, the neighboring planet might look like a habitable world (SN: 10/4/19). But up close, Venus is a scorching hellscape with sulfuric acid rain and crushing atmospheric pressures.
      Still, Venus might have been more hospitable in the recent past (SN: 8/26/16). And the current harsh conditions haven’t stopped astrobiologists from speculating about niches on Venus where present-day life could hang on, such as the temperate cloud decks.
      “Fifty kilometers above the surface of Venus, the conditions are what you would find if you walk out of your door right now,” at least in terms of atmospheric pressure and temperature, says planetary scientist Sanjay Limaye of the University of Wisconsin–Madison, who was not involved in the new study. The chemistry is alien, but “that’s a hospitable environment for life.”
      Previous work led by astrochemist Clara Sousa-Silva at MIT suggested that phosphine could be a promising biosignature, a chemical signature of life that can be detected in the atmospheres of other planets using Earth-based or space telescopes.
      On Earth, phosphine is associated with microbes or industrial activity — although that doesn’t mean it’s pleasant. “It’s a horrific molecule. It’s terrifying,” Sousa-Silva says. For most Earthly life, phosphine is poisonous because “it interferes with oxygen metabolism in a variety of macabre ways.” For anaerobic life, which does not use oxygen, “phosphine is not so evil,” Sousa-Silva says. Anaerobic microbes living in such places as sewage, swamps and the intestinal tracts of animals from penguins to people are the only known life-forms on Earth that produce the molecule.  
      Still, when Greaves and colleagues searched Venus’ skies for signs of phosphine, the researchers didn’t expect to actually find any. Greaves looked at Venus with the James Clerk Maxwell Telescope in Hawaii over five mornings in June 2017, aiming to set a detectability benchmark for future studies seeking the gas in the atmospheres of exoplanets (SN: 5/4/20), but was startled to find the hints of phosphine. “That’s a complete surprise,” Greaves says. When she was analyzing the observations, “I thought ‘Oh, I must have done it wrong.’”
      Signs of phosphine first showed up in data taken with the James Clerk Maxwell Telescope in Hawaii.Will Montgomerie/JCMT/EAO
      So the team checked again with a more powerful telescope, the Atacama Large Millimeter/submillimeter Array in Chile, in March 2019. But the signature of phosphine — seen as a dip in the spectrum of light at about 1.12 millimeters — was still there. The gas absorbs light in that wavelength. Some other molecules also absorb light near that wavelength, but those either couldn’t explain the whole signal or seemed improbable, Greaves says. “One of those is a plastic,” she says. “I think a floating plastic factory is a less plausible explanation than just saying there’s phosphine.”
      Phosphine takes a fair amount of energy to create and is easily destroyed by sunlight or sulfuric acid, which is found in Venus’ atmosphere. So if the gas was produced a long time ago, it shouldn’t still be detectable. “There has to be a source,” Greaves says.
      Greaves, Sousa-Silva and colleagues considered every explanation they could think of apart from life: atmospheric chemistry; ground and subsurface chemistry; volcanoes outgassing phosphine from the Venusian interior; meteorites peppering the atmosphere with phosphine from the outside; lightning; solar wind; tectonic plates sliding against each other. Some of those processes could produce trace amounts of phosphine, the team found, but orders of magnitude less than the team detected.
      “We’re at the end of our rope,” Sousa-Silva says. She hopes other scientists will come up with other explanations. “I’m curious what kind of exotic geochemistry people will come up with to explain this abiotically.”
      The idea of searching for life on Venus “has been regarded as a pretty out-there concept,” says Planetary Science Institute astrobiologist David Grinspoon, who is based in Washington, D.C. Grinspoon has been publishing about the prospects for life on Venus since 1997, but was not involved in the new discovery.
      “So now I hear about this, and I’m delighted,” he says. “Not because I want to declare victory and say this is definite evidence of life on Venus. It’s not. But it’s an intriguing signature that could be a sign of life on Venus. And it obligates us to go investigate further.”
      Because of the planet’s acidic atmosphere, extreme pressures and lead-melting temperatures, sending spacecraft to Venus is a challenge (SN: 2/13/18). But several space agencies are considering missions that could fly in the next few decades.
      In the meantime, Greaves and colleagues want to confirm the new phosphine detection in other wavelengths of light. Observations they had planned for the spring were put on hold by the coronavirus pandemic. And now, Venus is in a part of its orbit where it’s on the other side of the sun.
      “Maybe when Venus comes around on the other side of the sun again,” Greaves says, “things will be better for us here on Earth.” More

