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    NASA’s InSight lander has recorded the largest Marsquake yet

    Any Martians out there should learn to duck and cover.

    On May 4, the Red Planet was rocked by a roughly magnitude 5 temblor, the largest Marsquake detected to date, NASA’s Jet Propulsion Laboratory in Pasadena, Calif., reports. The shaking lasted for more than six hours and released more than 10 times the energy of the previous record-holding quake.

    The U.S. space agency’s InSight lander, which has been studying Mars’ deep interior since touching down on the planet in 2018 (SN: 11/26/18), recorded the event. The quake probably originated near the Cerberus Fossae region, which is more than 1,000 kilometers from the lander.

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    Cerberus Fossae is known for its fractured surface and frequent rockfalls. It makes sense that the ground would be shifting there, says geophysicist Philippe Lognonné, principal investigator of the Seismic Experiment for Interior Structure, InSight’s seismometer. “It’s an ancient volcanic bulge.”

    Just like earthquakes reveal information about our planet’s interior structure, Marsquakes can be used to probe what lies beneath Mars’ surface (SN: 7/22/21). And a lot can be learned from studying this whopper of a quake, says Lognonné, of the Institut de Physique du Globe de Paris. “The signal is so good, we’ll be able to work on the details.” More

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    We finally have an image of the black hole at the heart of the Milky Way

    There’s a new addition to astronomers’ portrait gallery of black holes. 

    Astronomers announced May 12 that they have finally assembled an image of the supermassive black hole at the center of our galaxy. 

    “This image shows a bright ring surrounding the darkness, the telltale sign of the shadow of the black hole,” astrophysicist Feryal Özel of the University of Arizona in Tucson said at a news conference announcing the result.

    The black hole, known as Sagittarius A*, appears as a faint silhouette amidst the glowing material that surrounds it. The image reveals the turbulent, twisting region immediately surrounding the black hole in new detail. The findings also were published May 12 in 6 studies in the Astrophysical Journal Letters.

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    A planet-spanning network of radio telescopes, known as the Event Horizon Telescope, worked together to create this much-anticipated look at the Milky Way’s giant. Three years ago, the same team released the first-ever image of a supermassive black hole (SN: 4/10/19). That object sits at the center of the galaxy M87, about 55 million light-years from Earth. 

    But Sagittarius A*, or Sgr A* for short, is “humanity’s black hole,” says astrophysicist Sera Markoff of the University of Amsterdam, and a member of the EHT collaboration. 

    At 27,000 light-years away, the behemoth is the closest giant black hole to Earth. That proximity means that Sgr A* is the most-studied supermassive black hole in the universe. Yet Sgr A* and others like it remain some of the most mysterious objects ever found. 

    That’s because, like all black holes, Sgr A* is an object so dense that its gravitational pull won’t let light escape. Black holes are “natural keepers of their own secrets,” says physicist Lena Murchikova of the Institute for Advanced Study in Princeton, N.J., who is not part of the EHT team. Their gravity traps light that falls within a border called the event horizon. EHT’s images of Sgr A* and the M87 black hole skirt up to that inescapable edge.

    [embedded content]
    This sonification is a translation into sound of the Event Horizon Telescope’s image of the supermassive black hole Sagittarius A*. The sonification sweeps clockwise around the black hole image. Material closer to the black hole orbits faster than material farther away. Here, the faster-moving material is heard at higher frequencies. Very low tones represent material outside the black hole’s main ring. Louder volume indicates brighter spots in the image.

    Sgr A* feeds on hot material pushed off of massive stars at the galactic center. That gas, drawn toward Sgr A* by its gravitational pull, flows into a surrounding disk of glowing material, called an accretion disk. The disk, the stars and an outer bubble of X-ray light “are like an ecosystem,” says astrophysicist Daryl Haggard of McGill University in Montreal and a member of the EHT collaboration. “They’re completely tied together.”

    That accretion disk is where the action is — as the gas moves within immensely strong magnetic fields — so astronomers want to know more about how the disk works.

