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      December’s stunning Geminid meteor shower is born from a humble asteroid

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      On Sunday night, December 13, countless meteors will shoot across the sky as space particles burn up in our atmosphere and meet a fiery end. Most meteor showers occur when Earth slams into debris left behind by a comet.
      But not this meteor shower, which is likely to be the most spectacular of the year. Known as the Geminid shower, it strikes every December and arises not from a flamboyant comet but from an ordinary asteroid — the first, but not the last, linked to a meteor shower.
      Although both comets and asteroids are small objects orbiting the sun, icy comets sprout beautiful tails when their ice vaporizes in the heat of the sun. In contrast, asteroids have earned the name “vermin of the skies” for streaking through and ruining photographs of celestial vistas by reflecting the sun’s light.
      So how can a mere asteroid outdo all of the glamorous comets and spawn a meteor shower that surpasses its rivals? “It remains a mystery,” says David Jewitt, an astronomer at UCLA. It’s akin to an ugly duckling’s offspring usurping the beautiful swan’s to win first place in a beauty contest.
      Astronomers still don’t know the secret to the asteroid’s success in creating a shower that at its peak normally produces more meteors per hour than any other shower of the year. Three years ago, however, the asteroid swung extra close to Earth and gave scientists their best chance to study the humble space rock. They now look forward to the launch of a spacecraft that will image the asteroid’s surface.

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      Cosmic connections
      Astronomers first linked a meteor shower to a comet in 1866. They connected the well-known Perseid meteors, visible to most of the world every August, with a comet named Swift-Tuttle that had passed Earth four years earlier. Astronomers later matched most major meteor showers with one comet or another.
      When a comet’s ice vaporizes in sunlight, dust grains also fly off the comet. These dust particles, called meteoroids, sprinkle along the comet’s orbit like a dandelion gone to seed. If Earth plows into this long dust stream, we see a fiery shower as the particles hit our atmosphere. The typical meteoroid is no larger than a grain of sand, but it travels so fast that it energizes electrons both in its own atoms as it disintegrates and in atmospheric atoms and molecules. As these electrons lose energy, they emit the streak of light — the meteor — that looks as though a star has fallen from the sky.
      Still, as comet after comet was linked to different meteor showers, the Geminids remained apart; no one knew their source.
      The Geminid meteors stood out in other ways, too. Unlike the Perseid meteors, which people have been observing for nearly 2,000 years, the Geminids are relatively new. First reports of their existence came from England and the United States in 1862. The shower in those days was weak, producing at most only one or two dozen meteors an hour. During the 20th century, however, the shower strengthened. Nowadays, at the shower’s peak, a single observer under a dark sky can see more than 100 meteors an hour. That’s better than most Perseid performances.
      On top of that, the Geminid meteoroid stream, the ribbon of dust that traces the asteroid’s orbit around the sun, is newer than many other streams. Over time, streams spread out, but this one is so narrow it must have formed less than 2,000 years ago and maybe only a few hundred years ago. And based on how little the meteoroids slow down when they hit the air, astronomers deduced that Geminid meteoroids are fairly dense, about three times as dense as water and twice as dense as the Perseid meteoroids.