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      Dark matter clumps in galaxy clusters bend light surprisingly well

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      Dark matter just got even more puzzling.
      This unidentified stuff, which makes up most of the mass in the cosmos, is invisible but detectable by the way it gravitationally tugs on objects like stars. (SN: 11/25/19). Dark matter’s gravity can also bend light traveling from distant galaxies to Earth — but now some of this mysterious substance appears to be bending light more than it’s supposed to. A surprising number of dark matter clumps in distant clusters of galaxies severely warp background light from other objects, researchers report in the Sept. 11 Science.
      This finding suggests that these clumps of dark matter, in which individual galaxies are embedded, are denser than expected. And that could mean one of two things: Either the computer simulations that researchers use to predict galaxy cluster behavior are wrong, or cosmologists’ understanding of dark matter is.
      Very high concentrations of dark matter can act like a lens to bend light and drastically alter the appearance of background galaxies as seen from Earth — stretching them into arcs or splitting them into multiple images of the same object on the sky. “It’s totally cool. It’s like a fun house mirror,” says astrophysicist Priyamvada Natarajan of Yale University.

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      Judging by computer simulations of galaxy clusters, clumps of dark matter around individual galaxies that are dense enough to cause such dramatic gravitational lensing effects should be rare (SN: 10/4/15). Based on cluster simulations run by Natarajan and colleagues, “we would expect to see 1 [strong lensing] event in every 10 clusters or so,” says study coauthor Massimo Meneghetti, an astrophysicist at the Astrophysics and Space Science Observatory of Bologna in Italy.
      But telescope images told a different story. The researchers used observations from the Hubble Space Telescope and the Very Large Telescope in Chile to investigate 11 galaxy clusters from about 2.8 billion to 5.6 billion light-years away. In that set, the team identified 13 cases of severe gravitational lensing by dark matter clumps around individual galaxies. These observations indicate there are more high-density dark matter clumps in real galaxy clusters than in simulated ones, Meneghetti says.
      The simulations could be missing some physics that leads dark matter in galaxy clusters to glom tightly together, Natarajan says. “Or … there’s something fundamentally off about our assumptions about the nature of dark matter,” she says, like the notion that gravity is the only attractive force that dark matter feels.
      Richard Ellis, a cosmologist at the University College London who was not involved in the work, thinks the crux of the problem is more likely in the computer simulations than in the nature of dark matter. “A cluster of galaxies is a very dangerous place. It’s like the Manhattan of the universe,” he says — busy with galaxies whizzing past one another, colliding and getting torn up. “There’s awful physics that goes into predicting how many of these little lensed things they should find,” Ellis says, so the new result “is intriguing, but my suspicion is that there’s something in the simulations … that isn’t quite right.”
      Future observations with the upcoming Euclid space telescope (SN: 11/14/17), the Nancy Grace Roman Space Telescope and Vera C. Rubin Observatory (SN: 1/10/20) could help clear matters up, says Bhuvnesh Jain, an astrophysicist at the University of Pennsylvania who was not involved in the work. “These three telescopes are going to produce extremely large samples of galaxy clusters,” he says. That may lead to a new understanding of the physics in these turbulent environments, and help determine whether unrealistic simulations are to blame for this dark matter mystery. More

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      A weirdly warped planet-forming disk circles a distant trio of stars

