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      Passing through the Milky Way’s arms may have helped form Earth’s solid ground

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      Earth’s journey through the Milky Way might have helped create the planet’s first continents.

      Comets may have bombarded Earth every time the early solar system traveled through our galaxy’s spiral arms, a new study suggests. Those recurring barrages in turn helped trigger the formation of our planet’s continental crust, researchers propose August 23 in Geology.

      Previous theories have suggested that such impacts might have played a role in forming Earth’s landmasses. But there has been little research explaining how those impacts occurred, until now, the team says.

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      It’s an intriguing hypothesis, other scientists say, but it’s not the last word when it comes to explaining how Earth got its landmasses.

      To peer back in time, geochronologist Chris Kirkland and his colleagues turned to geologic structures known as cratons (SN: 12/3/10). These relics of Earth’s ancient continental crust are some of the planet’s oldest rocks. Using material from cratons in Australia and Greenland that are billions of years old, the team measured the chemistry of more than 2,000 bits of rock. The analysis let the researchers determine the exact ages of the rocks, and whether they had formed anew from molten material deep within the Earth or from earlier generations of existing crust.

      When Kirkland and his colleagues looked for patterns in their measurements, the team found that new crust seemed to form in spurts at roughly regular intervals. “Every 200 million years, we see a pattern of more crust production,” says Kirkland, of Curtin University in Perth, Australia.

      That timing rang a bell: It’s also the frequency at which the Earth passes through the spiral arms of the Milky Way (SN: 12/30/15). The solar system loops around the center of the galaxy a bit faster than the spiral arms move, periodically passing through and overtaking them. Perhaps cosmic encounters with more stars, gas and dust within the spiral arms affected the young planet, the team suggests.

      The idea makes sense, the researchers say, since the higher density of material in the spiral arms would have led to more gravitational tugs on the reservoir of comets at our solar system’s periphery (SN: 8/18/22). Some of those encounters would have sent comets zooming into the inner solar system, and a fraction of those icy denizens would have collided with Earth, Kirkland and his team propose.

      Earth was probably covered mostly by oceans billions of years ago, and the energy delivered by all those comets would have fractured the planet’s existing oceanic crust — the relatively dense rock present since even earlier in Earth’s history — and excavated copious amounts of material while launching shock waves into the planet. That mayhem would have primed the way for parts of Earth’s mantle to melt, Kirkland says. The resulting magma would have naturally separated into a denser part — the precursor to more oceanic crust — and a lighter, more buoyant liquid that eventually turned into continental crust, the researchers suggest.

      That’s one hypothesis, but it’s far from a slam dunk, says Jesse Reimink, a geoscientist at Penn State who was not involved in the research. For starters, comet and meteorite impacts are notoriously tough to trace, especially that far back in time, he says. “There’s very few diagnostics of impacts.” And it’s not well-known whether such impacts, if they occurred in the first place, would have resulted in the release of magma, he says.

      In the future, Kirkland and his colleagues hope to analyze moon rocks to look for the same pattern of crust formation (SN: 7/15/19). Our nearest celestial neighbor would have been walloped by about the same amount of stuff that hit Earth, Kirkland says. “You’d predict it’d also be subject to these periodic impact events.” More

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      Here’s the James Webb telescope’s first direct image of an exoplanet

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      This is the first picture of an exoplanet from the James Webb Space Telescope.

      “We’re actually measuring photons from the atmosphere of the planet itself,” says astronomer Sasha Hinkley of the University of Exeter in England. Seeing those particles of light, “to me, that’s very exciting.”

      The planet is about seven times the mass of Jupiter and lies more than 100 times farther from its star than Earth sits from the sun, direct observations of exoplanet HIP 65426 b show. It’s also young, about 10 million or 20 million years old, compared with the more than 4-billion-year-old Earth, Hinkley and colleagues report in a study submitted August 31 at arXiv.org.

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      Those three features — size, distance and youth — made HIP 65426 b relatively easy to see, and so a good planet to test JWST’s observing abilities. And the telescope has once again surpassed astronomers’ expectations (SN: 7/11/22).

      “We’ve demonstrated really how powerful JWST is as an instrument for the direct imaging of exoplanets,” says exoplanet astronomer and coauthor Aarynn Carter of the University of California, Santa Cruz.

      Astronomers have found more than 5,000 planets orbiting other stars (SN: 3/22/22). But almost all of those planets were detected indirectly, either by the planets tugging on the stars with their gravity or blocking starlight as they cross between the star and a telescope’s view.

