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      Pluto’s dark side reveals clues to its atmosphere and frost cycles

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      Pluto’s dark side has come into dim view, thanks to the light of the dwarf planet’s moon.

      When NASA’s New Horizons spacecraft flew past Pluto in 2015, almost all the images of the dwarf planet’s unexpectedly complex surface were of the side illuminated by the sun (SN: 7/15/15). Darkness shrouded the dwarf planet’s other hemisphere. Some of it, like the area near the south pole, hadn’t seen the sun for decades.

      Now, mission scientists have finally released a grainy view of the dwarf planet’s dark side. The researchers describe the process to take the photo and what it tells them about how Pluto’s nitrogen cycle affects its atmosphere October 20 in the Planetary Science Journal.

      Before New Horizons passed by Pluto, the team suspected the dwarf planet’s largest moon, Charon, might reflect enough light to illuminate the distant world’s surface. So the researchers had the spacecraft turn back toward the sun to take a parting peek at Pluto.

      New Horizons captured this view of the backlit dark side of Pluto as the spacecraft receded from the dwarf planet in 2015. Some light and dark splotches were illuminated by the dim light of Pluto’s moon Charon.NOIRLab, SwRI, JHUAPL, NASA

      At first, the images just showed a ring of sunlight filtering through Pluto’s hazy atmosphere (SN: 7/24/15). “It’s very hard to see anything in that glare,” says planetary scientist John Spencer of the Southwest Research Institute in Boulder, Colo. “It’s like trying to read a street sign when you’re driving toward the setting sun and you have a dirty windshield.”

      Spencer and colleagues took a few steps to make it possible to pull details of Pluto’s dark side out of the glare. First, the team had the spacecraft take 360 short snapshots of the backlit dwarf planet. Each was about 0.4 seconds long, to avoid overexposing the images. The team also took snapshots of the sun without Pluto in the frame so that the sun could be subtracted out after the fact.

      Tod Lauer of the National Optical Astronomy Observatory in Tucson, Ariz., tried to process the images when he got the data in 2016. At the time, the rest of the data from New Horizons was still fresh and took up most of his attention, so he didn’t have the time to tackle such a tricky project.

      But “it was something that just sat there and ate away at me,” Lauer says. He tried again in 2019. Because the spacecraft was moving as it took the images, each image was a little bit smeared or blurred. Lauer wrote a computer code to remove that blur from each individual frame. Then he added the reflected Charon light in each of those hundreds of images together to produce a single image.

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      “When Tod did that painstaking analysis, we finally saw something emerging in the dark there … giving us a little bit of a glimpse of what the dark pole of Pluto looks like,” Spencer says.

      That the team got anything at all is impressive, says planetary scientist Carly Howett, also of the Southwest Research Institute and who is on the New Horizons team but was not involved in this work. “This dataset is really, really hard to work with,” she says. “Kudos to this team. I wouldn’t have wanted to do this.”

      The image, Howett says, can help scientists understand how Pluto’s frigid nitrogen atmosphere varies with its decades-long seasons. Pluto’s atmosphere is controlled by how much nitrogen is in a gas phase in the air and how much is frozen on the surface. The more nitrogen ice that evaporates, the thicker the atmosphere becomes. If too much nitrogen freezes to the ground, the atmosphere could collapse altogether.

      In this image of Pluto’s sunlit side from NASA’s New Horizons spacecraft, different colors represent different kinds of ices. A faint glimpse of Pluto’s dark hemisphere in newly released mission images reveals some new details about how those ices behave.SwRI, JHUAPL, NASA

      When New Horizons was there, Pluto’s south pole looked darker than the north pole. That suggests there was not a lot of fresh nitrogen frost freezing out of the atmosphere there, even though it was nearing winter. “The previous summer ended decades ago, but Pluto cools off pretty slowly,” Spencer says. “Maybe it’s still so warm [that] the frost can’t condense there, and that keeps the atmosphere from collapsing.”

      There was a bright spot in the middle of the image, which could be a fresh ice deposit. That’s also not surprising, Howett says. The ices may still be moving from the north pole to the south pole as Pluto moves deeper into its wintertime.