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      In one of the most complex cosmic dances astronomers have yet spotted, three rings of gas and dust circle a trio of stars.
      The star system GW Orionis, located about 1,300 light-years away in the constellation Orion, includes a pair of young stars locked in a close do-si-do with a third star making loops around both. Around all three stars is a broken-apart disk of dust and gas where planets could one day form. Unlike the flat disk that gave rise to the planets in our solar system, GW Orionis’ disk consists of three loops, with a warped middle ring and an inner ring even more twisted at a jaunty angle to the other two.
      The bizarre geometry of this system, the first known of its kind, is reported in two recent studies by two groups of astronomers. But how GW Orionis formed is a mystery, with the two teams providing competing ideas for the triple-star-and-ring system’s birth.
      In a Sept. 4 study in Science, astronomer Stefan Kraus of the University of Exeter in England and colleagues suggest that gravitational tugs and torques from the triple-star ballet tore apart and deformed the primordial disk. But in a May 20 study in the Astrophysical Journal Letters, Jiaqing Bi of the University of Victoria in Canada and colleagues think that a newborn planet is to blame.
      “The question is how do you actually form such systems,” says theoretical physicist Giuseppe Lodato of the University of Milan, who was not on either team. “There could be different mechanisms that could do that.”

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      Astronomers have seen tilted disks of gas and dust around binary star systems, but not systems of more than two stars (SN: 7/30/14). Around half of the stars in the galaxy have at least one stellar companion, and their planets often have tilted orbits with respect to their stars, going around more like a jump rope than a Hula-Hoop (SN: 11/1/13). That misalignment could originate with the disk in which the planets were born: If the disk was askew, the planets would be too.
      About a decade ago, astronomers first realized that GW Orionis has three stars and a planet-forming disk, and the scientists scrambled to get a closer look. (At the time, it was impossible to tell if that disk was a single loop or not.) Bi’s team and Kraus’ team aimed the Atacama Large Millimeter/submillimeter Array in Chile at the triple-star system.
      Both groups spotted the trio of stars: one about 2.5 times and another about 1.4 times the sun’s mass orbiting each other once every 242 days, and another 1.4 solar mass star orbiting the inner pair about every 11 years.
      The observations also revealed three distinct rings of dust and gas encircling the stars. The closest ring to the star trio lies about 46 times the distance from Earth to the sun; the middle one about 185 times the Earth-sun distance; and the outermost ring about 340 times that distance. For perspective, Neptune is about 30 times the distance from Earth to the sun.
      That innermost ring is strongly misaligned with respect to the other rings and the stars, the teams found. Kraus’ group added observations from the European Southern Observatory’s Very Large Telescope to show the shadow of the inner ring on the inside of the middle loop. That shadow revealed that the middle ring is warped, swooping up on one side and down on the other.
      Astronomers looked at GW Orionis with the ALMA telescope array (left, blue) and the SPHERE instrument on the Very Large Telescope (right, red), both in Chile. The ALMA observations revealed the disk’s tri-ringed structure, while the SPHERE images showed the shadow cast by the innermost ring, allowing scientists to describe the rings’ deformed shapes in detail.Left image: ALMA/ESO, NAOJ, NRAO; Right image: ESO, S. Kraus et al, Univ. of Exeter
      Next, both groups ran computer simulations to figure out how the system formed. This is where their conclusions begin to differ, Bi says. His team suggests that a newly formed, not-yet-discovered planet cleared its orbit of gas and dust, splitting the inner ring off from the rest of the disk (SN: 7/16/19). Once the disk was split, the inner ring was free to swing around the stars, settling into its skewed alignment.
      Simulations from Kraus’s team, though, found that the chaotic gravity from the triple stars’ orbital dance alone was enough to break up the disk, a phenomenon called disk tearing. Each star tends to keep the disk aligned with itself, and the tug-of-war warped and sheared the disk, and twisted the inner ring even further. Theoretical studies had suggested disk tearing might happen in multiple star systems, but this is the first time it’s been seen in real life, Kraus contends.
      “I think it’s plausible that there could be planets somewhere in the system, but they’re not needed to explain the misalignment,” he says. “We don’t need to invoke undiscovered planets to explain what we see.”
      [embedded content]
      A trio of stars in GW Orionis are surrounded by an enormous, warped disk of gas and dust, new observations reveal. This animation, which is based on computer simulations and observational data, shows the complex geometry of the deformed and broken-apart disk.
      The difference may lie in the assumptions that the groups made about the disk’s properties, in particular its viscosity, says astrophysicist Nienke van der Marel, Bi’s colleague at the University of Victoria. A more viscous disk would tear like how Kraus and colleagues propose, but a less viscous disk needs a planet to break apart, she says. She thinks her team’s work is more realistic based on observations of other star systems. But with current technology, there’s no way to tell what the properties of GW Orionis’ disk are really like.
      And neither group could explain what made the disk split into three. “We don’t really know what’s causing the outer ring,” Klaus says.
      Lodato, who predicted the disk-tearing effect in 2013, thinks GW Orionis is proof that the phenomenon really exists. Back then, Lodato and colleagues were “very worried” that their simulations showed an effect that was introduced by the computations, not real physics, he says. “Now observations tell us that it does happen in reality.”
      Future telescopes may also be able to spot the planet if it exists, van der Marel says. More