      To see a planet directly, astronomers have to block out the light from its star and let the planet’s own light shine, a tricky process. It’s been done before, but for only about 20 planets total (SN: 11/13/08; SN: 3/14/13; SN: 7/22/20).

      “In every area of exoplanet discovery, nature has been very generous,” says MIT astrophysicist Sara Seager, who was not involved in the JWST discovery. “This is the one area where nature didn’t really come through.”

      In 2017, astronomers discovered HIP 65426 b and took a direct image of it using an instrument on the Very Large Telescope in Chile. But because that telescope is on the ground, it can’t see all the light coming from the exoplanet. Earth’s atmosphere absorbs a lot of the planet’s infrared wavelengths — exactly the wavelengths JWST excels at observing. The space telescope observed the planet on July 17 and July 30, capturing its glow in four different infrared wavelengths.

      “These are wavelengths of light that we’ve never ever seen exoplanets in before,” Hinkley says. “I’ve literally been waiting for this day for six years. It feels amazing.”

      Pictures in these wavelengths will help reveal how planets formed and what their atmospheres are made of.

      “Direct imaging is our future,” Seager says. “It’s amazing to see the Webb performing so well.”

      While the team has not yet studied the atmosphere of HIP 65426 b in detail, it did report the first spectrum — a measurement of light in a range of wavelengths — of an object orbiting a different star. The spectrum allows a deeper look into the object’s chemistry and atmosphere, astronomer Brittany Miles of UC Santa Cruz and colleagues reported September 1 at arXiv.org.

      That object is called VHS 1256 b. It’s as heavy as 20 Jupiters, so it may be more like a transition object between a planet and a star, called a brown dwarf, than a giant planet. JWST found evidence that the amounts of carbon monoxide and methane in the atmosphere of the orb are out of equilibrium. That means the atmosphere is getting mixed up, with winds or currents pulling molecules from lower depths to its top and vice versa. The telescope also saw signs of sand clouds, a common feature in brown dwarf atmospheres (SN: 7/8/22).

      “This is probably a violent and turbulent atmosphere that is filled with clouds,” Hinkley says.

      HIP 65426 b and VHS 1256 b are unlike anything we see in our solar system. They’re more than three times the distance of Uranus from their stars, which suggests they formed in a totally different way from more familiar planets. In future work, astronomers hope to use JWST to image smaller planets that sit closer to their stars.

      “What we’d like to do is get down to study Earths, wouldn’t we? We’d really like to get that first image of an Earth orbiting another star,” Hinkley says. That’s probably out of JWST’s reach — Earth-sized planets are still too small see. But a Saturn? That may be something JWST could focus its sights on.  More

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      The James Webb telescope spotted CO2 in an exoplanet’s atmosphere

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      The James Webb Space Telescope has gotten the first sniff of carbon dioxide in the atmosphere of a planet in another solar system.

      “It’s incontrovertible. It’s there. It’s definitely there,” says planetary scientist and study coauthor Peter Gao of the Carnegie Institution for Science in Washington, D.C. “There have been hints of carbon dioxide in previous observations, but never confirmed to such an extent.”

      The finding, submitted to arXiv.org on August 24, marks the first detailed scientific result published from the new telescope. It also points the way to finding the same greenhouse gas in the atmospheres of smaller, rockier planets that are more like Earth.

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      The planet, dubbed WASP-39b, is huge and puffy. It’s a bit wider than Jupiter and about as massive as Saturn. And it orbits its star every four Earth days, making it scorching hot. Those features make it a terrible place to search for evidence of extraterrestrial life (SN: 4/19/16). But that combination of puffy atmosphere and frequent passes in front of its star makes it easy to observe, a perfect planet to put the new telescope through its paces.

      James Webb, or JWST, launched in December 2021 and released its first images in July 2022 (SN: 7/11/22). For about eight hours in July, the telescope observed starlight that filtered through the planet’s thick atmosphere as the planet crossed between its star and JWST. As it did, molecules of carbon dioxide in the atmosphere absorbed specific wavelengths of that starlight.

      Previous observations of WASP-39b with NASA’s now-defunct Spitzer Space Telescope had detected just a whiff of absorption at that same wavelength. But it wasn’t enough to convince astronomers that carbon dioxide was really there.

      “I would not have bet more than a beer, at most a six pack, on that weird tentative hint of carbon dioxide from Spitzer,” says astronomer Nicolas Cowan of McGill University in Montreal, who was not involved with the new study. The JWST detection, on the other hand, “is rock solid,” he says. “I wouldn’t bet my firstborn because I love him too much. But I would bet a nice vacation.”