    Like the majority of supermassive black holes,  Sgr A* is quiet and faint (SN: 6/5/19 ). The black hole eats only a few morsels fed to it by its accretion disk. Still, “it’s always been a little bit of a puzzle why it’s so, so faint,” says astrophysicist Meg Urry of Yale University, who is not part of the EHT collaboration. M87’s black hole, in comparison, is a monster gorging on nearby material and shooting out enormous, powerful jets (SN: 11/10/21). But that doesn’t mean Sgr A* isn’t producing light. Astrophysicists have seen its region feebly glowing in radio waves, jittering in infrared and burping in X-rays.

    In fact, the accretion disk around Sgr A* seems to constantly flicker and simmer. This variability, the constant flickering, is like a froth on top of ocean waves, Markoff says. “​​And so we’re seeing this froth that is coming up from all this activity, and we’re trying to understand the waves underneath the froth.” 

    The big question, she adds, has been if astronomers would be able to see something changing in those waves with EHT. In the new work, they’ve seen hints of those changes below the froth, but the full analysis is still ongoing.

    By combining about 3.5 petabytes of data, or the equivalent of about 100 million TikTok videos, captured in April 2017, researchers could begin to piece together the picture. To tease out an image from the initial massive jumble of data, the EHT team needed years of work, complicated computer simulations and observations in various types of light from other telescopes. 

    [embedded content]
    Scientists created a vast library of computer simulations of Sagittarius A* (one shown) to explore the turbulent flow of hot gas that rings the black hole. That rapid flow causes the ring’s appearance to vary in brightness on timescales of minutes. Scientists compared these simulations with the newly released observations of the black hole to better understand its true properties.

    Those “multiwavelength” data from the other telescopes were crucial to assembling the image. “By looking at these things simultaneously and all together, we’re able to come up with a complete picture,” says theorist Gibwa Musoke of the University of Amsterdam. 

    Sgr A*’s variability, the constant simmering, complicated the analysis because the black hole changes on timescales of just a few minutes, changing as the researchers were imaging it. “It was like trying to take a clear picture of a running child at night,” astronomer José L. Gómez of Instituto de Astrofísica de Andalucía in Granada, Spain, said at a news conference announcing the result. M87 was easier to analyze because it changed over the course of weeks.

    Ultimately, a better understanding of what is happening in the disk so close to Sgr A* could help scientists learn how many other similar supermassive black holes work. 

    The new EHT observations also confirm the mass of Sgr A* at 4 million times that of the sun. If the black hole replaced our sun, the shadow EHT imaged would sit within Mercury’s orbit. 

    The researchers also used the image of Sgr A* to put general relativity to the test (SN: 2/3/21). Einstein’s steadfast theory of gravity passed: The size of the shadow matched the predictions of general relativity. By testing the theory in extreme conditions — like those around black holes — scientists hope to pinpoint any hidden weaknesses.

    Scientists have previously tested general relativity by following the motions of stars that orbit very close to Sgr A* — work that also helped confirm that the object truly is a black hole (SN: 7/26/18). For that discovery, researchers Andrea Ghez and Reinhard Genzel won a share of the Nobel Prize in physics in 2020 (SN: 10/6/20).

    The two types of tests of general relativity are complementary,  says astrophysicist Tuan Do of UCLA. “With these big physics tests, you don’t want to use just one method.” If one test appears to contradict general relativity, scientists can check for a corresponding discrepancy in the other.

    The Event Horizon Telescope, however, tests general relativity much nearer to the black hole’s edge, which could highlight subtle effects of physics beyond general relativity. “The closer you get, the better you are in terms of being able to look for these effects,” says physicist Clifford Will of the University of Florida in Gainesville.

    However, some researchers have criticized a similar test of general relativity made using the EHT image of M87’s black hole (SN: 10/1/20). That’s because the test relies on relatively shaky assumptions about the physics of how material swirls around a black hole, says physicist Sam Gralla of the University of Arizona in Tucson. Testing general relativity in this way “would only make sense if general relativity were the weakest link,” but scientists’ confidence in general relativity is stronger than the assumptions that went into the test, he says.

    The observations of Sgr A* provide more evidence that the object is in fact a black hole, says physicist Nicolas Yunes of the University of Illinois Urbana-Champaign. “It’s really exciting to have the first image of a black hole that is in our own Milky Way. It’s fantastic.” It sparks the imagination, like early pictures astronauts took of Earth from the moon, he says.

    This won’t be the last eye-catching image of Sgr A* from EHT. Additional observations, made in 2018, 2021 and 2022, are still waiting to be analyzed. 