      “We’ve thought this for a long time. It makes sense,” she says. “But it’s nice to see it happening.” More

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      A rush to watch a supernova exposed its last gasp before exploding

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      A mad scramble to observe the moments after a star’s death is helping scientists understand how the star lived out its last year.

      Astronomers reported the exploding star just 18 hours after it flared up on March 31, 2020, in a galaxy about 60 million light-years away from Earth in the Virgo cluster. The supernova occurred in part of the sky already watched by NASA’s Transiting Exoplanet Survey Satellite, which images large portions of the sky every 30 minutes (SN: 1/8/19). And a team of scientists quickly realized that data would track precisely how the eruption brightened over time, making it ideal for further study. 

      To learn even more, the team leapt into action, viewing the supernova with a variety of telescopes in the hours and days that followed, even orchestrating a last-minute change of plans for the Hubble Space Telescope. That provided the supernova’s spectrum, an accounting of its light broken up by wavelength, at various moments after the blast.

      All that data revealed that in the last year of its life, the star had spewed some of its outer layers into space, researchers report October 26 in Monthly Notices of the Royal Astronomical Society. The amount of material ejected was about 0.23 times the mass of the sun, the team estimates. When the supernova went off, it launched a shock wave that plowed through that material shortly after the explosion, generating light picked up by the telescopes.

      As large stars get closer to death, they may start behaving erratically. Aging stars fuse heavier and heavier elements in their cores. For this star, the switch to fusing oxygen could have triggered that shedding in its last year, astrophysicist Samaporn Tinyanont of the University of California, Santa Cruz and colleagues suggest. “These stars have a roller coaster last few years of their life,” Tinyanont says.

      Scientists hope that understanding that roller coaster ride could help them recognize when other stars are about to blow. More

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      What the Perseverance rover’s quiet landing reveals about meteor strikes on Mars

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      The lander was listening. On February 18, NASA’s InSight lander on Mars turned its attention to the landing site for another mission, Perseverance, hoping to detect its arrival on the planet.

      But InSight heard nothing.

      Tungsten blocks ejected by Perseverance during entry landed hard enough to create craters on the Martian surface. Collisions like these — whether from space missions or meteor strikes — send shock waves through the ground. Yet in the first experiment of its kind on another world, InSight failed to pick up any seismic waves from the blocks’ impacts, researchers report October 28 in Nature Communications.

      As a result, researchers think that less than 3 percent of the energy from the impacts made its way into the Martian surface. The intensity of impact-generated rumblings varies from planet to planet and is “really important for understanding how the ground will change from a big impact event,” says Ben Fernando, a geophysicist at the University of Oxford.

      Perseverance left behind several craters (one indicated with the arrow) after pieces of the mission disengaged as planned during entry, creating a rare opportunity to see how Mars absorbs energy from impacts. Univ. of Arizona, JPL-Caltech/NASA

      But getting these measurements is tricky. Scientists need sensitive instruments placed relatively near an impact site. Knowing when and where a meteor will strike is nearly impossible, especially on another world.

      Enter Perseverance: a hurtling space object set to hit Mars at an exact time and place (SN: 2/17/21). To help with its entry, Perseverance dropped about 78 kilograms of tungsten as the rover landed about 3,450 kilometers from InSight. The timing and weight of the drop provided a “once-in-a-mission opportunity” to study the immediate seismic effects of an impact from space, Fernando says.

      The team had no idea whether InSight would be able to detect the blocks’ impacts or not, but the quiet arrival speaks volumes. “It lets us put an upper limit on how much energy from the tungsten blocks turned into seismic energy,” Fernando says. “We’ve never been able to get that number for Mars before.”

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      Astronomers may have spotted the first known exoplanet in another galaxy

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      Astronomers may have spotted the first known planet in another galaxy.

      The potential world, called M51-ULS-1b, orbits both a massive star and a dead star in the Whirlpool galaxy, about 28 million light-years from Earth. The object’s existence, if confirmed, suggests that there could be many other extragalactic exoplanets waiting to be discovered, astronomers report in a study to appear in Nature Astronomy.

      “We probably always assumed there would be planets” in other galaxies, says astrophysicist Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “But to actually find something, it’s a beautiful thing. It’s a humbling experience.”