      In 1983, astronomers finally found the Geminids’ parent. Jewitt, then a graduate student at Caltech, remembers walking home one January evening when he happened to see a rocket lift off from a military base. “I assumed it was an ICBM or something that the Air Force was launching to test,” he says. Instead, it was a heat-seeking spacecraft named the Infrared Astronomical Satellite.
      In October of that year, the satellite discovered a small asteroid. To Harvard astronomer Fred Whipple, best known for his “dirty snowball” model of comets (SN: 3/14/92, p. 170), that small object stood out. It followed the same path around the sun as the particles in the Geminid meteoroid stream. Half a century earlier, Whipple himself had determined the orbit of the meteoroids by photographing the paths of the meteors against the sky. The newfound asteroid, Whipple declared, must be their long-sought source. The find also explained why the meteoroids were so dense: They come from a space rock rather than an icy comet.
      The asteroid revolves around the sun every 1.43 years and comes very close to the sun, cutting well inside the orbit of Mercury, the innermost planet. Astronomers therefore christened the asteroid Phaethon, a son of Helios the sun god in Greek mythology. At its farthest, Phaethon ventures beyond the orbit of Mars and reaches the asteroid belt, home of the largest space rocks, between the paths of Mars and Jupiter.
      For a quarter century after Phaethon’s discovery, though, no one saw it shedding any dust particles or pebbles that could account for the many meteors that make up December’s show. Because of the sun’s glare, astronomers couldn’t observe Phaethon when it was closest to the sun. Observing during a close pass might be especially interesting because calculations indicated that the intense sunlight caused Phaethon’s surface temperature to soar to roughly 1,000 kelvins (1,340° Fahrenheit), hotter than any planet in the solar system. The torrid temperature might cause the asteroid to shoot particles into space.
      A lucky break came about because Jewitt married an astrophysicist who studies the sun. “Really, the key was talking to my wife about this,” he says. Jing Li, also at UCLA, and Jewitt realized that a solar spacecraft might be able to pick up details about the asteroid when it’s nearest the sun and thus offer clues to why the space rock is such a fertile meteor-maker.
      Sure enough, in 2009 and again in 2012, images taken by a NASA solar spacecraft named STEREO A caught Phaethon brightening when near the sun, which suggested the asteroid was throwing off dust particles. Then, in 2013, Jewitt and Li noticed a short dust tail in that data. The tail lasted only two days. “It’s really, really faint in basically the world’s crappiest data,” Jewitt says. The bright background sky makes the tail hard to see.
      The researchers attribute Phaethon’s dust production to the extreme heat, which breaks rocks on the asteroid’s surface and sends particles aloft. Phaethon has so little gravity that those particles can escape into space. Additional dust may result from desiccation, Jewitt says: In the presence of such heat, hydrated minerals on the asteroid may dry out and crack, the way empty lake beds do on Earth, releasing more particles.
      As seen from California’s Mojave Desert in 2009, a Geminid meteor streaks past Orion’s Belt. Walter Pacholka, Astropics/Science Source
      Phaethon’s fast spin causes further stress. The asteroid makes a full turn every three hours and 36 minutes. Such rapid rotation is typical of small asteroids, and it means the surface freezes and then fries over a short period of time. The spin also creates a centrifugal force that might help lift particles into space.
      Yet these findings don’t solve the mystery of how a modest asteroid produces such a stunning meteor shower, Jewitt says. For one thing, as he and colleagues noted in 2013 in the Astrophysical Journal Letters, the particles in Phaethon’s temporary tail are much too small.
      Most of the Geminid meteors we see come from particles roughly a millimeter across. But the particles in the tail are even tinier, spanning only about one one-thousandth of a millimeter. Jewitt and Li deduced the small size because sunlight exerts radiation pressure, which is weak, that pushes the tail straight back away from the sun; if the particles were larger, they would resist the weak pressure and the tail would be curved.
      Plus, Phaethon’s close passages to the sun don’t eject nearly enough particles to populate the Geminid stream. This suggests that some catastrophe hit the asteroid in the recent past and made so many meteoroids that they continue to delight meteor observers today.
      In 2014, astronomer Richard Arendt of the University of Maryland, Baltimore County reported the first direct sighting of the Geminid meteoroid stream itself. He had reanalyzed old data from a spacecraft whose chief mission had nothing to do with the solar system: the Cosmic Background Explorer, which NASA had launched a quarter century earlier to study the Big Bang’s afterglow and probe the universe’s birth.
      “They didn’t really have the tools to look at the data in the right way back then,” Arendt says. With modern computers, he made movies of the data and glimpsed glowing strands of dust threading the solar system that emit infrared light as the sun heats them. He used this approach to view the never-before-seen dust trail along the orbit of Halley’s comet, as well as Phaethon’s dust trail: the Geminid meteoroid stream, which looked like a narrow filament along Phaethon’s orbit. Arendt published his work in the Astronomical Journal.
      More recently, NASA’s Parker Solar Probe also detected the stream (SN: 1/18/20, p. 6). “This is the first time it’s been seen in visible light,” says Karl Battams, an astrophysicist at the U.S. Naval Research Laboratory in Washington, D.C. Sunlight hits the dust, reflecting the light to the probe. The observations put the stream’s mass at roughly 1 percent that of Phaethon itself. This is much more material than the asteroid produces when closest to the sun, which Battams says again favors the idea that the bulk of the Geminid meteoroid stream owes its existence to some past catastrophe.
      Phaethon visits Earth
      In December 2017, the asteroid helped astronomers by flying only 10 million kilometers from Earth, the closest the rock will come until 2093. “This was a great opportunity to look at Phaethon,” says Patrick Taylor, an astronomer then at Arecibo Observatory in Puerto Rico.
      Hurricane Maria had devastated the island and damaged the radio telescope just three months earlier, yet the observations succeeded. “That was the result of a tremendous amount of effort by the observatory staff, the community and the local government,” Taylor says. The telescope was repaired, and commercial power was restored to the observatory by clearing roads and replacing downed poles and cables to the site. “Everyone was aware how important this observation was going to be,” he says.
      Over a period of five days, his team bounced radar signals off the asteroid, watching different features come into view as the rock rotated. As published in 2019 in Planetary and Space Science, the observations indicate that Phaethon’s equatorial diameter is about 6.25 kilometers, which means the asteroid is a bit more than half the size of the one that hit Earth and did in the dinosaurs (SN: 2/15/20, p. 7). The images show what may be craters, one more than a kilometer across, on Phaethon’s surface. There’s also a possible boulder 300 meters wide.
      The radar images suggest Phaethon isn’t perfectly round. Instead, it may resemble a spinning top, like Bennu and Ryugu, two even smaller asteroids that spacecraft have recently visited. Both of those asteroids have equatorial diameters larger than their polar diameters. More than a thousand Bennus could fit inside Phaethon, but the two asteroids have similar shapes, Taylor notes. He thinks Phaethon may owe its shape to its rapid spin.

      Jewitt also tried to take advantage of Phaethon’s close visit. “It was a bit of a letdown,” he says, laughing. “We saw absolutely nothing at all.” Neither the Hubble Space Telescope nor the Very Large Telescope in Chile discerned any dust or rocks coming off the asteroid.
      But the future should hold much better views. In 2024, Japan will launch the DESTINY+ spacecraft, which will fly past Phaethon several years later. Japan has already sent spacecraft to two other small asteroids, and the new mission promises sharp images that should reveal Phaethon’s shape, structure, geologic features and dust trail. The spacecraft may even see the asteroid emit particles in real time, as NASA’s OSIRIS-REx mission did for Bennu (SN: 4/13/19, p. 10).
      The DESTINY+ spacecraft will search for signs of a recent catastrophe that could have excavated enough material to create the Geminid meteoroid stream. The most obvious possibility — an impact with another asteroid — is also the least likely, Jewitt says, because Phaethon is a small target and the impact would have had to occur less than 2,000 years ago. Nevertheless, if such an impact did happen, it surely carved a fresh scar, which a spacecraft might pick up.
      Perhaps some other catastrophe made the meteoroids. Maybe the asteroid was once a larger object that broke apart, because sunlight stressed it or it spun too fast. In fact, one or two other asteroids, smaller than Phaethon, follow similar paths around the sun and could be remnants of a super-Phaethon. After DESTINY+ zips by Phaethon, it may visit one of these other asteroids to investigate.
      There’s another question the spacecraft might address: The Geminids come from Phaethon, all right, but where did Phaethon come from? It wasn’t born where it is, because it crosses the paths of four planets. Within just a few tens of millions of years, it will either crash into one of them or else their gravity will hurl the rock into the sun or far away from it.
      Some astronomers have proposed that Phaethon is really a chunk kicked off of the large asteroid Pallas, a resident of the asteroid belt. “Could Phaethon be a piece of Pallas? Yes,” Jewitt says. “Is it likely to be a piece of Pallas? I’m not really sure about that.” The two asteroids resemble each other in composition, but there are also differences. Those distinctions may merely mean that strong sunlight has altered Phaethon’s surface. Or they may indicate the two asteroids have nothing to do with each other.
      Whatever the case, this month’s show should be especially good because moonlight won’t interfere. Any astronomers watching may make a wish on the falling stars for greater insight into how those meteors and their unlikely parent came to be. More

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      China is about to collect the first moon rocks since the 1970s

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      For the first time in almost half a century, scientists are going to get their hands on new moon rocks.
      The Chinese space agency’s Chang’e-5 spacecraft, which landed on the moon around 10:15 a.m. EST December 1, will scoop up lunar soil from a never-before-visited region and bring it back to Earth a few weeks later. Those samples could provide details about an era of lunar history not touched upon by previous moon missions.
      “We’ve been talking since the Apollo era about going back and collecting more samples from a different region,” says planetary scientist Jessica Barnes of the University of Arizona in Tucson, who works with lunar samples from the American and Soviet Union missions of the 1960s and 1970s. “It’s finally happening.”
      Chang’e-5, the latest in a series of missions named for the Chinese moon goddess (SN: 11/11/18), took off from the China National Space Administration’s launch site in the South China Sea on November 23 and landed in volcanic flatlands on the northwest region of the moon’s nearside.