      The JWST data also showed an extra bit of absorption at wavelengths close to those absorbed by carbon dioxide. “It’s a mystery molecule,” says astronomer Natalie Batalha of the University of California, Santa Cruz, who led the team behind the observation. “We have several suspects that we are interrogating.”

      The amount of carbon dioxide in an exoplanet’s atmosphere can reveal details about how the planet formed (SN: 5/11/18). If the planet was bombarded with asteroids, that could have brought in more carbon and enriched the atmosphere with carbon dioxide. If radiation from the star stripped away some of the planet atmosphere’s lighter elements, that could make it appear richer in carbon dioxide too.

      Despite needing a telescope as powerful as JWST to detect it, carbon dioxide might be in atmospheres all over the galaxy, hiding in plain sight. “Carbon dioxide is one of the few molecules that is present in the atmospheres of all solar system planets that have atmospheres,” Batalha says. “It’s your front-line molecule.”

      Eventually, astronomers hope to use JWST to find carbon dioxide and other molecules in the atmospheres of small rocky planets, like the ones orbiting the star TRAPPIST-1 (SN: 12/13/17). Some of those planets, at just the right distances from their star to sustain liquid water, might be good places to look for signs of life. It’s yet to be seen whether JWST will detect those signs of life, but it will be able to detect carbon dioxide.

      “My first thought when I saw these data was, ‘Wow, this is gonna work,’” Batalha says. More

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      The discovery of the Kuiper Belt revamped our view of the solar system

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      On a Hawaiian mountaintop in the summer of 1992, a pair of scientists spotted a pinprick of light inching through the constellation Pisces. That unassuming object — located over a billion kilometers beyond Neptune — would rewrite our understanding of the solar system.

      Rather than an expanse of emptiness, there was something, a vast collection of things in fact, lurking beyond the orbits of the known planets.

      The scientists had discovered the Kuiper Belt, a doughnut-shaped swath of frozen objects left over from the formation of the solar system.

      As researchers learn more about the Kuiper Belt, the origin and evolution of our solar system is coming into clearer focus. Closeup glimpses of the Kuiper Belt’s frozen worlds have shed light on how planets, including our own, might have formed in the first place. And surveys of this region, which have collectively revealed thousands of such bodies, called Kuiper Belt objects, suggest that the early solar system was home to pinballing planets.

      The humble object that kick-started it all is a chunk of ice and rock roughly 250 kilometers in diameter. It was first spotted 30 years ago this month.

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      Staring into space

      In the late 1980s, planetary scientist David Jewitt and astronomer Jane Luu, both at MIT at the time, were several years into a curious quest. The duo had been using telescopes in Arizona to take images of patches of the night sky with no particular target in mind. “We were literally just staring off into space looking for something,” says Jewitt, now at UCLA.

      An apparent mystery motivated the researchers: The inner solar system is relatively crowded with rocky planets, asteroids and comets, but there was seemingly not much out beyond the gas giant planets, besides small, icy Pluto. “Maybe there were things in the outer solar system,” says Luu, who now works at the University of Oslo and Boston University. “It seemed like a worthwhile thing to check out.”

      David Jewitt and Jane Luu, shown in Honolulu in the early 2000s, discovered the Kuiper Belt.D. Jewitt/UCLA

      Poring over glass photographic plates and digital images of the night sky, Jewitt and Luu looked for objects that moved extremely slowly, a telltale sign of their great distance from Earth. But the pair kept coming up empty. “Years went by, and we didn’t see anything,” Luu says. “There was no guarantee this was going to work out.”

      The tide changed in 1992. On the night of August 30, Jewitt and Luu were using a University of Hawaii telescope on the Big Island. They were employing their usual technique for searching for distant objects: Take an image of the night sky, wait an hour or so, take another image of the same patch of sky, and repeat. An object in the outer reaches of the solar system would shift position ever so slightly from one image to the next, primarily because of the movement of Earth in its orbit. “If it’s a real object, it would move systematically at some predicted rate,” Luu says.

      By 9:14 p.m. that evening, Jewitt and Luu had collected two images of the same bit of the constellation Pisces. The researchers displayed the images on the bulbous cathode-ray tube monitor of their computer, one after the other, and looked for anything that had moved. One object immediately stood out: A speck of light had shifted just a touch to the west.