    “This is our closest supermassive black hole,” Haggard says. “It is like our closest friend and neighbor. And we’ve been studying it for years as a community. [This image is a] really profound addition to this exciting black hole we’ve all kind of fallen in love with in our careers.” More

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    The sun’s searing radiation led to the shuffling of the solar system’s planets

    In the solar system’s early years, the still-forming giant planets sidestepped, did a do-si-do and then swung one of their partners away from the sun’s gravitational grasp. Things settled, and our planetary system was in its final configuration.

    What triggered that planetary shuffle has been unknown. Now, computer simulations suggest that the hot radiation of the young sun evaporating its planet-forming disk of gas and dust led to the scrambling of the giant planets’ orbits, researchers report in the April 28 Nature.

    As a result, the four largest planets may have been in their final configuration within 10 million years of the solar system’s birth about 4.6 billion years ago. That’s much quicker than the 500 million years that previous work had suggested.

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    The planetary-shuffling mechanism that the team uncovered in the computer simulations is very innovative, says Nelson Ndugu, an astrophysicist who studies forming planetary systems at North-West University in Potchefstroom, South Africa, and Muni University in Arua, Uganda. “It has huge potential.”

    Heaps of evidence, including observations of extrasolar planetary systems forming (SN: 7/2/18), had already indicated that something in our solar system’s early history jumbled the giant planets’ orbits, which scientists call the giant-planet instability (SN: 5/25/05).

    “The evidence for the giant-planet instability is really robust,” says Seth Jacobson, a planetary scientist at Michigan State University in East Lansing. “It explains many features of the outer solar system,” he says, like the large number of rocky objects beyond Neptune that make up the Kuiper Belt (SN: 12/31/09).

    To figure out what triggered that instability, Jacobson and colleagues ran computer simulations of the thousands of ways that the early solar system could have developed. All started with a young star and a planet-forming disk of gas and dust surrounding the star. The team then altered the disk parameters, such as its mass, density and how fast it evolved.

    The simulations also included the still-forming giant planets — five of them, in fact. Astronomers think a third ice giant, in addition to Uranus and Neptune, was originally a solar system member (SN: 4/20/12). Jupiter and Saturn round out the final tally of these massive planets.

    When the sun officially became a star, that is, the moment it began burning hydrogen at its core — roughly 4.6 billion years ago — its ultraviolet emission would have hit the disk’s gas, ionizing it and heating it to tens of thousands of degrees. “This is a very well-documented process,” Jacobson says. As the gas heats, it expands and flows away from the star, beginning with the inner portion of the disk.

    “The disk disperses its gas from inside out,” says Beibei Liu, an astrophysicist at Zhejiang University in Hangzhou, China. He and Jacobson collaborated with astronomer Sean Raymond of Laboratoire d’Astrophysique de Bordeaux in France in the new research.

    In the team’s simulations, as the inner part of the disk dissolves, that area loses mass, so the embedded, still-forming planets feel less gravity from that region, Jacobson says. But the planets still feel the same amount of pull from the disk’s outer region. This gravitational torquing, as the team calls it, can trigger a rebound effect: “Originally, the planets migrate in, and they reach the [inner] edge of this disk, and they reverse their migration,” Liu says.

    Because of Jupiter’s large mass, it’s mostly unaffected. Saturn, though, moves outward and into the region, which, in the simulations, holds the three ice giant planets. That area becomes crowded, Liu says, and close planetary interactions follow. One ice giant gets kicked out of the solar system entirely, Uranus and Neptune shift a bit farther from the sun, and “they gradually form the orbits close to our solar system’s configuration,” Liu says.

    In their computer simulations, the researchers found that as the sun’s radiation evaporates the disk, a planetary reshuffle nearly always ensues. “We can’t avoid this instability,” Jacobson says.

    Now that the researchers have an idea of what may have caused this solar system shuffle, the next step is to simulate how the evaporation of the disk could affect other objects.

    “We’ve focused really heavily on the giant planets, because their orbits were the original motivation,” Jacobson says. “But now, we have to do the follow-up work to show how this trigger mechanism relates to the small bodies.” More

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    Gravitational waves gave a new black hole a high-speed ‘kick’

    This black hole really knows how to kick back.