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      The lander, equipped with a scoop and a drill, will collect about two kilograms of soil and small rocks, possibly from as deep as two meters below the moon’s surface, says planetary scientist Long Xiao of China University of Geosciences in Wuhan.
      The spacecraft has to work fast. With no internal heating mechanism, it has no defenses against the extremely cold lunar night, which can reach –170° Celsius. The entire mission has to fit within one lunar day, about 14 Earth days.
      After the lander collects the sample, a small rocket will bring the lander and the sample back to the orbiter perhaps as early as December 3, although the Chinese space agency has not released the official schedule.
      Once in orbit, the moon material will be packaged into a return capsule and sent back to Earth. The capsule is expected to land in the Inner Mongolia region by December 17.
      The last time new lunar samples were sent back to Earth was 1976, with the end of the Soviet Union’s Luna program. Between those missions and NASA’s Apollo missions, scientists on Earth have about 380 kilograms of moon material to study (SN: 7/15/19). “Perhaps for a long time people thought, been there, done that, when it comes to the moon,” Barnes says.
      Two kilograms of new stuff might not sound like much next to what’s already in hand. But Chang’e-5 is returning samples from an entirely unexplored region. The landing site is in the Mons Rümker region in the northwest region of the nearside of the moon. Like the Apollo and Luna landing sites, Rümker is flat. “The engineering consideration is first, to be safe,” Xiao says.

      All the Apollo and Luna missions visited ancient volcanic plains, where the rocks are between 3 billion and 4 billion years old. Rümker’s volcanic rocks are much younger, around 1.3 billion to 1.4 billion years old. In the ‘60s, scientists didn’t think the moon was still volcanically active that late. More recent studies from lunar orbit and from telescopes have suggested a more complicated volcanic past.
      “With these new samples, we potentially add another pinpoint in our geologic history of the moon,” says Barnes. “We’ll get an idea of, what was the volcanic history like on the moon a billion years ago? That’s something we don’t have access to in the returned samples we already have.”
      The Rümker region is also rich in potassium, rare-Earth elements and phosphorous, often called KREEP elements. Those elements were some of the last to crystalize out of the magma ocean that covered the young moon and can help reveal details of how that process happened. It’s an “exotic flavor” of material, says Barnes. “It’s a really different area, geochemically, to the rest of the moon.”
      One of the biggest challenges for the mission will be drilling that material. The drill can’t change direction once it’s deployed, so it has to attempt to drill through anything directly below it. If the drill hits a large rock, it could fail. So the Chang’e-5 team is hoping for fine, loose soil, Long says.
      Once the sample is back on Earth, it will be stored and cataloged at a curation center in Beijing. Then it will be distributed for scientists to do research.
      “You can’t breathe easy on these types of missions until the samples are back and are safe in the curation place where they’re going to be held,” Barnes says.
      The Chinese space agency plans to share samples with international scientists. A 2011 congressional rule makes it difficult for U.S. scientists to collaborate directly with China, so it’s unclear who will get to work with the rocks. But the discoveries that the new samples will enable go beyond international borders.
      “It doesn’t matter who’s doing it,” says Barnes. “The whole world should be behind this mission and this endeavor. It’s a piece of history.” More

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      Astronomers spotted colliding neutron stars that may have formed a magnetar

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      A surprisingly bright cosmic blast might have marked the birth of a magnetar. If so, it would be the first time that astronomers have witnessed the formation of this kind of rapidly spinning, extremely magnetized stellar corpse.
      That dazzling flash of light was made when two neutron stars collided and merged into one massive object, astronomers report in an upcoming issue of the Astrophysical Journal. Though the especially bright light could mean that a magnetar was produced, other explanations are possible, the researchers say.
      Astrophysicist Wen-fai Fong of Northwestern University in Evanston, Ill., and colleagues first spotted the site of the neutron star crash as a burst of gamma-ray light detected with NASA’s orbiting Neil Gehrels Swift Observatory on May 22. Follow-up observations in X-ray, visible and infrared wavelengths of light showed that the gamma rays were accompanied by a characteristic glow called a kilonova.
      Kilonovas are thought to form after two neutron stars, the ultradense cores of dead stars, collide and merge. The merger sprays neutron-rich material “not seen anywhere else in the universe” around the collision site, Fong says. That material quickly produces unstable heavy elements, and those elements soon decay, heating the neutron cloud and making it glow in optical and infrared light (SN: 10/23/19).
      [embedded content]
      A new study finds that two neutron stars collided and merged, producing an especially bright flash of light and possibly creating a kind of rapidly spinning, extremely magnetized stellar corpse called a magnetar (shown in this animation). 
      Astronomers think that kilonovas form every time a pair of neutron stars merge. But mergers produce other, brighter light as well, which can swamp the kilonova signal. As a result, astronomers have seen only one definitive kilonova before, in August 2017, though there are other potential candidates (SN: 10/16/17).
      The glow that Fong’s team saw, however, put the 2017 kilonova to shame. “It’s potentially the most luminous kilonova that we’ve ever seen,” she says. “It basically breaks our understanding of the luminosities and brightnesses that kilonovae are supposed to have.”
      The biggest difference in brightness was in infrared light, measured by the Hubble Space Telescope about 3 and 16 days after the gamma-ray burst. That light was 10 times as bright as infrared light seen in previous neutron star mergers.
      “That was the real eye-opening moment, and that’s when we scrambled to find an explanation,” Fong says. “We had to come up with an extra source [of energy] that was boosting that kilonova.”
      Her favorite explanation is that the crash produced a magnetar, which is a type of neutron star. Normally, when neutron stars merge, the mega-neutron star that they produce is too heavy to survive. Almost immediately, the star succumbs to intense gravitational forces and produces a black hole.
      But if the supermassive neutron star is spinning rapidly and is highly magnetically charged (in other words, is a magnetar), it could save itself from collapsing. Both the support of its own rotation and dumping energy, and thus some mass, into the surrounding neutron-rich cloud could keep the star from turning into a black hole, the researchers suggest. That extra energy in turn would make the cloud give off more light — the extra infrared glow that Hubble spotted.