      More than 4,800 planets have been discovered orbiting stars other than the sun, all of them inside the Milky Way. There’s no reason to think that other galaxies don’t also host planets. But the most popular exoplanet hunting techniques are difficult to do with such faraway stars, which blend together too much to observe them one by one.

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      In 2018, Di Stefano and astrophysicist Nia Imara of the University of California, Santa Cruz suggested searching for planets around extragalactic X-ray binaries.

      X-ray binaries usually consist of a massive star and the remains of a second massive star, which has collapsed into a neutron star or a black hole. The dead star steals material from the living star and heats it to such hot temperatures that it emits bright X-rays that stand out from the crowd of other stars.

      That X-ray region can be smaller than a giant planet, meaning if a planet crosses, or transits, in front of such a system from astronomers’ perspective on Earth, it could temporarily block all the X-rays, revealing the planet’s presence.

      Di Stefano and colleagues searched archived data from NASA’s Chandra X-ray telescope for signs of blinking X-ray sources (SN: 7/25/19). The team looked at a total of 2,624 possible transits in three galaxies: M51 (the Whirlpool galaxy), M101 (the Pinwheel galaxy) and M104 (the Sombrero galaxy).

      Only one turned up a clear planetlike signal. On September 20, 2012, an object had blocked all of the X-rays from the X-ray binary M51-ULS-1 for about three hours.

      “We said, ‘Wow. Could this be it?’” Di Stefano says.

      After ruling out gas clouds passing in front of the binary, fluctuations in the X-ray source itself or other explanations for the dip, Di Stefano and colleagues conclude that the object was most likely a Saturn-sized planet orbiting the X-ray binary hundreds of times the distance between Earth and the sun.

      This isn’t a comfortable environment for the planet. “You don’t want to be there,” Di Stefano notes. Despite its distance from the X-ray binary, the planet receives as much energy in X-rays and ultraviolet radiation as a hot Jupiter exoplanet, which orbits its star in just a few days, receives from an ordinary star (SN: 6/5/17).

      “The possibility that the team discovered the transit of an extragalactic planet is quite intriguing and would be a great discovery,” says astrophysicist Ignazio Pillitteri of the Italian National Institute for Astrophysics in Palermo. He would like to see the transit happen again to confirm it.

      Not everyone finds the result convincing. “I find the paper very speculative,” says astrophysicist Matthew Bailes of the Swinburne University of Technology in Melbourne, Australia. If the planet is real, finding it relied on a lot of coincidences: Its orbit needed to be perfectly aligned with the point of view from Earth, and it needed to just happen to be passing in front of the X-ray binary while Chandra was looking.

      Di Stefano counters that the fact that her team saw a signal within such a small number of observations suggests there are lots of extragalactic planets out there. “Maybe we were lucky,” she acknowledges. “But I think it’s very likely that we were not special. We looked and we found something because there was something to find.”

      Di Stefano doesn’t expect to see this particular planet again in her lifetime. It could take decades for it to pass in front of its host stars again. “The real test,” she says, “is finding more planets.” More

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      5 cool things to know about NASA’s Lucy mission to the Trojan asteroids

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      For the first time, a spacecraft is headed to Jupiter’s odd Trojan asteroids. What Lucy finds there could provide a fresh peek into the history of the solar system.

      “Lucy will profoundly change our understanding of planetary evolution in our solar system,” Adriana Ocampo, a planetary scientist at NASA Headquarters in Washington, D.C., said at a news briefing October 14.

      The mission is set to launch from the Kennedy Space Center at Cape Canaveral, Fla., as early as October 16. Live coverage will air on NASA TV beginning at 5 a.m. EDT, in anticipation of a 5:34 a.m. blast off.

      The Trojan asteroids are two groups of space rocks that are gravitationally trapped in the same orbit as Jupiter around the sun. One group of Trojans orbits ahead of Jupiter; the other follows the gas giant around the sun. Planetary scientists think the Trojans could have formed at different distances from the sun before getting mixed together in their current homes. The asteroids could also be some of the oldest and most pristine objects in the solar system.

      The mission will mark several other firsts, from the types of objects it will visit to the way it powers its instruments. Here are five cool things to know about our first visit to the Trojans.