      But it was too early to celebrate. Spurious signals from high-energy particles zipping through space — cosmic rays — appear in images of the night sky all of the time. The real test would be whether this speck showed up in more than two images, the researchers knew.

      Jewitt and Luu nervously waited until 11 p.m. for the telescope’s camera to finish taking a third image. The same object was there, and it had moved a bit farther west. A fourth image, collected just after midnight, revealed the object had shifted position yet again. This is something real, Jewitt remembers thinking. “We were just blown away.”

      The way the circled object shifted position in the sky (time stamps at right) told Jewitt and Luu that the object, dubbed 1992 QB1, was distant. It was the first evidence of the icy zone called the Kuiper Belt.D. Jewitt/UCLA

      Based on the object’s brightness and its leisurely pace — it would take nearly a month for it to march across the width of the full moon as seen from Earth — Jewitt and Luu did some quick calculations. This thing, whatever it was, was probably about 250 kilometers in diameter. That’s sizable, about one-tenth the width of Pluto. It was orbiting far beyond Neptune. And in all likelihood, it wasn’t alone.

      Although Jewitt and Luu had been diligently combing the night sky for years, they had observed only a tiny fraction of it. There were possibly thousands more objects out there like this one just waiting to be found, the two concluded.

      The realization that the outer solar system was probably teeming with undiscovered bodies was mind-blowing, Jewitt says. “We expanded the known volume of the solar system enormously.” The object that Jewitt and Luu had found, 1992 QB1 (SN: 9/26/92, p. 196), introduced a whole new realm.

      Just a few months later, Jewitt and Luu spotted a second object also orbiting far beyond Neptune (SN: 4/10/93, p. 231). The floodgates opened soon after. “We found 40 or 50 in the next few years,” Jewitt says. As the digital detectors that astronomers used to capture images grew in size and sensitivity, researchers began uncovering droves of additional objects. “So many interesting worlds with interesting stories,” says Mike Brown, an astronomer at Caltech who studies Kuiper Belt objects.

      Finding all of these frozen worlds, some orbiting even beyond Pluto, made sense in some ways, Jewitt and Luu realized. Pluto had always been an oddball; it’s a cosmic runt (smaller than Earth’s moon) and looks nothing like its gas giant neighbors. What’s more, its orbit takes it sweeping far above and below the orbits of the other planets. Maybe Pluto belonged not to the world of the planets but to the realm of whatever lay beyond, Jewitt and Luu hypothesized. “We suddenly understood why Pluto was such a weird planet,” Jewitt says. “It’s just one object, maybe the biggest, in a set of bodies that we just stumbled across.” Pluto probably wouldn’t be a member of the planet club much longer, the two predicted. Indeed, by 2006, it was out (SN: 9/2/06, p. 149).

      Up-close look

      The discovery of 1992 QB1 opened the world’s eyes to the Kuiper Belt, named after Dutch-American astronomer Gerard Kuiper. In a twist of history, however, Kuiper predicted that this region of space would be empty. In the 1950s, he proposed that any occupants that might have once existed there would have been banished by gravity to even more distant reaches of the solar system.

      In other words, Kuiper anti-predicted the existence of the Kuiper Belt. He turned out to be wrong.

      Today, researchers know that the Kuiper Belt stretches from a distance of roughly 30 astronomical units from the sun — around the orbit of Neptune — to roughly 55 astronomical units. It resembles a puffed-up disk, Jewitt says. “Superficially, it looks like a fat doughnut.”

      The frozen bodies that populate the Kuiper Belt are the remnants of the swirling maelstrom of gas and dust that birthed the sun and the planets. There’s “a bunch of stuff that’s left over that didn’t quite get built up into planets,” says astronomer Meredith MacGregor of the University of Colorado Boulder. When one of those cosmic leftovers gets kicked into the inner solar system by a gravitational shove from a planet like Neptune and approaches the sun, it turns into an object we recognize as a comet (SN: 9/12/20, p. 14). Comets that circle the sun once only every 200 years or more typically derive from the solar system’s even more distant repository of icy bodies known as the Oort cloud.

      There are many places in the solar system where icy bodies congregate: the asteroid belt roughly between Jupiter and Mars (top), the doughnut-shaped Kuiper Belt beyond the gas giant planets (middle) and the most distant zone, the Oort cloud (bottom).Mark Garlick/Science Source

      In scientific parlance, the Kuiper Belt is a debris disk (SN Online: 7/28/21). Distant solar systems contain debris disks, too, scientists have discovered. “They’re absolutely directly analogous to our Kuiper Belt,” MacGregor says.