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      But there are other possible explanations for the extra bright light, Fong says. If the colliding neutron stars produced a black hole, that black hole could have launched a jet of charged plasma moving at nearly the speed of light (SN: 2/22/19). The details of how the jet interacts with the neutron-rich material surrounding the collision site could also explain the extra kilonova glow, she says.
      If a magnetar was produced, “that could tell us something about the stability of neutron stars and how massive they can get,” Fong says. “We don’t know the maximum mass of neutron stars, but we do know that in most cases they would collapse into a black hole [after a merger]. If a neutron star did survive, it tells us about under what conditions a neutron star can exist.”
      Finding a baby magnetar would be exciting, says astrophysicist Om Sharan Salafia of Italy’s National Institute for Astrophysics in Merate, who was not involved in the new research. “A newborn highly magnetized, highly rotating neutron star that forms from the merger of two neutron stars has never been observed before,” he says.
      But he agrees that it’s too soon to rule out other explanations. What’s more, recent computer simulations suggest that it might be difficult to see a newborn magnetar even if it formed, he says. “I wouldn’t say this is settled.”
      Observing how the object’s light behaves over the next four months to six years, Fong and her colleagues have calculated, will prove whether or not a magnetar was born.
      Fong herself plans to keep following up on the mysterious object with existing and future observatories for a long time. “I’ll be tracking this till I’m old and grey, probably,” she says. “I’ll train my students to do it, and their students.” More

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      Runaway stars may create the mysterious ultraviolet glow around some galaxies

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      Hot blue stars kicked out of their cradles may explain a mysterious ultraviolet glow that surrounds the disks of many spiral galaxies.
      A new computer simulation demonstrates that these runaway stars can populate the vast expanses beyond a galaxy’s visible disk (SN: 3/23/20). These distant regions have gas that is too warm and tenuous to make new stars, yet young stars nevertheless exist there.
      “It’s a big problem for classical star formation theory,” says Eric Andersson, an astrophysicist at Lund Observatory in Sweden.
      The mystery of the far-flung young stars has persisted for some time. In 2003, NASA launched the Galaxy Evolution Explorer space telescope, which surprised astronomers by discovering diffuse far-ultraviolet light in the hinterlands of nearby spiral and irregular galaxies (SN: 2/15/05). Unlike ordinary ultraviolet radiation, far-ultraviolet light has such a short wavelength that most of it doesn’t penetrate the Earth’s atmosphere.

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      Stars that emit profuse amounts of this energetic radiation are hot, blue and usually much more massive than the sun. These stars don’t live long, so they must have formed recently. But the gas on the galactic outskirts isn’t cold and dense enough to collapse and create new stars.
      Andersson and his colleagues propose a solution to the paradox: Many of these far-out far-ultraviolet-emitting stars weren’t born where they are now. Instead, they arose closer to the galaxy’s center and ran away from their homes.
      The researchers conducted a computer simulation to model the motion of massive stars in a spiral galaxy. Some of the runaway stars in the simulation dart across thousands of light-years of space to take up residence beyond the visible edge of the galaxy’s disk, thereby explaining the far-ultraviolet light there, the researchers report online at arXiv.org on October 22.
      The Milky Way has many of these runaway stars. A star can become a runaway when other massive stars fling it away through their gravity. Or, if the star orbits close to a massive star that explodes, the surviving star races away at the same speed it had been dashing around its companion. Most runaway stars are hot and blue, radiating just the type of far-ultraviolet light seen beyond the visible edges of galactic disks.
      Mark Krumholz, an astronomer at the Australian National University in Canberra, calls the idea “a plausible explanation.” He also offers a way to test it: by exploiting the properties of different types of massive stars.
      The rarest and most massive blue stars are so hot they ionize hydrogen gas, causing it to emit red light as electrons settle back into position around protons. But these very massive stars don’t live long, so any that reside on a galaxy’s outskirts must have been born there. After all, the stars didn’t have time to travel from elsewhere in the galaxy during their brief lives.
      In contrast, less massive blue stars live longer and therefore could have reached the galactic periphery from elsewhere during their lifetimes. If the ratio of far-ultraviolet light to red light from ionized gas is much greater beyond the galaxy’s visible edge than in its disk, Krumholz says, that would suggest much of the far-ultraviolet glow in the exurbs does indeed come from runaway stars. More

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      Betelgeuse went dark, but didn’t go supernova. What happened?

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      Astrophysicist Miguel Montargès has a clear memory of the moment the stars became real places to him. He was 7 or 8 years old, looking up from the garden of his parents’ apartment in the south of France. A huge, red star winked in the night. The young space fan connected the star to a map he had studied in an astronomy magazine and realized he knew its name: Betelgeuse.
      Something shifted for him. That star was no longer an anonymous speck floating in a vast uncharted sea. It was a destination, with a name.
      “I thought, wow, for the first time … I can name a star,” he says. The realization was life-changing.
      Since then, Montargès, now at the Paris Observatory, has written his Ph.D. thesis and about a dozen papers about Betelgeuse. He considers the star an old friend, observing it many times a year, for work and for fun. He says good-bye every May when the star slips behind the sun from the perspective of Earth, and says hello again in August when the star comes back.

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      So in late 2019, when the bright star suddenly dimmed for no apparent reason, Montargès was a little alarmed. Some people speculated that Betelgeuse was about to explode in a brilliant supernova that would outshine the full moon. Astronomers know the star is old and its days are numbered, but Montargès wasn’t ready to see it go.
      “It’s my favorite star,” he says. “I don’t want it to die.”
      Other researchers, though, were eager to watch Betelgeuse explode in real time. Supernovas mark the violent deaths of stars that are at least eight times as massive as the sun (SN: 11/7/20, p. 20). But astronomers still don’t know what would signal that one is about to blow. The outbursts sprinkle interstellar space with elements that ultimately form the bulk of planets and people — carbon, oxygen, iron (SN: 2/18/17, p. 24). So the question of how supernovas occur is a question of our own origins.