    Scientists recently observed two black holes that united into one, and in the process got a “kick” that flung the newly formed black hole away at high speed. That black hole zoomed off at about 5 million kilometers per hour, give or take a few million, researchers report in a paper in press in Physical Review Letters. That’s blazingly quick: The speed of light is just 200 times as fast.

    Ripples in spacetime, called gravitational waves, launched the black hole on its breakneck exit. As any two paired-up black holes spiral inward and coalesce, they emit these ripples, which stretch and squeeze space. If those gravitational waves are shot off into the cosmos in one direction preferentially, the black hole will recoil in response.

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    It’s akin to a gun kicking back after shooting a bullet, says astrophysicist Vijay Varma of the Max Planck Institute for Gravitational Physics in Potsdam, Germany.

    Gravitational wave observatories LIGO and Virgo, located in the United States and Italy, detected the black holes’ spacetime ripples when they reached Earth on January 29, 2020. Those waves revealed details of how the black holes merged, hinting that a large kick was probable. As the black holes orbited one another, the plane in which they orbited rotated, or precessed, similar to how a top wobbles as it spins. Precessing black holes are expected to get bigger kicks when they merge.

    So Varma and colleagues delved deeper into the data, gauging whether the black hole got the boot. To estimate the kick velocity, the researchers compared the data with various predicted versions of black hole mergers, created based on computer simulations that solve the equations of general relativity, Einstein’s theory of gravity (SN: 2/3/21). The recoil was so large, the researchers found, that the black hole was probably ejected from its home and kicked to the cosmic curb.

    Dense groups of stars and black holes called globular clusters are one locale where black holes are thought to partner up and merge. The probability that the kicked black hole would stay within a globular cluster home is only about 0.5 percent, the team calculated. For a black hole in another type of dense environment, called a nuclear star cluster, the probability of sticking around was about 8 percent.

    The black hole’s great escape could have big implications. LIGO and Virgo detect mergers of stellar-mass black holes, which form when a star explodes in a supernova and collapses into a black hole. Scientists want to understand if black holes that partner up in crowded clusters could partner up again, going through multiple rounds of melding. If they do, that could help explain some surprisingly bulky black holes previously seen in mergers (SN: 9/2/20). But if merged black holes commonly get rocketed away from home, that would make multiple mergers less likely.

    “Kicks are very important in understanding how heavy stellar-mass black holes form,” Varma says.

    Previously, astronomers have gleaned evidence of gravitational waves giving big kicks to supermassive black holes, the much larger beasts found at the centers of galaxies (SN: 3/28/17). But that conclusion hinges on observations of light, rather than gravitational waves. “Gravitational waves, in a way, are cleaner and easier to interpret,” says astrophysicist Manuela Campanelli of the Rochester Institute of Technology in New York, who was not involved in the new study.

    LIGO and Virgo data had already revealed some evidence of black holes getting small kicks. The new study is the first to report using gravitational waves to spot a black hole on the receiving end of a large kick.

    That big kick isn’t a surprise, Campanelli says. Earlier theoretical predictions by Campanelli and colleagues suggested that such powerful kicks were possible. “It’s always exciting when someone can measure from observations what you predicted from calculations.” More

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    ‘Goldilocks’ stars may pose challenges for any nearby habitable planets

    If you’re an aspiring life-form, you might want to steer clear of planets around orange dwarf stars.

    Some astronomers have called these orange suns “Goldilocks stars” (SN: 11/18/09). They are dimmer and age more slowly than yellow sunlike stars, thus offering an orbiting planet a more stable climate. But they are brighter and age faster than red dwarfs, which often spew large flares. However, new observations show that orange dwarfs emit lots of ultraviolet light long after birth, potentially endangering planetary atmospheres, researchers report in a paper submitted March 29 at arXiv.org.

    Using data from the Hubble Space Telescope, astronomer Tyler Richey-Yowell and her colleagues examined 39 orange dwarfs. Most are moving together through the Milky Way in two separate groups, either 40 million or 650 million years old.

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    To Richey-Yowell’s surprise, she and her team found that the ultraviolet flux didn’t drop off from the younger orange stars to the older ones — unlike the case for yellow and red stars. “I was like, `What the heck is going on?’” says Richey-Yowell, of Arizona State University in Tempe.