      In 2015, scientists got their first close look at a Kuiper Belt object when NASA’s New Horizons spacecraft flew by Pluto (SN Online: 7/15/15). The pictures that New Horizons returned in the following years were thousands of times more detailed than previous observations of Pluto and its moons. No longer just a few fuzzy pixels, the worlds were revealed as rich landscapes of ice-spewing volcanoes and deep, jagged canyons (SN: 6/22/19, p. 12; SN Online: 7/13/18). “I’m just absolutely ecstatic with what we accomplished at Pluto,” says Marc Buie, an astronomer at the Southwest Research Institute in Boulder, Colo., and a member of the New Horizons team. “It could not possibly have gone any better.”

      But New Horizons wasn’t finished with the Kuiper Belt. On New Year’s Day of 2019, when the spacecraft was almost 1.5 billion kilometers beyond Pluto’s orbit, it flew past another Kuiper Belt object. And what a surprise it was. Arrokoth — its name refers to “sky” in the Powhatan/Algonquian language — looks like a pair of pancakes joined at the hip (SN: 12/21/19 & 1/4/20, p. 5; SN: 3/16/19, p. 15). Roughly 35 kilometers long from end to end, it was probably once two separate bodies that gently collided and stuck. Arrokoth’s bizarre structure sheds light on a fundamental question in astronomy: How do gas and dust clump together and grow into larger bodies?

      One long-standing theory, called planetesimal accretion, says that a series of collisions is responsible. Tiny bits of material collide and stick together on repeat to build up larger and larger objects, says JJ Kavelaars, an astronomer at the University of Victoria and the National Research Council of Canada. But there’s a problem, Kavelaars says.

      In 2019, New Horizons flew by Arrokoth (above), a roughly 35-kilometer-long Kuiper Belt object.NASA, JHU-APL, SWRI

      As objects get large enough to exert a significant gravitational pull, they accelerate as they approach one another. “They hit each other too fast, and they don’t stick together,” he says. It would be unusual for a large object like Arrokoth, particularly with its two-lobed structure, to have formed from a sequence of collisions.

      More likely, Arrokoth was born from a process known as gravitational instability, researchers now believe. In that scenario, a clump of material that happens to be denser than its surroundings grows by pulling in gas and dust. This process can form planets on timescales of thousands of years, rather than the millions of years required for planetesimal accretion. “The timescale for planet formation completely changes,” Kavelaars says.

      If Arrokoth formed this way, other bodies in the solar system probably did too. That may mean that parts of the solar system formed much more rapidly than previously believed, says Buie, who discovered Arrokoth in 2014. “Already Arrokoth has rewritten the textbooks on how solar system formation works.”

      What they’ve seen so far makes scientists even more eager to study another Kuiper Belt object up close. New Horizons is still making its way through the Kuiper Belt, but time is running out to identify a new object and orchestrate a rendezvous. The spacecraft, which is currently 53 astronomical units from the sun, is approaching the Kuiper Belt’s outer edge. Several teams of astronomers are using telescopes around the world to search for new Kuiper Belt objects that would make a close pass to New Horizons. “We are definitely looking,” Buie says. “We would like nothing better than to fly by another object.”

      All eyes on the Kuiper Belt

      Astronomers are also getting a wide-angle view of the Kuiper Belt by surveying it with some of Earth’s largest telescopes. At the Canada-France-Hawaii Telescope on Mauna Kea — the same mountaintop where Jewitt and Luu spotted 1992 QB1 — astronomers recently wrapped up the Outer Solar System Origins Survey. It recorded more than 800 previously unknown Kuiper Belt objects, bringing the total number known to roughly 3,000.

      The Canada-France-Hawaii Telescope, near the summit of Mauna Kea on Hawaii’s Big Island, has revealed hundreds of Kuiper Belt objects.Gordon W. Myers/Wikimedia Commons (CC BY-SA 4.0)

      This cataloging work is revealing tantalizing patterns in how these bodies move around the sun, MacGregor says. Rather than being uniformly distributed, the orbits of Kuiper Belt objects tend to be clustered in space. That’s a telltale sign that these bodies got a gravitational shove in the past, she says.

      The cosmic bullies that did that shoving, most astronomers believe, were none other than the solar system’s gas giants. In the mid-2000s, scientists first proposed that planets like Neptune and Saturn probably pinballed toward and away from the sun early in the solar system’s history (SN: 5/5/12, p. 24). That movement explains the strikingly similar orbits of many Kuiper Belt objects, MacGregor says. “The giant planets stirred up all of the stuff in the outer part of the solar system.”

