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      Astronomers identified a second possible exomoon

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      Some of the same researchers who found the first purported exomoon now say that they’ve found another.

      Dubbed Kepler 1708 b i, the satellite has a radius about 2.6 times that of Earth, and circles a Jupiter-sized exoplanet that orbits its parent star about once every two Earth years, the team reports January 13 in Nature Astronomy. That sunlike star lies about 5,700 light-years from Earth.

      To find this nugget, the team sorted through a database of more than 4,000 exoplanets detected by NASA’s now-retired Kepler space telescope. Because large planets orbiting far from their parent star are more likely to have moons large enough to be detected, the team focused on a subset of 70 exoplanets.

      Each of these planets is between half and twice the size of Jupiter. They all either take more than 400 Earth days to orbit their star or have an estimated average surface temperature less than 300 kelvins (around 27° Celsius), slightly higher than that of Earth.

      After further screening, including tossing out exoplanets that don’t have near-circular orbits (which are statistically less likely to host moons), the team identified a strong candidate for an exomoon. It, like its host planet, caused detectable dimming of the parent star’s light when moving across the face of the star.

      Discovery of the first possible exomoon, dubbed Kepler 1625 b, has faced a lot of skepticism (SN: 4/30/19). Both proposed exomoons need to be confirmed by further observations by other instruments, such as the recently launched James Webb Space Telescope, the team notes (SN: 10/6/21).

      But fresh observations will need to wait: The newfound exomoon candidate and its planet won’t pass in front of the parent star again until March 24, 2023, the researchers calculate. More

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      Oxygen-rich exoplanets may be geologically active

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      Humble oxygen is more than just a building block of life. The element could also help scientists sneak a peek into the innards of planets orbiting faraway stars, a new study suggests.

      Laboratory experiments show that rocks exposed to higher concentrations of oxygen melt at lower temperatures than rocks exposed to lower amounts. The finding suggests that oxygen-rich rocky exoplanets could have a thick layer of soupy mantle, possibly giving rise to a geologically active world, researchers report in the Nov. 9 Proceedings of the National Academy of Sciences.

      A gooey interior is thought to have profound effects on a rocky planet. Molten rock deep within a planet is the magma that powers geologic activity on the surface, like what happens on Earth (SN: 7/31/13). During volcanic eruptions, volatiles such as water vapor and carbon dioxide can fizzle out of the magmatic ooze, setting up atmospheres that are potentially friendly to life (SN: 9/3/19). But the factors that drive mantle melting on Earth aren’t well-understood, and scientists have tended to focus on the role of metals, such as iron.

      The impact of oxygen on rock melting has been overlooked, says Yanhao Lin, a planetary scientist at the Center for High Pressure Science and Technology Advanced Research in Beijing. Oxygen is one of the most abundant elements on Earth and probably on rocky exoplanets too, he says. As such, other scientists may have previously thought that it is just too common of an element to play such a literally earthshaking role, adds Lin.

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      In the new study, Lin and colleagues measured the melting temperatures of synthetic, iron-free basalt rock under rock in two environments: under oxygen-starved conditions and exposed to oxygen-rich air. The team used the faux rock to isolate oxygen’s effect on melting and rule out the effects of iron, which can also influence rock melting.

      As the molten rocks cooled to less than 1000° Celsius, the minerals in the oxygenated basalt stayed melted longer than the oxygen-depleted samples, the team observed. The oxygenated rocks consistently solidified at temperatures 100° Celsius lower than their counterparts.

      Just as salt lowers the melting temperature of ice, oxygen similarly makes it easier for rocks to melt, the researchers conclude. Lin hypothesizes that oxygen can break up long chains of silicon and oxygen atoms in solid rock, coaxing them to form smaller bits. These fragments are more mobile and can flow more easily compared to the longer, tangly groups.

      The degree of oxidation could determine how a young exoplanet’s syrupy insides eventually settle into subterranean layers. A more oxidized and more melt-prone gut at lower temperatures may lead to a smaller solid core, a thicker sludgy mantle and a more metal-deprived crusty shell, the researchers say.

      A caveat to the work is that the researchers tested the impact of only oxygen on the melting temperature of rocks. The team has yet to consider other factors such as iron concentration and high pressure, which are also probably part of many real-world exoplanet interiors. These additional factors will further induce melting, Lin predicts.