      1. The Trojan asteroids are a solar system time capsule.

      The Trojans occupy spots known as Lagrangian points, where the gravity from the sun and from Jupiter effectively cancel each other out. That means their orbits are stable for billions of years.

      “They were probably placed in their orbits by the final gasp of the planet formation process,” the mission’s principal investigator Hal Levison, a planetary scientist at Southwest Research Institute in Boulder, Colo.,  said September 28 in a news briefing.

      But that doesn’t mean the asteroids are all alike. Scientists can tell from Earth that some Trojans are gray and some are red, indicating that they might have formed in different places before settling in their current orbits. Maybe the gray ones formed closer to the sun, and the red ones formed farther from the sun, Levison speculated.

      Studying the Trojans’ similarities and differences can help planetary scientists tease out whether and when the giant planets moved around before settling into their present positions (SN: 4/20/12). “This is telling us something really fundamental about the formation of the solar system,” Levison said.

      2. The spacecraft will visit more individual objects than any other single spacecraft. 

      Lucy will visit eight asteroids, including their moons. Over its 12-year mission, it will visit one asteroid in the main asteroid belt between Mars and Jupiter, and seven Trojans, two of which are binary systems where a pair of asteroids orbit each other.

      “We are going to be visiting the most asteroids ever with one mission,” planetary scientist Cathy Olkin, Lucy’s deputy principal investigator, said in the Oct. 14 briefing.

      The spacecraft will observe the asteroids’ composition, shape, gravity and geology for clues to where they formed and how they got to the Lagrangian points.

      The spacecraft’s first destination, in April 2025, will be an asteroid in the main belt. Next, it will visit five asteroids in the group of Trojans that orbit the sun ahead of Jupiter: Eurybates and its satellite Queta in August 2027; Polymele in September 2027; Leucus in April 2028; and Orus in November 2028. Finally, the spacecraft will shift to Jupiter’s other side and visit the twin asteroids Patroclus and Menoetius in the trailing group of space rocks in March 2033.

      The spacecraft won’t land on any of its targets, but it will swoop within 965 kilometers of their surfaces at speeds of 3 to 5 meters per second relative to the asteroids’ speed through space.

      There’s no need to worry about collisions while zipping through these asteroid clusters, Levison said. Although there are about 7,000 known Trojans, they’re very far apart. “If you were standing on any one of our targets, you wouldn’t be able to tell you were part of the swarm,” he said.

      The Trojan asteroids trail and follow Jupiter in its orbit around the sun, but they’re actually quite far from the giant planet. In fact, Earth is closer to Jupiter than either swarm of Trojans is.NASA, adapted by T. TibbittsThe Trojan asteroids trail and follow Jupiter in its orbit around the sun, but they’re actually quite far from the giant planet. In fact, Earth is closer to Jupiter than either swarm of Trojans is.NASA, adapted by T. Tibbitts

      3. Lucy will have a weird flight path.

      In order to make so many stops, Lucy will need to take a complex path. First, the spacecraft will swoop past Earth twice to get a gravitational boost from our planet that will help propel it onward to its first asteroid.

      The closest Earth flyby, in October 2022, will take it within 300 kilometers of the planet’s surface, closer than the International Space Station, the Hubble Space Telescope and many satellites, Olkin said. Observers on Earth might even be able to see it. “I’m hoping to go near where it flies past and look up and see Lucy flying by a year from now,” she said.

      Then in December 2030, after more than a year exploring the “leading” swarm of Trojans, Lucy will come back to the vicinity of Earth for one more boost. That final gravitational slingshot will send the spacecraft to the other side of the sun to visit the “trailing” swarm. This will make Lucy the first spacecraft ever to venture to the outer solar system and come back near Earth again.

      4. Lucy will travel farther from the sun than any other solar-powered craft.

      Another record Lucy will break has to do with its power source: the sun. Lucy will run on solar power out to 850 million kilometers away from the sun, making it the farthest-flung solar powered spacecraft ever.

      To accomplish that, Lucy has a pair of enormous solar arrays. Each 10-sided array is more than 7.3 meters across and includes about 4,000 solar cells per panel, Lucy project manager Donya Douglas-Bradshaw said in a news briefing on October 13. Standing on one end, Lucy and its solar panels would be as tall as a five-story building.