    In a stroke of luck, another team of researchers supplied part of the answer. As yellow sunlike stars age, they spin more slowly, causing them to be less active and emit less UV radiation. But for orange dwarfs, this steady spin-down stalls when the stars are roughly a billion years old, astronomer Jason Lee Curtis at Columbia University and colleagues reported in 2019.

    “[Orange] stars are just much more active for a longer time than we thought they were,” Richey-Yowell says. That means these possibly not-so-Goldilocks stars probably maintain high levels of UV light for more than a billion years.

    And that puts any potential life-forms inhabiting orbiting planets on notice. Far-ultraviolet light — whose photons, or particles of light, have much more energy than the UV photons that give you vitamin D — tears molecules in a planet’s atmosphere apart. That leaves behind individual atoms and electrically charged atoms and groups of atoms known as ions. Then the star’s wind — its outflow of particles — can carry the ions away, stripping the planet of its air.

    But not all hope is lost for aspiring life-forms that have an orange dwarf sun. Prolonged exposure to far-ultraviolet light can stress planets but doesn’t necessarily doom them to be barren, says Ed Guinan, an astronomer at Villanova University in Pennsylvania who was not involved in the new work. “As long as the planet has a strong magnetic field, you’re more or less OK,” he says.

    Though far-ultraviolet light splits water and other molecules in a planet’s atmosphere, the star’s wind can’t remove the resulting ions if a magnetic field as strong as Earth’s protects them. “That’s why the Earth survived” as a life-bearing world, Guinan says. In contrast, Venus might never have had a magnetic field, and Mars lost its magnetic field early on and most of its air soon after.

    “If the planet doesn’t have a magnetic field or has a weak one,” Guinan says, “the game is over.”

    What’s needed, Richey-Yowell says, is a study of older orange dwarfs to see exactly when their UV output declines. That will be a challenge, though. The easiest way to find stars of known age is to study a cluster of stars, but most star clusters get ripped apart well before their billionth birthday (SN: 7/24/20). As a result, star clusters somewhat older than this age are rare, which means the nearest examples are distant and harder to observe. More

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    Crumbling planets might trigger repeating fast radio bursts

    Fragmenting planets sweeping extremely close to their stars might be the cause of mysterious cosmic blasts of radio waves.

    Milliseconds-long fast radio bursts, or FRBs, erupt from distant cosmic locales. Some of these bursts blast only once and others repeat. A new computer calculation suggests the repetitive kind could be due to a planet interacting with its magnetic host star, researchers report in the March 20 Astrophysical Journal.

    FRBs are relative newcomers to astronomical research. Ever since the first was discovered in 2007, researchers have added hundreds to the tally. Scientists have theorized dozens of ways the two different types of FRBs can occur, and nearly all theories include compact, magnetic stellar remnants known as neutron stars. Some ideas include powerful radio flares from magnetars, the most magnetic neutron stars imaginable (SN: 6/4/20). Others suggest a fast-spinning neutron star, or even asteroids interacting with magnetars (SN: 2/23/22).

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    “How fast radio bursts are produced is still up for debate,” says astronomer Yong-Feng Huang of Nanjing University in China.

    Huang and his colleagues considered a new way to make the repeating flares: interactions between a neutron star and an orbiting planet (SN: 3/5/94). Such planets can get exceedingly close to these stars, so the team calculated what might happen to a planet in a highly elliptical orbit around a neutron star. When the planet swings very close to its star, the star’s gravity pulls more on the planet than when the planet is at its farthest orbital point, elongating and distorting it. This “tidal pull,” Huang says, will rip some small clumps off the planet. Each clump in the team’s calculation is just a few kilometers wide and maybe one-millionth the mass of the planet, he adds.

    Then the fireworks start. Neutron stars spew a wind of radiation and particles, much like our own sun but more extreme. When one of these clumps passes through that stellar wind, the interaction “can produce really strong radio emissions,” Huang says. If that happens when the clump appears to pass in front of the star from Earth’s perspective, we might see it as a fast radio burst. Each burst in a repeating FRB signal could be caused by one of these clumps interacting with the neutron star’s wind during each close planet pass, he says. After that interaction, what remains of the clump drifts in orbit around the star, but away from Earth’s perspective, so we never see it again.

