Space

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

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

Jupiter’s icy moon Europa could give the word “moonlight” a whole new meaning. New lab experiments suggest the nightside of this moon glows in the dark.
Europa’s surface, thought to be mostly water ice laced with various salts, is continually bombarded with energetic electrons by Jupiter’s intense magnetic field (SN: 5/19/15). When researchers simulated that interaction in the lab by shooting electrons at salty ice samples, the ice glowed. The brightness of that glow depended on the kind of salt in the ice, researchers report online November 9 in Nature Astronomy.
If the same interaction on Europa creates this never-before-seen kind of moonlight, a future mission there, such as NASA’s planned Europa Clipper spacecraft, may be able to use this ice glow map Europa’s surface composition. That, in turn, could give insight into the salinity of the ocean thought to lurk under Europa’s icy crust (SN: 6/14/19).
“That has implications for the temperature of that liquid water — the freezing point; it has implications for the thickness of the ice shell; it has implications for the habitability of that liquid water,” says Jennifer Hanley, a planetary scientist at Lowell Observatory in Flagstaff, Ariz. not involved in the new work. Europa’s subsurface ocean is considered one of the most promising places to look for extraterrestrial life in the solar system (SN: 4/8/20).

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The discovery of Europa’s potential ice glow “was serendipity,” says Murthy Gudipati, who studies the physics and chemistry of ices at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Gudipati and colleagues originally set out to investigate how electron bombardment might change the chemistry of Europa’s surface ice. But in video footage of their initial experiments, the team noticed that ice samples pelted with electrons gave off an unexpected glow.
Intrigued, the researchers turned their electron beam on samples of pure water ice, as well as water ice mixed with different salts. Each ice core was cooled to the surface temperature of Europa (about –173° Celsius) and showered with electrons that had the same energies as those that strike Europa. Over 20 seconds of irradiation, a spectrometer measured the wavelengths of light, or spectrum, given off by the ice.
The ice samples all gave off a whitish glow, because they emitted light at many different wavelengths. But the brightness of each ice sample depended on its composition. Ice containing sodium chloride, also known as table salt, or sodium carbonate appeared dimmer than pure water ice. Ice mixed with magnesium sulfate, on the other hand, was brighter.

“I was doing some back of the envelope calculations [of] what would be the brightness of Europa, if we were to be standing on it in the dark,” Gudipati says. “It’s approximately … as bright as me walking on the beach in full moonlight.”
Based on the specs proposed for a camera to fly on the Europa Clipper mission, Gudipati and colleagues estimate that the spacecraft could see Europa’s ice glow during a flyby of the dark side of the moon. Dark patches of Europa could reveal sodium-rich regions, while brighter areas may be rich in magnesium.
But seeing ice glow in the lab does not necessarily mean it happens the same way on Europa, Hanley cautions. Jupiter’s icy moon has been barraged by high-energy electrons for a lot longer than 20 seconds. “Is there ever a point where you might break down the salts, and this glow stops happening?” she wonders.
Other planetary scientists, meanwhile, are not convinced that Europa’s surface is highly salted. These researchers, including Roger Clark of the Planetary Science Institute in Lakewood, Colo., think the apparent hints of salts on Europa are actually created by acids, such as sulfuric acid. Europa’s surface may be coated in both salts and acids, Clark says. “What [the researchers] need to do next is irradiate acids … to see if they can tell the difference between salt with water ice and acids with water ice.” More

If you’re a small galaxy and want to mint new stars, come to the Milky Way — but don’t get too close if you want a long-lasting star-making career. New observations with the Gaia space telescope show that our galaxy is both friend and foe to the lesser galaxies that revolve around it.
Some 60 known galaxies orbit the Milky Way. About a dozen of these satellite galaxies are dim dwarf spheroidals, which each emit just 0.0005 to 0.1 percent as much light as the Milky Way (SN: 12/22/14). Their few stars are spread out from one another, giving the galaxies such a ghostly appearance that the first one found was initially suspected to be only a fingerprint on a photographic plate.
But these ghostly galaxies once sparkled with young stars. A new study finds that most of these galaxies lit up when they first crossed into our galaxy’s gravitational domain as fresh stars arose. But then, in most cases, the little galaxies stopped making stars soon afterward, because the Milky Way stripped the dwarf galaxies of gas, the raw material for star formation.
Astronomer Masashi Chiba of Tohoku University in Sendai, Japan, and his then-graduate student Takahiro Miyoshi studied seven of the dwarf spheroidal galaxies orbiting the Milky Way. The researchers used the European Space Agency’s Gaia spacecraft, which had measured the galaxies’ motions, to compute their orbits around the Milky Way’s center. The orbits are elliptical, so the galaxies approach and then recede from our galaxy’s center. The astronomers then compared those paths to the times when the galaxies formed their stars.