      Refining the solar system’s early history requires observations of even more Kuiper Belt objects, says Meg Schwamb, an astronomer at Queen’s University Belfast in Northern Ireland. Researchers expect that a new astronomical survey, slated to begin next year, will find roughly 40,000 more Kuiper Belt objects. The Vera C. Rubin Observatory, being built in north-central Chile, will use its 3,200-megapixel camera to repeatedly photograph the entire Southern Hemisphere sky every few nights for 10 years. That undertaking, the Legacy Survey of Space and Time, or LSST, will revolutionize our understanding of how the early solar system evolved, says Schwamb, a cochair of the LSST Solar System Science Collaboration.

      The Vera C. Rubin Observatory in Chile is expected to spot about 40,000 Kuiper Belt objects with its 8.4-meter mirror and the world’s largest digital camera.Rubin Observatory/NSF and AURA

      It’s exciting to think about what we might learn next from the Kuiper Belt, Jewitt says. The discoveries that lay ahead will be possible, in large part, because of advances in technology, he says. “One picture with one of the modern survey cameras is roughly a thousand pictures with our setup back in 1992.”

      But even as we uncover more about this distant realm of the solar system, a bit of awe should always remain, Jewitt says. “It’s the largest piece of the solar system that we’ve yet observed.” More

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      Oort cloud comets may spin themselves to death

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      Comets from the solar system’s deep freezer often don’t survive their first encounter with the sun. Now one scientist thinks he knows why: Solar warmth makes some of the cosmic snowballs spin so fast, they fall apart.

      This suggestion could help solve a decades-old mystery about what destroys many “long-period” comets, astronomer David Jewitt reports in a study submitted August 8 to arXiv.org. Long-period comets originate in the Oort cloud, a sphere of icy objects at the solar system’s fringe (SN: 8/18/08). Those that survive their first trip around the sun tend to swing by our star only once every 200 years.

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      “These things are stable out there in the Oort cloud where nothing ever happens. When they come toward the sun, they heat up, all hell breaks loose, and they fall apart,” Jewitt says.

      The Dutch astronomer Jan Oort first proposed the Oort cloud as a cometary reservoir in 1950. He realized that many of its comets that came near Earth were first-time visitors, not return travelers. Something was taking the comets out, but no one knew what.

      One possibility was that the comets die by sublimating all of their water away as they near the heat of the sun until there’s nothing left. But that didn’t fit with observations of comets that seemed to physically break up into smaller pieces. The trouble was, those breakups are hard to watch in real time.

      “The disintegrations are really hard to observe because they’re unpredictable, and they happen quickly,” Jewitt says.

      He ran into that difficulty when he tried to observe Comet Leonard, a bright comet that put on a spectacular show in winter 2021–2022. Jewitt had applied for time to observe the comet with the Hubble Space Telescope in April and June 2022. But by February, the comet had already disintegrated. “That was a wake-up call,” Jewitt says.

      So Jewitt turned to historical observations of long-period comets that came close to the sun since the year 2000. He selected those whose water vapor production had been indirectly measured via an instrument called SWAN on NASA’s SOHO spacecraft, to see how quickly the comets were losing mass. He also picked out comets whose movements deviating from their orbits around the sun had been measured. Those motions are a result of water vapor jets pushing the comet around, like a spraying hose flopping around a garden.

      That left him with 27 comets, seven of which did not survive their closest approach to the sun.

      Jewitt expected that the most active comets would disintegrate the fastest, by puffing away all their water. But he found the opposite: It turns out that the least active comets with the smallest dirty snowball cores were the most at risk of falling apart.

      “Basically, being a small nucleus near the sun causes you to die,” Jewitt says. “The question is, why?”

      It wasn’t that the comets were torn apart by the sun’s gravity — they didn’t get close enough for that. And simply sublimating until they went poof would have been too slow a death to match the observations. The comets are also unlikely to collide with anything else in the vastness of space and break apart that way. And a previous suggestion that pressure builds up inside the comets until they explode like a hand grenade doesn’t make sense to Jewitt. Comets’ upper few centimeters of material would absorb most of the sun’s heat, he says, so it would be difficult to heat the center of the comet enough for that to work.

      The best remaining explanation, Jewitt says, is rotational breakup. As the comet nears the sun and its water heats up enough to sublimate, jets of water vapor form and make the core start to spin like a catherine wheel firework. Smaller cores are easier to push around than a larger one, so they spin more easily.