    Comparing the calculated bursts to two known repeaters — the first ever discovered, which repeats roughly every 160 days, and a more recent discovery that repeats every 16 days, the team found the fragmenting planet scenario could explain how often the bursts happened and how bright they were (SN: 3/2/16).

    The star’s strong gravitational “tidal” pull on the planet during each close pass might change the planet’s orbit over time, says astrophysicist Wenbin Lu of Princeton University, who was not involved in this study but who investigates possible FRB scenarios. “Every orbit, there is some energy loss from the system,” he says. “Due to tidal interactions between the planet and the star, the orbit very quickly shrinks.” So it’s possible that the orbit could shrink so fast that FRB signals wouldn’t last long enough for a chance detection, he says.

    But the orbit change could also give astronomers a way to check this scenario as an FRB source. Observing repeating FRBs over several years to track any changes in the time between bursts could narrow down whether this hypothesis could explain the observations, Lu says. “That may be a good clue.” More

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    This is the biggest known comet in our solar system

    The nucleus of a comet discovered in 2014 is the largest ever spotted.

    The “dirty snowball” at the center of comet C/2014 UN271 is about 120 kilometers across, researchers report in the April 10 Astrophysical Journal Letters. That makes this comet — also known as Bernardinelli-Bernstein, after its discoverers — about twice as wide as Rhode Island, says David Jewitt, an astronomer at UCLA.

    Though the comet is big — and vastly larger than Halley’s comet, which measures a little more than 11 kilometers across — it will never be visible to the naked eye from Earth because it’s too far away, Jewitt says (SN: 12/14/15). The object is now about 3 billion kilometers from Earth. At its closest approach in 2031, the comet will come no closer to the sun than 1.6 billion kilometers, about the same distance as Saturn.

    Jewitt and colleagues sized up the comet with the help of new images from the Hubble Space Telescope, combined with images taken by another team at far-infrared wavelengths. The analysis also revealed that the comet’s nucleus reflects only about 3 percent of the light that strikes it. That makes the object “blacker than coal,” Jewitt says.

    Comet Bernardinelli-Bernstein takes about 3 million years to circle the sun in a highly elliptical orbit. At its farthest, the comet may reach about half a light-year from the sun — about one-eighth of the distance to the next nearest star.

    The comet is likely “just the tip of the iceberg” as far as undiscovered comets of this size go, Jewitt says. And for every comet this size, he suggests, there could be tens of thousands of smaller objects circling the sun undetected.

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    New thermal maps of Neptune reveal surprising temperature swings

    Neptune’s atmospheric temperature is on an unexpected roller-coaster ride, and it could take decades for scientists to piece together what’s happening at the distant planet.

    The ice giant’s global temperature dropped about 8 degrees Celsius between 2003 and 2012 at the start of Neptune’s summer, researchers report April 11 in Planetary Sciences Journal. Then from 2018 to 2020, thermal images show that the planet’s south pole brightened dramatically, indicating a spike of 11 degrees C (SN: 10/2/07).

    Naomi Rowe-Gurney, a planetary scientist at NASA Goddard Space Flight Center in Greenbelt, Md., and colleagues looked at 17 years of mid-infrared data from ground-based telescopes and the no-longer-functioning Spitzer Space Telescope (SN: 7/18/18; SN: 1/28/20). The researchers used infrared light to pierce Neptune’s top cloud layer and peer at its stratosphere, where the planet’s atmospheric chemistry comes into view.

    Each Neptune year lasts 165 Earth years, so the time period analyzed — from 2003 to 2020 — is essentially equivalent to five weeks on Earth. The wildest temperature shift occurred from 2018 to 2020, when the atmospheric temperature at Neptune’s south pole rose from –121° C to –110° C.

    “We weren’t expecting any seasonal changes to happen in this short time period, because we’re not even seeing a full season,” says Rowe-Gurney. “It’s all very strange and interesting.”

    The researchers don’t yet know what’s causing the temperature changes. The sun’s ultraviolet rays break up methane molecules in the stratosphere, so that chemistry or even the sun’s activity cycle could be a trigger. Nailing down specifics requires more observations. “We need to keep observing over the next 20 years to see a full season and see if something else changes,” says Rowe-Gurney.

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