      “It’s a very intricate, sophisticated design,” she said. The advantage of using solar power is that the team can adjust how much power the spacecraft needs based on how far from the sun it is.

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      5. The inspiration for Lucy’s name is decidedly earthbound.

      NASA missions are often named for famous scientists, or with acronyms that describe what the mission will do. Lucy, on the other hand, is named after a fossil.

      The idea that the Trojans hold secrets to the history of the solar system is part of how the mission got its unusual name. To understand, go back to 1974, when paleoanthropologist Donald Johanson and a graduate student discovered a fossil of a human ancestor who had lived 3.2 million years ago. After listening to the Beatles song “Lucy in the Sky with Diamonds” at camp that night, Johanson’s team named the fossil hominid “Lucy.” (In a poetic echo, the first asteroid the Lucy spacecraft will visit is named Donaldjohanson.)

      Planetary scientists hope the study of the Trojans will revolutionize our understanding of the solar system’s history in the same way that studying Lucy’s fossil revolutionized our understanding of human history.

      “We think these asteroids are fossils of solar system formation,” Levison said. So his team named the spacecraft after the fossil. 

      The spacecraft even carries a diamond in one of its instruments, to help split beams of light. Said planetary scientist Phil Christensen of Arizona State University in Tempe at the Oct. 14 briefing: “We truly are sending a diamond into the sky with Lucy.” More

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      Earth is reflecting less light. It’s not clear if that’s a trend

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      The amount of sunlight that Earth reflects back into space — measured by the dim glow seen on the dark portions of a crescent moon’s face — has decreased measurably in recent years. Whether the decline in earthshine is a short-term blip or yet another ominous sign for Earth’s climate is up in the air, scientists suggest.

      Our planet, on average, typically reflects about 30 percent of the sunlight that shines on it. But a new analysis bolsters previous studies suggesting that Earth’s reflectance has been declining in recent years, says Philip Goode, an astrophysicist at Big Bear Solar Observatory in California. From 1998 to 2017, Earth’s reflectance declined about 0.5 percent, the team reported in the Sept. 8 Geophysical Research Letters.

      Using ground-based instruments at Big Bear, Goode and his colleagues measured earthshine — the light that reflects off our planet, to the moon and then back to Earth — from 1998 to 2017. Because earthshine is most easily gauged when the moon is a slim crescent and the weather is clear, the team collected a mere 801 data points during those 20 years, Goode and his colleagues report.

      Much of the decrease in reflectance occurred during the last three years of the two-decade period the team studied, Goode says. Previous analyses of satellite data, he and his colleagues note, hint that the drop in reflectance stems from warmer temperatures along the Pacific coasts of North and South America, which in turn reduced low-altitude cloud cover and exposed the underlying, much darker and less reflective seas.

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      “Whether or not this is a long-term trend [in Earth’s reflectance] is yet to be seen,” says Edward Schwieterman, a planetary scientist at University of California, Riverside, who was not involved in the new analysis. “This strengthens the argument for collecting more data,” he says.

      Decreased cloudiness over the eastern Pacific isn’t the only thing trimming Earth’s reflectance, or albedo, says Shiv Priyam Raghuraman, an atmospheric scientist at Princeton University. Many studies point to a long-term decline in sea ice (especially in the Arctic), ice on land, and tiny pollutants called aerosols — all of which scatter sunlight back into space to cool Earth.

      With ice cover declining, Earth is absorbing more radiation. The extra radiation absorbed by Earth in recent decades goes toward warming the oceans and melting more ice, which can contribute to even more warming via a vicious feedback loop, says Schwieterman.

      Altogether, Goode and his colleagues estimate, the decline in Earth’s reflectance from 1998 to 2017 means that each square meter of our planet’s surface is absorbing, on average, an extra 0.5 watts of energy. For comparison, the researchers note in their study, planet-warming greenhouse gases and other human activity over the same period boosted energy input to Earth’s surface by an estimated 0.6 watts of energy per square meter. That means the decline in Earth’s reflectance has, over that 20-year period, almost doubled the warming effect our planet experienced. More

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      A Jupiter-like planet orbiting a white dwarf hints at our solar system’s future

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      A glimpse of our solar system’s future has appeared thousands of light-years away in the constellation Sagittarius. There a giant planet like Jupiter orbits a white dwarf, a dim, dense star that once resembled the sun.