      “It just spins faster and faster, until it doesn’t have enough tensile strength to hold together,” Jewitt says. “I’m pretty sure that’s what’s happening.”

      That deadly spin speed is actually quite slow. Spinning at about half a meter per second could spell curtains for a kilometer-sized comet, he calculates. “You can walk faster.”

      But comets are fragile. If you held a fist-sized comet in front of your face, a sneeze would destroy it, says planetary astronomer Nalin Samarasinha of the Planetary Science Institute in Tucson, who was not involved in the study.

      Samarasinha thinks Jewitt’s proposal is convincing. “Even though the sample size is small, I think it is something really happening.” But other things might be destroying these comets too, he says, and Jewitt agrees.

      Samarasinha is holding out for more comet observations, which could come when the Vera Rubin Observatory begins surveying the sky in 2023. Jewitt’s idea “is something which can be observationally tested in a decade or two.” More

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      Asteroid impacts might have created some of Mars’ sand

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      Sand on Earth is continuously being created by the slow erosion of rocks. But on Mars, violent asteroid impacts may play an important role in making new sand.

      As much as a quarter of Martian sand is composed of spherical bits of glass forged in the intense heat of impacts, a new study shows. Since windblown sand sculpts the Martian landscape, this discovery reveals how asteroid impacts contribute to shaping Mars, even long after the collisions occur, Purdue University planetary scientist Briony Horgan and colleagues suggest. The team will present their results August 18 at the 85th Annual Meeting of the Meteoritical Society in Glasgow, Scotland.

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      Using data collected by spacecraft orbiting Mars, Horgan and collaborators looked at different wavelengths of visible and infrared light reflected from the planet’s surface to determine the minerals present in Martian sand. The team found signatures of glass all over the planet, particularly at higher latitudes.

      One explanation for all that glass is volcanic eruptions, which are known to produce glass when magma mixes with water. But the most glass-rich swath of Mars — the planet’s northern plains — is conspicuously bereft of volcanoes, the researchers note. That rules out volcanic eruptions as the culprit in that location and instead suggests that far more cataclysmic events — asteroid impacts — might be involved.

      That’s a plausible argument, says Steven Goderis, a geochemist at the Vrije Universiteit Brussel in Belgium who was not involved in the research. “Often Mars is seen as a volcanic planet. But there’s also a very strong impact component, and this is often overlooked.”

      When an asteroid moving at several kilometers per second slams into a rocky planet like Mars, the energy of the event melts nearby rocks and launches them skywards. That molten shrapnel fragments and produces sand grain–sized pieces that are roughly spherical. Those bits of glass — called impact spherules — eventually rain back onto the planet (SN: 3/31/21).

      Martian sand, imaged by NASA’s Phoenix Mars Lander, contains dark, spherical grains that were most likely created by asteroid impacts.Briony Horgan/ICL/UA/JPL/NASA

      Over the last 3 billion years, asteroid impacts could have plausibly blanketed the surface of Mars in a layer of impact spherules roughly half a meter thick, Horgan and her colleagues calculate. All that material added to the sand on Mars that formed through normal erosion. “Impacts helped supply sand to the surface continuously over time,” Horgan says.

      Scientists might have the opportunity to analyze Martian impact spherules in the future. NASA’s Perseverance rover is currently storing samples of Martian sand and rocks for eventual return to Earth (SN: 9/10/21). That’s exciting, Horgan says. “The record of all this is in the sand.” More

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      in Space & Astronomy

      Over time, Betelgeuse changed color. Now it’s also lost its rhythm

      by

      The star Betelgeuse has always been a diva.

      Astronomers from antiquity through the present day have watched the red supergiant pulsing at the shoulder of the constellation Orion, and the star has continually put on a show, two new studies suggest. Betelgeuse may still be recovering from a deep dimming episode a few years ago, one team reports. And the star appears to have put on its reddish stage makeup just 2,000 years ago, before which it wore yellow, another team says.

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      Together, these studies could tell researchers about how stars spew their guts into space and hint at how long it will be before Betelgeuse explodes in a supernova.

      “This star always fools you,” says astronomer Edward Guinan of Villanova University in Pennsylvania, who has studied Betelgeuse for decades and was not involved in the new works. “You think you have it, and all of a sudden, it changes.”

      The “Great Dimming”

      In late 2019, Betelgeuse captured astronomers’ attention when it suddenly grew dark for several months — an event astronomers now call the Great Dimming. Months of subsequent observations led researchers to an explanation: The star had coughed out a big bubble of plasma. That material cooled, condensed into dust and blocked the star’s face from the perspective of Earth months later (SN: 11/29/20). The surface of the star also cooled down, contributing to the dimming (SN: 6/16/21).