      The findings are “a very good effort,” says planetary scientist Tim Lichtenberg of the University of Oxford who was not involved in the study. Other caveats to mantle melting may surpass oxygen’s contribution, but the new results are still useful, he says. Understanding oxygen’s potential impact, for example, could be valuable for explaining the inner workings and history of any exoplanet that scientists come across in their astronomical observations. That understanding could be even more valuable — and opportune — as scientists prepare to use the newly launched James Webb Space Telescope to probe the atmospheres of other worlds (SN: 10/6/21).

      Lab experiments, of course, can’t capture all the nuances of real-life planetary interiors. But the work is necessary to guide — and confirm — the formulation of theories about how certain types of exoplanets came to be, Lichtenberg says. Simulations can then extend the reach of experimental results when combined with other techniques, such as modeling.

      “Observations, the modeling and the experiments,” Lichtenberg says, “there’s a trifecta.” These three prongs feed off each other to advance exoplanet science as a whole, long before humankind ever sets foot on such distant worlds. More

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      Two stars’ close encounter may explain a cosmic flare that has barely faded

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      A newborn star whizzing past another stellar youngster triggered a cosmic flare-up that began nearly a century ago and is still going strong today, researchers say.

      In late 1936, a dim star in the constellation Orion started to erupt in our sky and soon shone over 100 times as brightly as it had before. Only telescopes could detect the star prior to the outburst, but afterward, the star was so bright it was visible through binoculars. The star even lit up part of the previously dark interstellar cloud called Barnard 35 that presumably gave the star birth (SN: 1/10/76).

      Amazingly, the star, now named FU Orionis, is still shining almost as brightly today, 85 years later. That means the star wasn’t a nova, a stellar explosion that quickly fades from view (SN: 2/12/21). But the exact cause of the long-lasting flare-up has been a mystery.

      Now, computer simulations may offer a clue to what’s kept the celestial beacon shining so brightly. Located about 1,330 light-years from Earth, FU Orionis is actually a double star, consisting of two separate stars that probably orbit each other. One is about as massive as the sun, while the other is only 30 percent to 60 percent as massive. Because the stars are so young, each has a disk of gas and dust revolving around it. It’s the lesser star’s passage through the other star’s disk that triggered and sustains the great flare-up, the simulations suggest.

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      “The low-mass star is the one that is in outburst,” says Elisabeth Borchert, an astrophysicist at Monash University in Clayton, Australia.

      According to Borchert’s team, the outburst arose as the low-mass star passed 10 to 20 times as far from its mate as the Earth is from the sun — comparable to the distance between the sun and Saturn or Uranus. As the lesser star plowed through the other star’s disk, gas and dust from that disk rained down onto the intruder. In the simulations, this material got hot and glowed profusely, making the low-mass star hundreds of times brighter, behavior that mimicked FU Orionis’ outburst.

      The flare-up has endured so long because the gravitational pull of the lesser star captured material that began to orbit the star and is still falling onto it, the researchers explain in a paper submitted online November 24 at arXiv.org. The study will be published in Monthly Notices of the Royal Astronomical Society.

      “It is a plausible explanation,” says Scott Kenyon, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who was not involved with the study. The researchers “get a rise in luminosity about what the observations show,” he says, and “it lasts a long time.”

      Kenyon says one way to test the team’s theory is to track how the two stars move relative to each other in the future. That may reveal whether the stars were as close together in 1936 as the simulations suggest. Astronomers discovered the binary nature of FU Orionis only two decades ago, by which time the stars were much farther apart in their elliptical orbit around each other.

      Since the discovery of FU Orionis, several other newborn stars have flared up in a similar fashion. The binary model “could be a good explanation for all of them,” Borchert says, if those stars also have stellar companions that recently skirted past. More

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      Enceladus’ plumes might not come from an underground ocean

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      Saturn’s icy moon Enceladus sprays water vapor into space. Scientists have thought that the plumes come from a deep subsurface ocean — but that might not be the case, new simulations suggest.

      Instead, the water could come from pockets of watery mush in the moon’s icy shell, scientists report December 15 at the American Geophysical Union’s fall meeting.

      “Maybe we didn’t get the straw all the way through the ice shell to the ocean. Maybe we’re just getting this weird pocket,” says planetary scientist Jacob Buffo of Dartmouth College.

      The finding is “a cautionary tale,” Buffo says. The hidden ocean makes Enceladus one of the best places to search for life in the solar system (SN: 4/8/20). Concepts for future missions to Enceladus rely on the idea that taking samples of the plumes would directly test the contents of the ocean, without needing to drill or melt through the ice. “That could be true,” Buffo says. But the simulations suggest “you could be sampling this slushy region in the middle of the shell, and that might not be the same chemistry as is down in the ocean.”