      But the explosions are rare — astronomers estimate that one occurs in our galaxy just a few times a century. The last one spotted nearby, SN 1987A, was more than 33 years ago in a neighboring galaxy (SN: 2/18/17, p. 20). Betelgeuse is just one of the many aging, massive stars — called red supergiants — that could go supernova at any moment. But as one of the closest and brightest, Betelgeuse is the one that space enthusiasts know best.
      So when the star started acting strangely at the end of last year, Montargès and a small band of Betelgeuse diehards aimed every telescope they could at the dimming giant. Over the following months, the star returned to its usual brightness, and the excitement over an imminent supernova faded. But the flurry of data collected in the rush to figure out what was happening might help answer a different long-standing question: How do massive, old stars send their planet-building star stuff into the cosmos even before they explode?
      Orion’s shoulder
      If you’ve looked up at the stars during winter in the Northern Hemisphere, you’ve probably seen Betelgeuse, whether you realized it or not. The star is the second brightest in the constellation Orion, marking the hunter’s left shoulder from our perspective.
      And it’s huge. Estimates for Betelgeuse’s vital statistics vary, but if it sat at the center of our solar system, the star would fill much of the space between the sun and Jupiter. At about 15 to 20 times as massive as the sun, somewhere between 750 and 1,000 times its diameter and just about 550 light-years from Earth, Betelgeuse is typically between the sixth- and seventh-brightest star in the sky.
      Betelgeuse’s brightness varies, even under normal circumstances. Its outer layers are a bubbling cauldron of hot gas and plasma. As hot material rises to the surface, the star brightens; as material falls toward the core, the star dims. That convection cycle puts Betelgeuse on a semiregular dimmer switch that fluctuates roughly every 400 days or so. The star’s brightness also varies about every six years, though astronomers don’t know why.

      What they do know is that Betelgeuse is running out of time. It’s less than 10 million years old, a youngster compared with the roughly 4.6-billion–year-old sun. But because Betelgeuse is so massive and burns through its fuel so quickly, it’s already in the final life stage of a red supergiant. Someday in the not too distant future, the star won’t be able to support its own weight — it will collapse in on itself and rebound in a supernova.
      “We know one day it’s going to die and explode,” says Emily Levesque, an astrophysicist at the University of Washington in Seattle. But no one knows when. “In astronomical terms, ‘one day’ means sometime in the next 200,000 years.”
      In October 2019, Betelgeuse started dimming, which wasn’t too strange in and of itself. The change fit within the normal 400ish-day cycle, says astronomer Edward Guinan of Villanova University in Pennsylvania, who has been tracking Betelgeuse’s cycles of brightness since the 1980s.
      But by Christmas, Betelgeuse was the dimmest it had been in the 100-plus years that astronomers have measured it. And the dimming continued all the way through February.
      Guinan was one of the first to sound the alarm. On December 7, and again on December 23, he and colleagues posted a bulletin on The Astronomer’s Telegram website announcing the star’s “fainting” and encouraging fellow astronomers to take a look.
      There was no reason to think that the dimming was a harbinger of a supernova. “I never said it was going to be one,” Guinan says. But because these explosions are so rare, astronomers don’t know what the signals of an imminent supernova are. Dimming could be one of them.
      That report of odd behavior was all astronomers and amateur space enthusiasts needed to hear. Online, the story caught fire.
      “On Twitter, it was hysterical,” says Andrea Dupree, an astrophysicist at the Harvard & Smithsonian’s Center for Astrophysics in Cambridge, Mass. She recalls seeing one tweet suggesting that the explosion was going to happen that night, with the hashtag #HIDE. “Where am I going to hide? Under my desk?” (When Betelgeuse finally explodes, it probably won’t hurt life on Earth — it’s a safe distance away.)

      Most astronomers didn’t really believe that Betelgeuse’s end was nigh, even as they rushed to schedule telescope time. But some got caught up in the excitement.
      “I don’t expect it to blow,” Guinan recalls thinking. “But I don’t want to blink.” He signed up for phone alerts from telescopes that detect invisible particles called neutrinos and ripples in spacetime called gravitational waves. A detection of either one might be an early sign of a supernova. He found himself outside at 1 a.m. in January after a report of gravitational waves from the direction of Orion. “It was cloudy, but I thought I might see a brightening,” he says. “I’ve gotten crazy about it.”
      Others were believers too, until their data cast doubt on the notion.
      “I thought it might,” says astrophysicist Thavisha Dharmawardena of the Max Planck Institute for Astronomy in Heidelberg, Germany. “We knew there were other explanations, and we might have to look into it. But we know Betelgeuse is an old star, close to the end of its life. It was exciting.”
      Two camps
      Once the star started returning to its usual brightness in mid-February, talk of an imminent supernova faded. A paper published in the Oct. 10 Astrophysical Journal boosted confidence in Betelgeuse’s longevity, suggesting that the star is just at the beginning of its old age and has at least 100,000 years to go before it explodes. But what was it up to, if it was not on the verge of exploding?
      As results from telescopes all over the world and in space flooded in, most astronomers have fallen into two camps. One says Betelgeuse’s dimming was caused by a cloud of dust coughed out by the star itself, blocking its glow. The other camp isn’t sure what the explanation is, but says “no” to the dust speculation.
      One explanation for why Betelgeuse went dark in 2019 is that the star sneezed out a burst of gas and dust (illustrated, left), which condensed into a dark cloud. That cloud blocked the star’s face from the perspective of Earth (right).NASA, ESA, E. Wheatley/STScI
      If the dust theory proves true, it could have profound implications for the origins of complex chemistry, planets and even life in the universe. Red supergiants are surrounded by diffuse clouds of gas and dust that are full of elements that are forged only in stars — and those clouds form before the star explodes. Even before they die, supergiants seem to bequeath material to the next generation of stars.
      “The carbon, oxygen in our body, it’s coming from there — from the supernova and from the clouds around dying stars,” Montargès says. But it’s not clear how those elements escape the stars in the first place. “We have no idea,” he says.
      Montargès hoped studying Betelgeuse’s dimming would let scientists see that process in action.
      In December 2019, he and colleagues took an image of Betelgeuse in visible light with the SPHERE instrument on the Very Large Telescope in Chile. That image showed that, yes, Betelgeuse was much dimmer than it had been 11 months earlier — but only the star’s bottom half. Perhaps an asymmetrical dust cloud was to blame.
      Observations from February 15, 2020, seem to support that idea (SN: 4/11/20, p. 6). Levesque and Philip Massey of the Lowell Observatory in Flagstaff, Ariz., compared the February observations with similar ones from 2004. The star’s temperature hadn’t dropped as much as would be expected if the dimming was from something intrinsic to the star, like its convection cycles, the pair reported in the March 10 Astrophysical Journal Letters.
      That left dust as a reasonable explanation. “We know Betelgeuse sheds mass and produces dust around itself,” Levesque says. “Dust could have come toward us, cooled and temporarily blocked the light.”
      Dark cloud
      A strong vote for dust came from Dupree, who was watching Betelgeuse with the Hubble Space Telescope. Like Guinan, she has a decades-long relationship with Betelgeuse. In 1996, she and colleague Ronald Gilliland looked at Betelgeuse with Hubble to make the first real image of any star other than the sun. Most stars are too far and too faint to show up as anything but a point. Betelgeuse is one of the few stars whose surface can be seen as a two-dimensional disk — a real place.
      By the end of 2019, Dupree was observing Betelgeuse with Hubble several times a year. She had assembled an international team of researchers she calls the MOB, for Months of Betelgeuse, to observe the star frequently in a variety of wavelengths of light.