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“We found that there’s a very nice coincidence between the timing of the first infall of the satellite [toward the Milky Way] and the peak in the star formation history,” Chiba says. In work posted online at arXiv.org on October 23, the astronomers attribute the burst of star formation in the small galaxies to the Milky Way. Encountering the giant galaxy squeezes the dwarf galaxies’ gas, causing that gas to collapse and spawn lots of new stars.
As an example, the Draco dwarf galaxy first crossed into the Milky Way’s domain 11 billion years ago and formed numerous stars then — but never again. More recently, the Leo I dwarf galaxy entered our galaxy’s realm just 2 billion years ago, a time that coincided with its last burst of star birth. But today Leo I creates no new stars and, like Draco, has no gas to do so.
Dwarf galaxies that kept their distance also kept their gas longer, the researchers found. The galaxies that came closest to the Milky Way’s center, such as Draco and Leo I, ceased all star formation soon after crossing the Milky Way’s frontier. However, the galaxies that entered our galaxy’s domain but remained farther out, such as Fornax and Carina, fared better.
“Those two galaxies kept their interstellar gas inside them, so that the star formation still continued,” Chiba says. Both galaxies managed to eke out new stars for many billions of years after crossing into the Milky Way’s realm. Today, however, neither galaxy has any gas left.
“I think it all makes sense,” says Vasily Belokurov, an astronomer at the University of Cambridge, who notes how essential the Gaia spacecraft was to the discovery. “It’s a beautiful demonstration of what we’ve never been able to do before Gaia, and suddenly we can do these magical things.” More

Jupiter may be the first planet besides Earth known to host atmospheric light shows called “sprites” or “elves.”
Sprites (SN: 6/14/02) and elves (SN: 12/23/95) are two kinds of atmospheric glows that form when lightning alters the electromagnetic environment in the atmosphere above a storm. On Earth, these electromagnetic upsets cause nitrogen molecules in the upper atmosphere to emit a brief, reddish glow. Sprites can brighten a region of the sky tens of kilometers across, while elves can span hundreds of kilometers (SN: 12/21/96).
Scientists suspected these atmospheric phenomena might appear on other planets that crackle with lightning (SN: 6/19/18). But until now, no one had seen hints of sprites or elves on another world.
From 2016 to 2020, the ultraviolet spectrograph on NASA’s Juno spacecraft, in orbit around Jupiter, caught 11 superfast flashes of light across the giant planet. Those flares, reported online October 27 in the Journal of Geophysical Research: Planets, lasted an average 1.4 milliseconds, which is about as fleeting as sprites and elves on Earth. The ultraviolet light was at wavelengths emitted by molecular hydrogen — the type of glow expected of sprites or elves on Jupiter, whose atmosphere is made mostly of hydrogen, rather than nitrogen.
Juno would need to spot a lightning strike at the same place as one of these bright flares to confirm that they actually are sprites or elves, says study coauthor Rohini Giles, a planetary scientist at the Southwest Research Institute in San Antonio, Texas. “But there is reasonably good circumstantial evidence,” she says. The flashes originated a few hundred kilometers above Jupiter’s layer of water clouds, where lightning typically forms, and several appeared in known stormy regions.
Observations of these events when Juno is closer to Jupiter may reveal their size, and help determine whether it is sprites or elves (or both) lighting up Jupiter’s atmosphere. More

Past observations have suggested that there’s water on the moon. New telescope observations conclude that those findings hold water.
Spacecraft have seen evidence of water ice in permanently shadowed craters at the lunar poles (SN: 5/9/16), as well as hints of water molecules on the sunlit surface (SN: 9/23/09). But water sightings in sunlit regions have relied on detection of infrared light at a wavelength that could also be emitted by other hydroxyl compounds, which contain hydrogen and oxygen. 
Now, the Stratospheric Observatory for Infrared Astronomy, or SOFIA, has detected an infrared signal unique to water near the lunar south pole, researchers report online October 26 in Nature Astronomy. “This is the first unambiguous detection of molecular water on the sunlit moon,” says study coauthor Casey Honniball, a lunar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “This shows that water is not just in the permanently shadowed regions — that there are other places on the moon that we could potentially find it.”
These observations could inform future missions to the moon that will scout out lunar water as a potential resource for human visitors (SN: 12/16/19).