      In 2010, that star passed in front of a much more distant star. Like a magnifying glass, the white dwarf’s gravity bent the more distant star’s light rays so that they converged on Earth and made the distant star look hundreds of times brighter. A giant planet orbiting the white dwarf star also “microlensed” the distant star’s light, revealing the planet’s presence.

      In 2015, 2016 and again in 2018 astrophysicist Joshua Blackman of the University of Tasmania in Hobart, Australia and colleagues pointed the Keck II telescope in Hawaii at the far-off system, which lies some 5,000 to 8,000 light-years from Earth. The team was in search of the giant planet’s star, but saw, well, nothing.

      “We expected that we’d see a star similar to the sun,” Blackman says. “And so we spent quite a few years trying to figure out why on Earth we didn’t see the star which we expected to see.”

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      After failing to detect any light from the spot where the planet’s star should be, Blackman’s team concluded that the object can’t be a typical star like the sun — also known as a main sequence star, which generates energy by converting hydrogen into helium at its center. Instead, the star must be something much fainter. The microlensing data indicate that the star is roughly half as massive as the sun, so the object isn’t massive enough to be a neutron star or black hole. But a white dwarf star fits the bill perfectly, the researchers report online October 13 in Nature.

      “They’ve carefully ruled out the other possible lens stars — neutron stars and black holes and main sequence stars and whatnot,” says Ben Zuckerman, an astronomer at UCLA, who was not involved with the work. He notes that only a handful of planets have ever been found orbiting white dwarfs.

      The new planet is the first ever discovered that is orbiting a white dwarf and resembles Jupiter in both its mass and its distance from its star. Blackman’s team estimates that the planet is one to two times as massive as Jupiter and probably lies 2.5 to six times farther from the white dwarf star than Earth does from the sun. For comparison, Jupiter is 5.2 times farther out from the sun than Earth is. The white dwarf is somewhat larger than Earth, which means the planet is much bigger than its host star.

      The white dwarf formed after a sunlike star expanded and became a red giant star. Then the red giant ejected its outer layers, exposing its hot core. That former core is the white dwarf star.

      Our sun will turn into a white dwarf about 7.8 billion years from now, so the new discovery is “a snapshot into the future of our solar system,” Blackman says. As the sun becomes a red giant, it will engulf and destroy its innermost planet, Mercury, and perhaps Venus too. But Mars, Jupiter and more distant planets should survive.

      And Earth? No one yet knows what will happen to it. More

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      The fastest-spinning white dwarf ever seen rotates once every 25 seconds

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      The sun turns once a month and the Earth once a day, but a white dwarf star 2,000 light-years away spins every 25 seconds, beating the old champ by five seconds. That makes it the fastest-spinning star of any sort ever seen — unless you consider such exotic objects as neutron stars and black holes, some of which spin even faster, to be stars (SN: 3/13/07).  

      About as small as Earth but roughly as massive as the sun, a white dwarf is extremely dense. The star’s surface gravity is so great that if you dropped a pebble from a height of a few feet, it would smash into the surface at thousands of miles per hour. The typical white dwarf takes hours or days to spin.

      The fast-spinning white dwarf, named LAMOST J0240+1952 and located in the constellation Aries, got in a whirl because of its ongoing affair with a red dwarf star that revolves around it. Just as falling water makes a waterwheel turn, so gas falling from the red companion star made the white dwarf twirl.

      The discovery occurred the night of August 7, when astronomer Ingrid Pelisoli of the University of Warwick in Coventry, England, and her colleagues detected a periodic blip of light from the dim duo. The blip repeated every 24.93 seconds, revealing the white dwarf star’s record-breaking rotation period, the researchers report August 26 at arXiv.org.

      The star’s only known rival is an even faster-spinning object in orbit with the blue star HD 49798. But that rapid rotator’s nature is unclear, with some recent studies saying it is likely a neutron star, not a white dwarf. More