      The goal was the same as Montargès’: to answer fundamental questions about how Betelgeuse, and perhaps other red supergiants, lose material. The MOB had baseline observations from before the dimming and already had Hubble time scheduled to track the star’s brightness cycles.
      Those observations showed that in January and March 2019, Betelgeuse looked “perfectly normal,” Dupree says. But from September through November, just before the dimming event, the star gave out more ultraviolet light — up to four or five times its usual UV brightness — over its southern hemisphere.
      The temperature and electron density in that region went up, too. And material seemed to be moving outward, away from the star and toward Earth.
      Dupree and colleagues’ theory of what happened, reported in the Aug. 10 Astrophysical Journal, is that one of the giant bubbles of hot plasma always churning in the star’s outer layers rose to the edge of the star’s atmosphere and escaped, sending huge amounts of material flowing into interstellar space. That could be one way that red supergiants shed material before exploding.
      Once it had fled the star, that hot stuff cooled, condensed into dust and floated in front of Betelgeuse for several months. As the dust cleared, Betelgeuse appeared brighter again.
      “It seems to us that what we saw with the ultraviolet is kind of the smoking gun,” Dupree says. “This material moved on out, condensed and formed this dark, dark dust cloud.”
      Paul Hertz, director of NASA’s astrophysics division, shared the Hubble results in a NASA online town hall meeting on September 10 as if it were the final answer. “Mystery solved,” he said. “Not gonna supernova anytime soon.”
      Cycles and spots
      Maybe not — but that doesn’t mean dust explains the dimming.
      In the July 1 Astrophysical Journal Letters, Dharmawardena and colleagues published observations of Betelgeuse that ran counter to the dust explanation. Her team used the James Clerk Maxwell Telescope in Hawaii in January, February and March to look at Betelgeuse in submillimeter wavelengths of light. “If we think it’s a dust cloud, the submillimeter is the perfect wavelength to look at,” she says.
      Dust should have made Betelgeuse look brighter in those wavelengths, as floating grains absorbed and reemitted starlight. But it didn’t. If anything, the star dimmed slightly. “Our first thought was that we’d done something wrong — everyone in the community expected it to be dust,” she says. But “the fact that it didn’t increase or stay constant in the submillimeter was pretty much a dead giveaway that it’s not dust.”
      Infrared observations with the airborne SOFIA telescope should have found the glowing signature of dust too, if it existed. “It never showed up,” Guinan says. “I don’t think it’s dust.”
      Instead, Guinan thinks the dimming may have been part of Betelgeuse’s natural convection cycle. The star’s outer atmosphere constantly pulsates and “breathes” in and out as enormous bubbles of hot plasma rise to the surface and sink down again. “It’s driven by the internal core of the star,” he says. “You have hot blobs rising up, they cool, they get more dense, they fall back.”
      Multiple cycles syncing up could explain why the 2019 dimming was so extreme. Guinan and colleagues analyzed about 180 years of observations of Betelgeuse, dating back to astronomer John Herschel’s 1839 discovery that the star’s brightness varies. Guinan’s group found that, in addition to the roughly six-year and 400-day cycles, Betelgeuse might have a third, smaller cycle of about 187 days. It looks like all three cycles might have hit their brightness nadirs at the same time in late 2019, Guinan says.
      Or maybe the darkness in the southern hemisphere that Montargès’ team saw with SPHERE was an enormous star spot, Dharmawardena offers. In the sun’s case, those dark splotches, called sunspots, mark the sites of magnetic activity on the surface. Betelgeuse is one of a handful of stars on which star spots have been directly seen.
      But to cause Betelgeuse’s dimming, a star spot would have to be enormous. Typical star spots cover about 20 to 30 percent of a star’s surface, Dharmawardena says. This one would need to cover at least half, maybe up to 70 percent.
      “That’s rare,” Dharmawardena admits. “But so is this kind of dimming.”
      Pandemic disruptions
      Analyses are still coming in. But just as Betelgeuse was returning to its normal brightness, the COVID-19 pandemic hit.
      “We were hoping to have a lot more data,” Dharmawardena says.
      A few observations came in right under the wire. The SOFIA observations were made on one of the last flights before the pandemic grounded the plane that carries the telescope. And Montargès took another look with SPHERE just days before its observatory shut down in mid-March.
      In mid-July 2020, astronomers announced that STEREO, a sun-watching spacecraft, had seen signs that the star Betelgeuse was beginning to dim yet again. HI/Stereo/NASA
      In mid-July 2020, astronomers announced that STEREO, a sun-watching spacecraft, had seen signs that the star Betelgeuse was beginning to dim yet again. HI/Stereo/NASA
      But one of Montargès’ most hoped-for results may never come. Eager to solve the dust versus not-dust mystery, his plan was to combine two kinds of observations: making a 2-D picture of the whole star’s disk, like Dupree did with Hubble in the ’90s, but in longer wavelengths such as infrared or submillimeter, like Dharmawardena’s images from early 2020. That way, you could differentiate the dust from the star, he reasoned.
      Only one observatory can do both at once: the Atacama Large Millimeter/submillimeter Array, or ALMA, in Chile. Montargès had planned to ask to observe Betelgeuse with ALMA in June and July, when the winter skies in the Southern Hemisphere are most free of turbulence. But ALMA closed in March and was still closed in September.
      “When I realized ALMA will not get the time in June, I thought … we are never going to solve it,” he says. “We may never be completely certain, because of COVID.”
      Any other star
      Montargès and his colleagues have submitted their analysis of the SPHERE pictures from March for publication. Though he’s not yet willing to share the results, he thinks they could pull the two camps together.
      Ultimately, if Betelgeuse did cough out a cloud of dust last year, it could teach us about the origins of life in the universe, Montargès says. If the dust camp is even partially right, Betelgeuse’s dimming may have been the first time humans have watched the seeds of life being launched into the cosmos.
      In the meantime, he’s relieved to see his favorite star shining bright again. “I must admit that since [last] December, since this whole stuff started, every time I see it, I am like, phew, it’s still there,” he says.
      People keep asking him if he would like ​Betelgeuse to go supernova so he can study it. “I would like another star to go supernova,” he says. “Antares, I don’t care about it; it can explode anytime. But not Betelgeuse.” More