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SOFIA, operated by NASA and the German Aerospace Center, is a 2.5-meter telescope that rides aboard a jumbo jet to get clear views of the sky (SN: 2/17/16). During a flight in August 2018, the telescope detected 6-micrometer infrared light emanating from a region near the moon’s southern Clavius crater. This wavelength of light is generated by the vibrations of sunlight-heated water molecules, but not other compounds containing hydroxyl, which consists of an oxygen atom bound to a hydrogen atom.
“I thought it was really brilliant” to confirm the presence of water on the moon with observations at this wavelength, says Jessica Sunshine, a planetary scientist at the University of Maryland in College Park. Sunshine was involved in past observations that spotted hints of water on the moon, but was not involved in the new study.
Based on the brightness of the observed infrared light, Honniball’s team calculated a water concentration of about 100 to 400 parts per million around the Clavius crater. That’s less than half a liter of water per metric ton of lunar soil. This concentration was about what the researchers expected, based on past spacecraft observations.
These water molecules are not frozen in ice, like the water in permanently shadowed regions of the moon. Nor is it liquid, Sunshine says. “There’s no moon puddles.” Instead, the water molecules are thought to be bound inside some other material on the lunar surface.
“The only way for us to be seeing water on the [sunlit] moon is if it is sheltered from this harsh environment,” Honniball says. These water molecules could be encased in glass forged by micrometeorite impacts, or wedged between soil grains that shield the water from blistering solar radiation.
Water could have formed on the moon itself, from hydrogen ions in the continual outward flow of charged particles from the sun reacting with oxygen on the surface (SN: 10/6/14). Or, if the water is stored in impact glass, it could have been delivered to the moon by micrometeorites. More

NASA’s OSIRIS-REx spacecraft is a cosmic rock collector. Cheers erupted from mission control at 6:12 p.m. EDT on October 20 as scientists on Earth got word that the spacecraft had gently nudged a near-Earth asteroid called Bennu, and grabbed some of its rocks to return to Earth.
“The spacecraft did everything it was supposed to do,” said mission principal investigator Dante Lauretta of the University of Arizona in Tucson on a NASA TV webcast. “I can’t believe we actually pulled this off.”
OSIRIS-REx arrived at Bennu in December 2018, and spent almost two years making detailed maps of the 500-meter-wide asteroid’s surface features and composition (SN: 10/8/20). Observations from Earth suggested Bennu should be smooth and sandy, but when OSIRIS-REx arrived, it found a treacherous, rocky landscape.
The team selected a relatively smooth patch in a crater named Nightingale. The spot was not without hazards, though — the team was so worried about a particularly large rock nearby that they named it “Mount Doom” (SN: 12/12/19).

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Luckily, the spacecraft did not need to fully land in the crater to complete its mission. As it hovered just above the surface, OSIRIS-REx reached out a robotic arm with an instrument called TAGSAM at the end, for Touch-And-Go Sample Acquisition Mechanism. The instrument tapped the asteroid lightly for several seconds, and released a burst of nitrogen gas to disturb the surface dust and pebbles. Once those small rocks were lofted, some hopefully were blown into the sample collector.
Because signals from Earth took 18½ minutes to reach Bennu, the spacecraft performed the sampling sequence autonomously. When the mission team got the signal that the spacecraft had finished its job and retreated to a safe distance from Bennu, team members pumped their arms in the air, cheered and sent each other socially distant high-fives and hugs.
OSIRIS-REx is not the first spacecraft to grab samples from an asteroid. That distinction goes to Japan’s Hayabusa mission, which brought back grains of asteroid Itokawa in 2010 (SN: 6/14/10). An encore to that mission, Hayabusa2, collected samples of asteroid Ryugu last year, and is on track to land in Australia in December (SN: 2/22/19).
But OSIRIS-REx attempted to collect much more material than Hayabusa2 did. Hayabusa2 hoped to collect 100 milligrams; OSIRIS-REx is aiming for a minimum of 60 grams, or a little more than two ounces.
Hayabusa2’s scientists have no way to know how much material it actually collected until the spacecraft returns to Earth. But OSIRIS-REx’s team plans to find out using the spacecraft itself. On October 24, the spacecraft will extend its arm and spin its whole body. The difference in the way it spins before and after the sample collection will reveal the mass of the sample.
OSIRIS-REx will return to Earth in 2023, where scientists will analyze the rocks in hopes of unlocking details of the history of the solar system and the origins of water and life on Earth (SN: 1/15/19). More