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      Record-breaking gravitational waves reveal that midsize black holes do exist

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      The biggest. The farthest. The most energetic. A new detection of gravitational waves from two colliding black holes has racked up multiple superlatives.
      What’s more, it also marks the first definitive sighting of an intermediate mass black hole, one with a mass between 100 and 100,000 times the sun’s mass. That midsize black hole was forged when the two progenitor black holes coalesced to form a larger one with about 142 solar masses. It significantly outweighs all black holes previously detected via gravitational waves, ripples that wrinkle spacetime in the aftermath of extreme events.
      “This is the big guy we’ve been waiting for, for the longest time,” says Emanuele Berti, a physicist at Johns Hopkins University who was not involved with the research. One of the behemoth’s two progenitors was itself so massive that scientists are pondering how to explain its existence.
      Detected on May 21, 2019, the gravitational waves originated from a source about 17 billion light-years from Earth, making this the most distant detection confirmed so far. It’s also the most energetic event yet seen, radiating about eight times the equivalent of the sun’s mass in energy, says astrophysicist Karan Jani of Vanderbilt University in Nashville, a member of the LIGO Scientific Collaboration. “I hope it deserves its own entry in the record book.”

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      The new event dethrones the previous record-holder, a collision that occurred about 9 billion light-years away that radiated about five solar masses worth of energy, and created a black hole of 80 solar masses (SN: 12/4/18).
      Researchers with LIGO, or the Advanced Laser Interferometer Gravitational-Wave Observatory, in the United States and Advanced Virgo in Italy reported the new detection September 2 in two papers in Physical Review Letters and the Astrophysical Journal Letters.
      While scientists know of black holes with tens of solar masses and others with millions or billions of solar masses, the intermediate echelon has remained elusive. Previous purported sightings of intermediate mass black holes have been questioned (SN:1/22/16).
      But, for the new event, “there’s no doubt,” says astrophysicist Cole Miller of University of Maryland at College Park, who was not involved with the study. “This demonstrates that there is now at least one intermediate mass black hole in the universe.”
      The black hole’s two progenitors were themselves heftier than any seen colliding before — at about 85 and 66 times the mass of the sun. That has scientists puzzling over how this smashup came to be.

      Normally, physicists expect that the black holes involved in these mergers would each have formed in the collapse of a dying star. But in the new event, the larger of the pair is so big that it couldn’t have formed that way. The known processes that go on within a star’s core mean that stars that are the right mass to form such a big black hole would blow themselves apart completely, rather than leaving behind a corpse.
      Instead, it might be that one or both of the colliding black holes formed from an earlier round of black hole mergers, within a crowded cluster of stars and black holes (SN: 1/30/17). That would make for a family tree that began with black holes lightweight enough to form from collapsing stars.
      But there’s a problem with the multiple-merger explanation. Each time black holes merge, that coalescence provides a kick to their velocity, which would normally launch the resulting black hole out of the cluster, preventing further mergers.
      However, mergers as massive as the new event seem to be very rare, given that LIGO and Virgo have detected only one. That means, Miller says, “my gosh, you’re allowed to invoke a tooth fairy,” a relatively unlikely process. Perhaps, he says, the kick might sometimes be small enough that the black holes could stay within their cluster and merge again.
      The May 21 gravitational wave event had previously been publicly reported as an unconfirmed candidate, to allow astronomers to look for flashes of light in the sky that might have resulted from the collision. Some researchers had suggested that the waves might have been associated with a flare of light from the center of a distant galaxy (SN: 6/25/20). But that galaxy is significantly closer than the distance now pinpointed in the new papers, at about 8 billion light-years from Earth rather than 17 billion, making the explanation less plausible.
      The longer LIGO and Virgo observe the heavens, the more the bounty of unusual events can be expected to grow, Miller says. “We are going to have a set of ‘gosh, didn’t expect that’ type of events, which are thrilling to think about and extremely informative about the universe.” More