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      Arecibo Observatory, an ‘icon of Puerto Rican science,’ will be demolished

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      Arecibo’s days are done. After two support cables failed in recent months, the radio observatory’s 305-meter-wide dish is damaged beyond repair, the National Science Foundation announced on November 19. It will be decommissioned and dismantled.
      “It’s a death in the family,” says astronomer Martha Haynes of Cornell University, who has used the telescope in Puerto Rico to study hydrogen in the universe since she was fresh out of college in 1973. “For those of us who use Arecibo and had hoped to use it in the future, it’s a disaster.”
      The telescope, famous for appearances in movies like GoldenEye and Contact, consists of a wide dish to collect radio waves from space and focus them into detectors housed in a dome suspended above the dish. In August, one of the cables that holds up the dome slipped out of a socket and punched a hole in the dish.
      The NSF and the University of Central Florida, which manages the telescope, had plans to repair the cable, Haynes said. But then a second cable unexpectedly broke on November 6. If a third cable were to break, it could send the platform holding up the dome swinging, or the whole structure could collapse.

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      The NSF determined that there was no safe way to repair the telescope, the agency announced on November 19.
      “Until these assessments came in, our question was not if the observatory should be repaired but how,” said Ralph Gaume, director of NSF’s Division of Astronomical Sciences, in a statement. “But in the end, a preponderance of data showed that we simply could not do this safely. And that is a line we cannot cross.”
      The closure is the last in a series of near disasters for Arecibo. A different cable was damaged in an earthquake in 2014. Repairs on that cable were delayed by Hurricane Maria in 2017, which temporarily shut down the observatory as Puerto Rico weathered widespread power outages and humanitarian crises (SN: 9/29/17). And the observatory has been the victim of threatened or actual budget cuts for years (SN: 11/17/17).
      But its loss is a major blow for astronomy. Built in 1963, Arecibo was one of the best facilities in the world for observations ranging from mysterious blasts of radio waves from deep space (SN: 2/7/20) to tracking near-Earth asteroids that could potentially crash into our planet (SN: 1/20/20). It also was used in the early days of the search for extraterrestrial intelligence, or SETI (SN: 5/29/12).
      The Arecibo Observatory starred in major films, scanned the sky for hazardous asteroids and spotted mysterious radio bursts from space, among other things.University of Central Florida
      “Astronomers don’t have a lot of facilities,” Haynes says. Each new one is designed to have unique advantages over existing telescopes. “So when you lose one, it’s gone.”
      The observatory’s end is also a symbolic and practical loss for Puerto Rico, says radio astronomy researcher Kevin Ortiz Ceballos, a senior at the University of Puerto Rico at Arecibo who used the observatory to study the first known interstellar comet and stars that host exoplanets (SN: 10/14/19).
      “Arecibo is like an icon of Puerto Rican science,” he says. “This is absolutely devastating.”
      Ortiz Ceballos grew up watching Puerto Rican cartoons in which the characters went to Arecibo to use the telescope. His parents drove him an hour and a half to visit the telescope. He credits it with sparking his interest in astronomy, and he had hoped to come back to Puerto Rico to work at Arecibo after completing his Ph.D.
      “Puerto Rico has a huge mass emigration problem,” he says. “It’s a lot of people, and they’re all my age. It’s a huge brain drain. Being able to do what I love without having to leave, it was a huge dream for me.”
      And not just him, he notes: Dozens of students at the university and the observatory, plus more than 200 Puerto Rican students who went through the observatory’s high school program, have a similar story.
      “Losing this, especially after all that we’ve lost over the past half decade, makes me feel like we’re condemned to have our country just be ruins,” he says. “It becomes a signifier of a broader collapse. That’s just really tragic.” More

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      Farming on Mars will be a lot harder than ‘The Martian’ made it seem

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      In the film The Martian, astronaut Mark Watney (played by Matt Damon) survives being stranded on the Red Planet by farming potatoes in Martian dirt fertilized with feces.
      Future Mars astronauts could grow crops in dirt to avoid solely relying on resupply missions, and to grow a greater amount and variety of food than with hydroponics alone (SN: 11/4/11). But new lab experiments suggest that growing food on the Red Planet will be a lot more complicated than simply planting crops with poop (SN: 9/22/15).
      Researchers planted lettuce and the weed Arabidopsis thaliana in three kinds of fake Mars dirt. Two were made from materials mined in Hawaii or the Mojave Desert that look like dirt on Mars. To mimic the makeup of the Martian surface even more closely, the third was made from scratch using volcanic rock, clays, salts and other chemical ingredients that NASA’s Curiosity rover has seen on the Red Planet (SN: 1/31/19). While both lettuce and A. thaliana survived in the Marslike natural soils, neither could grow in the synthetic dirt, researchers report in the upcoming Jan. 15 Icarus.
      “It’s not surprising at all that as you get [dirt] that’s more and more accurate, closer to Mars, that it gets harder and harder for plants to grow in it,” says planetary scientist Kevin Cannon of the Colorado School of Mines in Golden, Colo., who helped make the synthetic Mars dirt but wasn’t involved in the new study.