A strange, newly measured clump of stars orbiting the nearby Andromeda galaxy has the lowest level of heavy chemical elements ever seen in one of these mysterious star clusters. Named RBC EXT8, this globular cluster is also surprisingly massive, challenging theories for how such clusters and some galaxies form, astronomers report online October 15 in Science.
“It’s a very unusual object,” says astrophysicist Oleg Gnedin of the University of Michigan in Ann Arbor, who was not involved in the new discovery.
Globular clusters are crowded, spherical collections of stars that orbit a galaxy’s center, though most, including RBC EXT8, live in the galactic outskirts. The clusters are typically billions of years old, so their stars tend to be chemically pristine, meaning they formed before the universe had time to create much of any of the elements heavier than hydrogen or helium, which astronomers lump together as “metals.”
Previous observations of these clusters in the Milky Way and other galaxies had suggested that there’s a limit to how low a globular cluster’s metal content can be. The most metal-poor clusters were about 300 times less rich in heavy elements like iron than the sun, but no less.
But spectra of RBC EXT8, some 2.5 million light-years away, show that the cluster’s metal content is about 800 times less than the sun’s. The globular cluster that held the previous record for lowest “metallicity” has three times that amount.
“It was completely unexpected that we would find a globular cluster that is so metal poor,” says astronomer Søren Larsen of Radboud University in Nijmegen, the Netherlands.
The bigger, fuzzy blob in the inset image at left is RBC EXT8, a globular cluster that orbits about 88,000 light-years from the center of the galaxy Andromeda (shown at right). The cluster has surprisingly few heavy elements for its size, a new study finds.© 2020 ESASky, CFHT
What’s more, given its metal-poor status, this cluster is surprisingly massive, weighing about 1.14 million times the mass of the sun. (A mid-weight globular cluster is about 100,000 solar masses, but some clusters reach 3 million solar masses. RBC EXT8 is heavy, but not the heaviest.)
That mass makes the cluster even harder to explain because across the cosmos, the more massive a galaxy or cluster is, the more heavy elements it normally has.
There are several potential explanations for that trend, but one is simply that more massive galaxies or globular clusters have more stars. A star fuses heavy elements in its core and sprinkles them around its host cluster or galaxy as it ages. Sufficiently massive stars can explode in a supernova, spreading those metals to become part of the next generation of stars (SN: 8/9/19). So more stars means more opportunity for metals to accumulate locally.
More massive objects also have the advantage of gravity, which lets them better hold on to the metals that they do have and remain a cohesive group for billions of years. Less massive globular clusters dissolve into their host galaxies over time.
Those trends together could have explained the apparent “metallicity floor” for globular clusters — all of the less massive, more metal-poor clusters have broken apart over the eons.
RBC EXT8 turns that conventional wisdom on its head. “It’s too big to have as low metallicity as it has,” Gnedin says. “That’s the conundrum.”

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Astronomers aren’t sure how globular clusters form in general, but they probably grow within galaxies, rather than forming outside of them and being pulled in later. And so the clusters reflect the characteristics of their galaxies: small, metal-poor galaxies end up with small metal-poor globular clusters, and vice versa. But based on RBC EXT8’s metal content, it’s galactic birthplace would be less than a million solar masses, so smaller than the globular cluster itself – which is a paradox.
As a result, the cluster challenges some simplified models of galaxy formation. But it doesn’t completely break them, Gnedin says. “It’s one object, it’s not going to overturn things,” he says. “It just makes us people working on these issues have to work harder” and be more open-minded about other ways that galaxies could form.
Open-mindedness and willingness to explore is perhaps responsible for the new finding about RBC EXT8’s metals. Larsen and colleagues spotted the globular cluster at the beginning of a night of observing with the Keck telescope in Hawaii in October 2019. “It was really a serendipitous discovery,” he says. He had a spare hour before the globular clusters in galaxy M33 that his team was planning to look at rose above the horizon, so the observers picked another cluster “more or less at random” to fill the time.
“At first, I couldn’t really believe that what was coming out [in the observations] was right,” Larsen says. “But I kept working on it, and it turned out to hold up.” More