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      Earth’s building blocks may have had far more water than previously thought

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      Earth’s deep stores of water may have been locally sourced rather than trucked in from far-flung regions of the solar system.
      A new analysis of meteorites from the inner solar system — home to the four rocky planets — suggests that Earth’s building blocks delivered enough water to account for all the H2O buried within the planet. What’s more, the water produced by the local primordial building material likely shares a close chemical kinship with Earth’s deep-water reserves, thus strengthening the connection, researchers report in the Aug. 28 Science.
      Earth is thought to have been born in an interplanetary desert, too close to the sun for water ice to survive. Many researchers suspect that ocean water got delivered toward the end of Earth’s formation by ice-laden asteroids that wandered in from cooler, more distant regions of the solar system (SN: 5/6/15). But the ocean isn’t the planet’s largest water reservoir. Researchers estimate that Earth’s interior holds several times as much water as is found at the surface.
      To test whether or not the material that formed Earth could have delivered this deep water, cosmochemist Laurette Piani of the University of Lorraine in Vandœuvre-lès-Nancy, France, and colleagues analyzed meteorites known as enstatite chondrites. Thanks to many chemical similarities with Earth rocks, these relatively rare meteorites are widely thought to be good analogs of the dust and space rocks from the inner solar system that formed Earth’s building blocks, Piani says.

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      She and her team measured the abundance of hydrogen in these meteorites — a proxy for how much H2O they could produce — and calculated that local interplanetary debris had the potential to deliver at least three times as much water as is found in all the oceans. The meteorites don’t contain water, Piani says. Rather, they house enough of the raw ingredients to create water when heated.
      In the meteorites, the team also found a close match to the type of water found in Earth’s mantle. A smattering of all water molecules on Earth contain a heavy variant of hydrogen known as deuterium. The ratio of deuterium to hydrogen in the enstatite chondrites lies within the range measured in Earth’s deep water. That similarity, the team argues, makes a strong case for local building blocks being the source of much of the planet’s water.
      “This work is something I wanted to do myself or had been waiting for someone to do,” says Lydia Hallis, a planetary scientist at the University of Glasgow in Scotland. In 2015, she led a team that measured the deuterium abundance in lava plumes that tap deep into Earth’s mantle (SN: 11/12/15). “I’m really happy that [the new data] sits within the region where our previous data from deep mantle samples is sitting.”
      Hallis and others stress that these new measurements are difficult. Once the meteorites hit the ground, they quickly absorb hydrogen from Earth’s environment. “They did a really good job of picking the right meteorites and making the right measurements,” she says. “This is pretty convincing that this hydrogen that’s measured is from the enstatite chondrites rather than from terrestrial contamination.”
      The enstatite chondrites could have also contributed a lot of water to the oceans as well — but they are not the full story. The deuterium-hydrogen ratio in ocean water, which is a bit higher than that of mantle water, is better matched to the ratio found in icy asteroids from the outer solar system. “We still need a bit of water coming from the outer solar system,” Piani says. So, while local materials may have delivered the bulk of Earth’s water, the oceans were likely topped off a bit later by collisions with remote space rocks. More

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      Check out the first-ever map of the solar corona’s magnetic field

