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      Here is the first direct look at Neptune’s rings in more than 30 years

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      Humankind is seeing Neptune’s rings in a whole new light thanks to the James Webb Space Telescope.

      In an infrared image released September 21, Neptune and its gossamer diadems of dust take on an ethereal glow against the inky backdrop of space. The stunning portrait is a huge improvement over the rings’ previous close-up, which was taken more than 30 years ago.

      Unlike the dazzling belts encircling Saturn, Neptune’s rings appear dark and faint in visible light, making them difficult to see from Earth. The last time anyone saw Neptune’s rings was in 1989, when NASA’s Voyager 2 spacecraft, after tearing past the planet, snapped a couple grainy photos from roughly 1 million kilometers away (SN: 8/7/17). In those photos, taken in visible light, the rings appear as thin, concentric arcs.

      As Voyager 2 continued to interplanetary space, Neptune’s rings once again went into hiding — until July. That’s when the James Webb Space Telescope, or JWST, turned its sharp, infrared gaze toward the planet from roughly 4.4 billion kilometers away (SN: 7/11/22).

      Neptune’s elusive rings appear as thin arcs of light in this 1989 image from the Voyager 2 spacecraft, taken shortly after the probe made its closest approach to the planet. JPL/NASA

      Neptune itself appears mostly dark in the new image. That’s because methane gas in the planet’s atmosphere absorbs much of its infrared light. A few bright patches mark where high-altitude methane ice clouds reflect sunlight.

      And then there are the ever-elusive rings. “The rings have lots of ice and dust in them, which are extremely reflective in infrared light,” says Stefanie Milam, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., and one of JWST’s project scientists. The enormity of the telescope’s mirror also makes its images extra sharp. “JWST was designed to look at the first stars and galaxies across the universe, so we can really see fine details that we haven’t been able to see before,” Milam says.

      Upcoming JWST observations will look at Neptune with other scientific instruments. That should provide new intel on the rings’ composition and dynamics, as well as on how Neptune’s clouds and storms evolve, Milam says. “There’s more to come.”

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      A protogalaxy in the Milky Way may be our galaxy’s original nucleus

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      The Milky Way left its “poor old heart” in and around the constellation Sagittarius, astronomers report. New data from the Gaia spacecraft reveal the full extent of what seems to be the galaxy’s original nucleus — the ancient stellar population that the rest of the Milky Way grew around — which came together more than 12.5 billion years ago.

      “People have long speculated that such a vast population [of old stars] should exist in the center of our Milky Way, and Gaia now shows that there they are,” says astronomer Hans-Walter Rix of the Max Planck Institute for Astronomy in Heidelberg, Germany.

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      The Milky Way’s ancient heart is a round protogalaxy that spans nearly 18,000 light-years and possesses roughly 100 million times the mass of the sun in stars, or about 0.2 percent of the Milky Way’s current stellar mass, Rix and colleagues report in a study posted September 7 at arXiv.org.

      “This study really helps to firm up our understanding of this very, very, very young stage in the Milky Way’s life,” says Vasily Belokurov, an astronomer at the University of Cambridge who was not involved in the work. “Not much is really known about this period of the Milky Way’s life,” he says. “We’ve seen glimpses of this population before,” but the new study gives “a bird’s-eye view of the whole structure.”

      Most stars in the Milky Way’s central region abound with metals, because the stars originated in a crowded metropolis that earlier stellar generations had enriched with those metals through supernova explosions. But Rix and his colleagues wanted to find the exceptions to the rule, stars so metal-poor they must have been born well before the rest of the galaxy’s stellar denizens came along — what Rix calls “a needle-in-a-haystack exercise.”

      His team turned to data from the Gaia spacecraft, which launched in 2013 on a mission to chart the Milky Way (SN: 6/13/22). The astronomers searched about 2 million stars within a broad region around the galaxy’s center, which lies in the constellation Sagittarius, looking for stars with metal-to-hydrogen ratios no more than 3 percent of the sun’s.

      The astronomers then examined how those stars move through space, retaining only the ones that don’t dart off into the vast halo of metal-poor stars engulfing the Milky Way’s disk. The end result: a sample of 18,000 ancient stars that represents the kernel around which the entire galaxy blossomed, the researchers say. By accounting for stars obscured by dust, Rix estimates that the protogalaxy is between 50 million and 200 million times as massive as the sun.

      “That’s the original core,” Rix says, and it harbors the Milky Way’s oldest stars, which he says probably have ages exceeding 12.5 billion years. The protogalaxy formed when several large clumps of stars and gas conglomerated long ago, before the Milky Way’s first disk — the so-called thick disk — arose (SN: 3/23/22).