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      Enceladus has beguiled planetary scientists since NASA’s Cassini spacecraft revealed the moon’s dramatic plumes in 2005 (SN: 8/23/05). At the time, researchers wondered if the spray originated on Enceladus’ icy surface, where friction from quakes could melt ice and let it escape as pure water vapor into space. But later evidence collected by Cassini convinced most scientists that the geysers are from fractures in the shell that reach all the way to a salty, subsurface sea (SN: 8/4/14).

      One of the most convincing pieces of evidence was the fact that the plumes contain salts, said physicist Colin Meyer of Dartmouth in a talk at the meeting, which was held virtually and in New Orleans. Early versions of the quake idea couldn’t account for those salts, and instead suggested that any salts in the melted ice would be left on the surface as the water escaped into space, like the sheen of salt left on your skin after you sweat, he says.

      But Meyer, who has studied the physics of sea ice on Earth, realized that pockets of meltwater in the ice shell could concentrate salts and other compounds. He, Buffo and colleagues applied computer simulations developed for sea ice on Earth to the observed icy conditions on Enceladus. The team found that Enceladus could easily generate pockets of mush within its shell and vent the contents of that mush out into space, salts and all.

      That does not mean Enceladus doesn’t have an ocean, Meyer says — it almost certainly does. And it does not mean the ocean isn’t habitable, Buffo adds.

      The implications of the results “are huge,” especially for proposed life-finding missions to Enceladus, says planetary scientist Emily Martin of the Smithsonian National Air and Space Museum in Washington, D.C., who was not involved in the work.

      “If those plumes aren’t tapping into the ocean, it will really shift our perspective on what that plume is telling us about the interior of Enceladus,” Martin says. “And that’s a big deal.” More

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      The only known pulsar duo sheds new light on general relativity and more

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      The only known duo of pulsars has just revealed a one-of-a-kind heap of cosmic insights.

      For over 16 years, scientists have been observing the pair of pulsars, neutron stars that appear to pulsate. The measurements confirm Einstein’s theory of gravity, general relativity, to new levels of precision, and hint at subtle effects of the theory, physicists report in a paper published December 13 in Physical Review X.

      Pulsars, spinning dead stars made of densely packed neutrons, appear to blink on and off due to their lighthouse-like beams of radiation that sweep past Earth at regular intervals. Variations in the timing of those pulses can expose pulsars’ movements and effects of general relativity. While physicists have found plenty of individual pulsars, there’s only one known pair orbiting one another. The 2003 discovery of the double-pulsar system, dubbed J0737-3039, opened up a new world of possible ways to test general relativity.

      One of the pulsars whirls around roughly 44 times per second while the other spins about once every 2.8 seconds. The slower pulsar went dark in 2008, due to a quirk of general relativity that rotated its beams out of view. But researchers kept monitoring the remaining visible pulsar, combining that new data with older observations to improve the precision of their measurements.

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      Now, astrophysicist Michael Kramer of the Max Planck Institute for Radio Astronomy in Bonn, Germany, and colleagues have dropped an exhaustive paper that “just lays it all out,” says physicist Clifford Will of the University of Florida in Gainesville. “To me, it’s just magnificent.”

      Here are five insights from the new study:

      1. Einstein was right, in so many ways.

      The pulsar duo allows for five independent tests of general relativity in one, checking whether various properties of the orbit match predictions of Einstein’s theory. For example, the researchers measure the rate at which the orbit’s ellipse rotates, or precesses, to see if it agrees with expectations. All of the parameters fell in line with Einstein.

      What’s more, says astrophysicist Scott Ransom of the National Radio Astronomy Observatory in Charlottesville, Va., “each of the individual tests of general relativity have gotten so precise that …  higher-order effects of general relativity have to be included to match the data.” That means that the measurements are so exacting that they hint at subtle peculiarities of gravity. “All of those things are really amazing,” says Ransom, who was not involved with the research.

      2. Gravitational waves are sapping energy.

      The observations reveal that the pulsars’ orbit is shrinking. By measuring how long the pulsars take to complete each orbit, the researchers determined that the pair get about seven millimeters closer every day.

      That’s because, as they orbit, the pulsars stir up gravitational waves, ripples in spacetime that vibrate outward, carrying away energy (SN: 12/18/15). This telltale shrinkage was seen for the first time in the 1970s in a system with one pulsar and one neutron star, providing early evidence for gravitational waves (SN: 12/16/78). But the new result is 25 times as precise as the earlier measurement.