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      The sun’s wispy upper atmosphere, called the corona, is an ever-changing jungle of sizzling plasma. But mapping the strength of the magnetic fields that largely control that behavior has proved elusive. The fields are weak and the brightness of the sun outshines its corona.
      Now though, observations taken using a specialized instrument called a coronagraph to block out the sun’s bright disk have allowed solar physicists to measure the speed and intensity of waves rippling through coronal plasma (SN: 3/19/09). “This is the first time we’ve mapped the coronal magnetic field on a large scale,” says Steven Tomczyk, a solar physicist at the High Altitude Observatory in Boulder, Colo., who designed the coronagraph.
      In 2017, Tomczyk had been part of a team that took advantage of a total solar eclipse crisscrossing North America to take measurements of the corona’s magnetic field (SN: 8/16/17). He trekked to a mountaintop in Wyoming with a special camera to snap polarized pictures of the corona just as the moon blocked the sun.  (I was there with them, reporting on the team’s efforts to help explain why the corona is so much hotter than the sun’s surface (SN: 8/21/17).) The team observed a tiny slice of the corona to test whether a particular wavelength of light could carry signatures of the corona’s magnetic field. It can (SN: 8/21/18).
      But it’s the observations from the coronagraph, made in 2016, that allowed researchers to look at the whole corona all at once. Theorists had shown decades ago that coronal waves’ velocities can be used to infer the strength of the magnetic field. Such waves might also help carry heat from the sun’s surface into the corona (SN: 11/14/19). But no one had measured them across the whole corona before.

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      The corona’s magnetic field strength is mostly between 1 and 4 gauss, a few times the strength of the Earth’s magnetic field at the planet’s surface, the researchers report in the Aug. 7 Science.
      Making a map is a big step, the team says. But what solar physicists would really like to do is track the corona’s magnetic field continuously, at least once a day.
      “The solar magnetic field is evolving all the time,” says solar physicist Zihao Yang of Peking University in Beijing. Sometimes the sun releases magnetic energy explosively, sending bursts of plasma can shooting out into space (SN: 3/7/19). Those ejections can wreak havoc on satellites or power grids when they strike Earth. Continuously monitoring coronal magnetism can help predict those outbursts. “Our work demonstrated that we can use this technique to map the global distribution of coronal magnetic field, but we only showed one map from a single dataset,” Yang says.
      Measuring the strength of the corona’s magnetic field is “a really big deal,” says solar physicist Jenna Samra of the Smithsonian Astrophysical Observatory in Cambridge, Mass. “Making global maps of the coronal magnetic field strength … is what’s going to allow us to eventually get better predictions of space weather events,” she says. “This is a really nice step in that direction.”
      Tomczyk and colleagues are working on an upgraded version of the coronagraph, called COSMO, for Coronal Solar Magnetism Observatory, that would use the same technique repeatedly with the ultimate goal of predicting the sun’s behavior.
      “It’s a milestone to do it,” Tomczyk says. “The goal is to do it regularly, do it all the time.” More

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      In a first, astronomers spotted a space rock turning into a comet

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      Like the mythical half-human, half-horse creatures, centaurs in the solar system are hybrids between asteroids and comets. Now, astronomers have caught one morphing from one type of space rock to the other, potentially giving scientists an unprecedented chance to watch a comet form in real time in the decades to come.
      “We have an opportunity here to see the birth of a comet as it starts to become active,” says planetary scientist Kat Volk of the University of Arizona in Tucson.
      The object, called P/2019 LD2, was discovered by the ATLAS telescope in Hawaii in May. Its orbit suggests that it’s a centaur, a class of rocky and icy objects with unstable orbits. Because of that mixed composition and potential to move around the solar system, astronomers have long suspected that centaurs are a missing link between small icy bodies in the Kuiper Belt beyond Neptune and comets that regularly visit the inner solar system (SN: 11/19/94).
      These “short-period” comets, which are thought to originate from icy objects in the Kuiper Belt, orbit the sun once a decade or so, and make repeat appearances in Earth’s skies. (Long-period comets, like Halley’s Comet, which visits the inner solar system once a century, probably originate even farther from the sun, in the Oort cloud (SN: 10/25/13).)