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      Soil on Earth is full of microbes and other organic matter that helps plants grow, but Mars dirt is basically crushed rock. The new result “tells you that if you want to grow plants on Mars using soil, you’re going to have to put in a lot of work to transform that material into something that plants can grow in,” Cannon says.
      Biochemist Andrew Palmer and colleagues at the Florida Institute of Technology in Melbourne planted lettuce and A. thaliana seeds in imitation Mars dirt under controlled lighting and temperature indoors, just as astronauts would on Mars. The plants were cultivated at 22° Celsius and about 70 percent humidity.
      Seeds of both species germinated and grew in dirt mined from Hawaii or the Mojave Desert, as long as the plants were fertilized with a cocktail of nitrogen, potassium, calcium and other nutrients. No seeds of either species could germinate in the synthetic dirt, so “we would grow up plants under hydroponic-like conditions, and then we would transfer them” to the artificial dirt, Palmer says. But even when given fertilizer, those seedlings died within a week of transplanting.
      In lab experiments, lettuce was able to grow in Marslike soil from the Mojave Desert (pictured) as long as the soil was fertilized with nitrogen, potassium, calcium and other nutrients.Nathan Hadland
      Palmer’s team suspected that the problem with the synthetic Mars dirt was its high pH, which was about 9.5. The two natural soils had pH levels around 7. When the researchers treated the synthetic dirt with sulfuric acid to lower the pH to 7.2, transplanted seedlings survived an extra week but ultimately died.
      The team also ran up against another problem: The original synthetic dirt recipe did not include calcium perchlorate, a toxic salt that recent observations suggest make up to about 2 percent of the Martian surface. When Palmer’s team added it at concentrations similar to those seen on Mars, neither lettuce nor A. thaliana grew at all in the dirt.
      “The perchlorate is a major problem” for Martian farming, says Edward Guinan, an astrobiologist at Villanova University in Pennsylvania who was not involved in the work. But calcium perchlorate may not have to be a showstopper. “There are bacteria on Earth that enjoy perchlorates as a food,” Guinan says. As the microbes eat the salt, they give off oxygen. If these bacteria were taken from Earth to Mars to munch on perchlorates in Martian dirt, Guinan imagines that the organisms could not only get rid of a toxic component of the dirt, but perhaps also help produce breathable oxygen for astronauts.
      What’s more, the exact treatment required to make Martian dirt farmable may vary, depending on where astronauts make their homestead. “It probably depends where you land, what the geology and chemistry of the soil is going to be,” Guinan says.
      To explore how that variety might affect future agricultural practices, geochemist Laura Fackrell of the University of Georgia in Athens and colleagues mixed up five new types of faux Mars dirt. The recipes for these fake Martian materials, also reported in the Jan. 15 Icarus, are based on observations of Mars’ surface from the Curiosity, Spirit and Opportunity rovers, as well as NASA’s Mars Global Surveyor spacecraft and Mars Reconnaissance Orbiter.
      Each new artificial Mars dirt represents a mix of materials that could be found or made on the Red Planet. One is designed to represent the average composition across Mars, similar to the synthetic material created by Cannon’s team. The other four varieties have slightly different makeups, such as dirt that is particularly rich in carbonates or sulfates. This collection “expands the palette of what we have available” as test-beds for agricultural experiments, Fackrell says.
      She’s now using her stock to run preliminary plant growth experiments. So far, a legume called moth bean, which has similar nutritional content to a soybean but is more drought resistant, has grown the best. “But they’re not necessarily super healthy,” Fackrell says. Future experiments could explore what nutrient cocktails help plants survive in the various fake Martian terrains. But this much is clear, Fackrell says: “It’s not quite as easy as it looks in The Martian.” More

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      Planets with many neighbors may be the best places to look for life

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      If you’re looking for life beyond the solar system, there’s strength in numbers.
      A new study suggests that systems with multiple planets tend to have rounder orbits than those with just one, indicating a calmer family history. Only child systems and planets with more erratic paths hint at past planetary sibling clashes violent enough to knock orbits askew, or even lead to banishment. A long-lasting abundance of sibling planets might therefore have protected Earth from destructive chaos, and may be part of what made life on Earth possible, says astronomer Uffe Gråe Jørgensen of the Niels Bohr Institute in Copenhagen.
      “Is there something other than the Earth’s size and position around the star that is necessary in order for life to develop?” Jørgensen says. “Is it required that there are many planets?”
      Most of the 4,000-plus exoplanets discovered to date have elongated, or eccentric, orbits. That marks a striking difference from the neat, circular orbits of the planets in our solar system. Rather than being an oddity, those round orbits are actually perfectly normal — for a system with so many planets packed together, Jørgensen and his Niels Bohr colleague Nanna Bach-Møller report in a paper  published online October 30 in the Monthly Notices of the Royal Astronomical Society.

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      Bach-Møller and Jørgensen analyzed the eccentric paths of 1,171 exoplanets orbiting 895 different stars. The duo found a tight correlation between number of planets and orbit shape. The more planets a system has, the more circular their orbits, no matter where you look or what kind of star they orbit.
      Earlier, smaller studies also saw a correlation between number of planets and orbit shapes, says astrophysicist Diego Turrini of the Italian National Astrophysics Institute in Rome. Those earlier studies used only a few hundred planets.
      “This is a very important confirmation,” Turrini says. “It is providing us an idea of … how likely it is there will be no fight in the family, no destructive events, and your planetary system will remain as it formed … long enough to produce life.”
      Systems with as many planets as ours are exceedingly rare, though. Only one known system comes close: the TRAPPIST-1 system, with seven roughly Earth-sized worlds (SN: 2/22/17). Astronomers have found no solar systems so far, other than ours, with eight or more planets. Extrapolating out to the number of stars expected to have planets in the galaxy, Jørgensen estimates that about 1 percent of planetary systems have as many planets as we do.
      “It’s not unique, but the solar system belongs to a rare type of planetary system,” he says.
      That could help explain why life seems to be rare in the galaxy, Jørgensen suggests. Exoplanet studies indicate that there are billions of worlds the same size as Earth, whose orbits would make them good places for liquid water. But just being in the so-called “habitable zone” is not enough to make a planet habitable (SN: 10/4/19).
      “If there are so many planets where we could in principle live, why are we not teeming with UFOs all the time?” Jørgensen says. “Why do we not get into traffic jams with UFOs?”
      The answer might lie in the different histories of planetary systems with eccentric and circular orbits. Theories of solar system formation predict that most planets are born in a disk of gas and dust that encircles a young star. That means young planets should have circular orbits, and all orbit in the same plane as the disk.
      “You want the planets to not come too close to each other, otherwise their interactions might destabilize the system,” says Torrini. “The more planets you have the more delicate the equilibrium is.”
      Planets that end up on elliptical orbits may have gotten there via violent encounters with neighboring planets, whether direct collisions that break both planets apart or near-misses that toss the planets about (SN: 2/27/15). Some of those encounters may have ejected planets from their solar systems altogether, possibly explaining why planets with eccentric orbits have fewer siblings (SN: 3/20/15).
      Earth’s survival may therefore have depended on its neighbors playing nice for billions of years (SN: 5/25/05). It doesn’t need to have escaped violence altogether, either, Jørgensen says. One popular theory holds that Jupiter and Saturn shifted in their orbits billions of years ago, a reshuffling that knocked the orbits of distant comets askew and send them careening into the inner solar system. Several lines of evidence suggest comets could have brought water to the early Earth (SN: 5/6/15).
      “It’s not the Earth that is important,” Jørgensen says. “It’s the whole configuration of the planetary system that’s important for life to originate on an earthlike planet.” More