      The protogalaxy is compact, which means little has disturbed it since its formation. Smaller galaxies have crashed into the Milky Way, augmenting its mass, but “we didn’t have any later mergers that deeply penetrated into the core and shook it up, because then the core would be larger now,” Rix says.

      The new data on the protogalaxy even capture the Milky Way’s initial spin-up — its transition from an object that didn’t rotate into one that now does. The oldest stars in the proto–Milky Way barely revolve around the galaxy’s center but dive in and out of it instead, whereas slightly younger stars show more and more movement around the galactic center. “This is the Milky Way trying to become a disk galaxy,” says Belokurov, who saw the same spin-up in research that he and a colleague reported in July.

      Today, the Milky Way is a giant galaxy that spins rapidly — each hour our solar system speeds through 900,000 kilometers of space as we race around the galaxy’s center. But the new study shows that the Milky Way got its start as a modest protogalaxy whose stars still shine today, stars that astronomers can now scrutinize for further clues to the galaxy’s birth and early evolution. More

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      Saturn’s rings and tilt might have come from one missing moon

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      A single, doomed moon could clear up a couple of mysteries about Saturn.

      This hypothetical missing moon, dubbed Chrysalis, could have helped tilt Saturn over, researchers suggest September 15 in Science. The ensuing orbital chaos might then have led to the moon’s demise, shredding it to form the iconic rings that encircle the planet today.

      “We like it because it’s a scenario that explains two or three different things that were previously not thought to be related,” says study coauthor Jack Wisdom, a planetary scientist at MIT. “The rings are related to the tilt, who would ever have guessed that?”

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      Saturn’s rings appear surprisingly young, a mere 150 million years or so old (SN: 12/14/17). If the dinosaurs had telescopes, they might have seen a ringless Saturn.  Another mysterious feature of the gas giant is its nearly 27-degree tilt relative to its orbit around the sun. That tilt is too large to have formed when Saturn did or to be the result of collisions knocking the planet over.

      Planetary scientists have long suspected that the tilt is related to Neptune, because of a coincidence in timing between the way the two planets move. Saturn’s axis wobbles, or precesses, like a spinning top. Neptune’s entire orbit around the sun also wobbles, like a struggling hula hoop.

      The periods of both precessions are almost the same, a phenomenon known as resonance. Scientists theorized that gravity from Saturn’s moons — especially the largest moon, Titan — helped the planetary precessions line up. But some features of Saturn’s internal structure were not known well enough to prove that the two timings were related.

      Wisdom and colleagues used precision measurements of Saturn’s gravitational field from the Cassini spacecraft, which plunged into Saturn in 2017 after 13 years orbiting the gas giant, to figure out the details of its internal structure (SN: 9/15/17). Specifically, the team worked out Saturn’s moment of inertia, a measure of how much force is needed to tip the planet over. The team found that the moment of inertia is close to, but not exactly, what it would be if Saturn’s spin were in perfect resonance with Neptune’s orbit.

      “We argue that it’s so close, it couldn’t have occurred by chance,” Wisdom says. “That’s where this satellite Chrysalis came in.”

      After considering a volley of other explanations, Wisdom and colleagues realized that another smallish moon would have helped Titan bring Saturn and Neptune into resonance by adding its own gravitational tugs. Titan drifted away from Saturn until its orbit synced up with that of Chrysalis. The enhanced gravitational kicks from the larger moon sent the doomed smaller moon on a chaotic dance. Eventually, Chrysalis swooped so close to Saturn that it grazed the giant planet’s cloud tops. Saturn ripped the moon apart, and slowly ground its pieces down into the rings.

      Calculations and computer simulations showed that the scenario works, though not all the time. Out of 390 simulated scenarios, only 17 ended with Chrysalis disintegrating to create the rings. Then again, massive, striking rings like Saturn’s are rare, too.

      The name Chrysalis came from that spectacular ending: “A chrysalis is a cocoon of a butterfly,” Wisdom says. “The satellite Chrysalis was dormant for 4.5 billion years, presumably. Then suddenly the rings of Saturn emerged from it.”

      The story hangs together, says planetary scientist Larry Esposito of the University of Colorado Boulder, who was not involved in the new work. But he’s not entirely convinced. “I think it’s all plausible, but maybe not so likely,” he says. “If Sherlock Holmes is solving a case, even the improbable explanation may be the right one. But I don’t think we’re there yet.” More

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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