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      All previously found short-period comets were spotted only after they had transitioned into comets (SN: 8/6/14). But LD2 just came in from the Kuiper Belt recently and will become a comet in as little as 43 years, Volk and colleagues report August 10 at arXiv.org.
      “It’s weird to think that this object should be becoming a comet when I’m retiring,” Volk says.
      In 2019, she and colleagues showed that there’s a region of space just beyond Jupiter that they call the “Gateway”.  In this area, small planetary objects hang out while warming up and transitioning from outer solar system ice balls to inner solar system comets with their long tails. It’s like a comet incubator, says planetary scientist Gal Sarid of the SETI Institute, who is based in Rockville, Md.
      After hearing about LD2, Volk, Sarid and their colleagues simulated thousands of possible trajectories to see where the object had been and where it is going. LD2’s orbit probably took it near Saturn around 1850, and it entered its current orbit past Jupiter after a close encounter with the gas giant in 2017, the team found. The object will leave its present orbit and move in toward the sun in 2063, where heat from the sun will probably sublimate LD2’s volatile elements, giving it a bright cometary tail, the researchers say.
      “This will be the first ever comet that we know its history, because we’ve seen it before being a comet,” Sarid says.
      The fact that LD2 is fairly new to the inner reaches of the solar system suggests that it’s made of relatively pristine material that has been in the back of the solar system’s freezer for billions of years, unaltered by heat from the sun. That would make it a time capsule of the early solar system. Studying its composition could help planetary scientists learn what the first planets were made of.
      The orbital analysis looks “very reasonable,” says Henry Hsieh, a planetary astronomer with the Planetary Science Institute who is based in Honolulu and was not involved in the study. But studying just one transition object is not enough to open the solar system time capsule.
      “What we really need to do is study many of these,” he says. “Study this one first, and then study more of them, and figure out whether this object is an outlier or whether we see a consistent picture.” Future sky surveys, like the ones planned using the future Vera Rubin Observatory (SN: 1/10/20), should discover more balls of ice shifting into comets.
      Sarid and colleagues think LD2 could be a good target for a spacecraft to visit. NASA has considered sending spacecraft to centaurs, although no missions have been selected for development yet. But considering that LD2 will become a comet in just a few decades, scientists don’t have much time to plan, build and launch a mission to visit it. “The windows are closing,” Sarid says. “We really need to be doing this now.” More

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      Hubble watched a lunar eclipse to see Earth from an alien’s perspective

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      To practice searching for extraterrestrial life, researchers have run a dress rehearsal with the one world they know to be habitable: Earth.
      While Earth was between the sun and moon for a lunar eclipse in January 2019, the Hubble Space Telescope observed how chemicals in Earth’s atmosphere blocked certain wavelengths of sunlight from reaching the moon. That observing setup mimicked the way astronomers plan to probe the atmospheres of Earthlike exoplanets as they pass in front of their stars, filtering out some starlight.
      “We basically pretend we’re alien observers looking at our planet,” says Giada Arney, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.
      Using Hubble, the researchers focused on spotting the effects of atmospheric ozone. Because ozone is both a chemical by-product of oxygen produced in photosynthesis and a shield that protects life from the sun’s harmful ultraviolet rays, astronomers think atmospheric ozone could be a key indicator that a distant world is habitable. During the lunar eclipse, Hubble examined sunlight that had passed through Earth’s atmosphere and reflected off of the moon for signatures of ozone.

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      “It’s safer for Hubble to observe sunlight reflected off the moon” than to look directly at the backlit Earth, explains Allison Youngblood, an astronomer at the University of Colorado Boulder. The telescope’s instruments are so sensitive and Earth is so bright that “even the nightside would fry Hubble’s detectors.” 
      Those observations revealed prominent dips in particular wavelengths of ultraviolet sunlight that had been absorbed by the ozone, Youngblood, Arney and colleagues report online August 6 in the Astronomical Journal.
      The data help confirm that chemicals in the Earth’s atmosphere filter light as expected, based on researchers’ understanding of atmospheric chemistry. That finding gives astronomers more confidence that they will be able to recognize potentially habitable exoplanets. More