      But what happened next was equally surprising, astrophysicist Andrea Dupree and colleagues report in a paper submitted August 2 to arXiv.org. The star’s regular pulsating brightness, it seems, went completely out of whack.

      In its non–Great Dimming life, Betelgeuse’s brightness was on a quasi-periodic dimmer switch. As the star breathed in and out — ballooning out before shrinking back down — its brightness went up and down. “For 200 years, it had a nice, 400-day oscillation in brightness,” says Dupree, of the Harvard & Smithsonian Center for Astrophysics in Cambridge, Mass. “But that’s gone now.”

      That regular drumbeat has since grown erratic. Instead of a regular thrum, the oscillations are “like an unbalanced washing machine, going ‘wonka wonka wonka,’” Dupree says.

      The wonkiness is a sign of the star struggling to recover from the loss of material in 2019, Dupree says. She calculates that Betelgeuse ejected several times the mass of the moon from its surface, leaving a large cool spot behind. The star’s surface plasma is sloshing around as it returns to equilibrium.

      If this picture is correct, it means red supergiants like Betelgeuse can spray material into interstellar space in discrete bursts, rather than a continuous stream. That’s important to know because many of the elements that make up planets and people were formed in stars undergoing what Betelgeuse is going through right now. Studying Betelgeuse’s growing pains and death throes can tell us about our own origins.

      But while this picture of Betelgeuse holds together, it is still speculative, Guinan cautions.

      One confounding factor is a new set of observations of Betelgeuse during the four-month period when it’s usually out of view. From May through August every year, Betelgeuse is too close to the sun from Earth’s perspective to be seen at night. Usually that leaves a hole in the datasets of astronomers who track its periodic behavior.

      But amateur observer Otmar Nickel of Mainz, Germany, developed a technique to measure Betelgeuse’s brightness using multiple images taken during the day. Dupree’s paper is the first to include those daytime data.

      “That’s cool,” Guinan says. “You can follow the star all year round.”

      Those extra observations might reveal recurring changes that have always been there, rather than picking up on something truly new. “Those little variations you’re seeing…could easily be present right before the Great Dimming,” Guinan says.

      Dupree’s team predicts that the dust Betelgeuse lost could become visible to some telescopes on Earth in 2023. “That would be proof” that the brightness changes were due to a single outburst, Guinan says.

      Seeing yellow

      The Great Dimming isn’t the first time humans have recorded a major change in Betelgeuse’s personality. Two millennia ago, the star was a completely different color, astrophysicist Ralph Neuhäuser and colleagues report in a paper in press in Monthly Notices of the Royal Astronomical Society.

      The team analyzed ancient descriptions of more than 200 stars whose colors should have been visible to the naked eye in the past few thousand years. Most stars observed over human history had the same color recorded in the past as they display today, the team found. But not Betelgeuse.

      The ancient Roman astronomer Gaius Julius Hyginus, who lived from about 64 B.C. to A.D. 17, and is thought to have written the Latin work De Astronomia, described the star in the right shoulder of Orion has having a similar color to Saturn ­— which is yellow. Astrologer and archivist Sima Qian, working during the Chinese Han dynasty around 100 B.C., independently described the star as yellow. Observers from other ancient cultures conspicuously left Betelgeuse out of their lists of red stars.

      “I thought, ‘Oh, how can this be?’” says Neuhäuser, of AIU Jena in Germany. “I was not expecting such a result … to find a star to change color in historical time.”

      A star’s color is a sign of its evolutionary stage (SN: 7/23/21). When stars burn through the hydrogen fuel in their cores, they puff up and expel gases into space. That expansion makes their surface temperatures drop, and they change color from blue to red in fairly short order — about 10,000 years for a giant star like Betelgeuse, which is around 14 times as massive as the sun.

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      Measuring a star’s age isn’t as easy as you’d think. Here’s how scientists get their ballpark estimates.

      That relatively recent color change suggests Betelgeuse has just reached the end of its hydrogen-burning life and became the red supergiant we know it as today while human observers were watching.

      “It’s fully consistent with astrophysical knowledge,” Neuhäuser says. “It could have been expected, but no one really checked.”

      That result means anyone waiting for Betelgeuse to go supernova will have a very long wait. If the star just became a supergiant in the last few millennia, it has more than 1 million years to go before the boom. More