      3. The pulsar is losing mass and that matters.

      There’s a subtler effect that tweaks that orbit, too. Pulsars gradually slow down over time, losing rotational energy. And because energy and mass are two sides of the same coin, that means the faster pulsar is losing about 8 million metric tons per second.

      “When I realized that for the first time, it really blew me away,” says Kramer. Although it sounds like a lot, that mass loss equates to only a tiny adjustment of the orbit. Previously, scientists could neglect this effect in calculations because the tweak was so small. But the measurement of the orbit is now precise enough that it makes sense to include.

      4. We can tell which way the pulsar spins and that hints at its origins.

      By studying the timing of the pulses as the light from one pulsar passes by its companion, scientists can tell in what direction the faster pulsar is spinning. The results indicate that the pulsar rotates in the same direction as it orbits, and that provides clues to how the pulsar duo formed.

      The two pulsars began as neighboring stars that exploded, one after the other. Often when a star explodes, the remnant it leaves behind gets kicked away, splitting apart such pairs. The fact that the faster pulsar spins in the same direction it orbits means the explosion that formed it didn’t give it much of a jolt, helping to explain how the union stayed intact.

      5. We have a clue to the pulsar’s radius.

      Gravitational effects are known to cause the orbit’s ellipse to precess, or rotate, by about 17 degrees per year. But there’s a subtle tweak that becomes relevant in the new study. The pulsar drags the fabric of spacetime behind it as it spins, like a twirling dancer’s twisting skirt, altering that precession.

      This dragging effect implies that the faster pulsar’s radius must be less than 22 kilometers, an estimate that, if made more precise with future work, could help unveil the physics of the extremely dense neutron star matter that makes up pulsars (SN: 4/20/21).

      “The authors have clearly been very meticulous in their study of this amazing system,” says astrophysicist Victoria Kaspi of McGill University in Montreal. “It is wonderful to see that the double pulsar … indeed is living up to its unique promise.” More

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      The cosmic ‘Cow’ may have produced a new neutron star or black hole

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      A cosmic flare-up called the Cow seems to have left behind a black hole or neutron star.

      When the flash was spotted in June 2018, astronomers debated its origins. Now, astrophysicist DJ Pasham of MIT and colleagues have seen the first direct evidence of what the Cow left behind. “We may be seeing the birth of a black hole or neutron star,” Pasham says.  

      The burst’s official, random designation is AT2018cow, but astronomers affectionately dubbed it the Cow. The light originated about 200 million light-years away and was 10 times as bright as an ordinary supernova, the explosion that marks the death of a massive star.

      Astronomers thought the flare-up could have been from an unusual star being eaten by a black hole or from a weird sort of supernova that left behind a black hole or neutron star (SN: 6/21/19).

      So Pasham and colleagues checked the Cow for flickering X-rays, which are typically produced close to a compact object, possibly in a disk of hot material around a black hole or on the surface of a neutron star.

      Flickers in these X-rays can reveal the size of their source. The Cow’s X-rays flicker roughly every 4 milliseconds, meaning the object that produces them must be no more than 1,000 kilometers wide. Only a neutron star or a black hole fits the bill, Pasham and colleagues report December 13 in Nature Astronomy.

      Because the Cow’s flash was from the explosion that produced either of these objects, a preexisting black hole was probably not responsible for the burst. Pasham admits he was hoping for a black hole eating an exotic star. “I was a little bit disappointed,” he says. “But I’m more blown away that this could be direct evidence of the birth of a black hole. This is an even cooler result.” More

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      This tiny, sizzling exoplanet could be made of molten iron

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      A newly discovered exoplanet is really making astronomers prove their mettle. Planet GJ 367b is smaller than Earth, denser than iron and hot enough to melt, researchers report in the Dec. 3 Science.

      “We think the surface of this exoplanet could be molten,” says astronomer Kristine Wei Fun Lam of the Institute of Planetary Research at the German Aerospace Center in Berlin.

      Signals of the planet were first spotted in data from NASA’s TESS telescope in 2019. The small world swung around its host star every 7.7 hours.

      Using data from TESS and the ground-based HARPS spectrograph at the European Southern Observatory in Chile, Lam and her colleagues measured the planet’s radius and mass. GJ 367b clocked in at about 0.72 times Earth’s radius and 0.55 times its mass. That makes it the first ultrashort-period planet — a class of worlds with years shorter than one Earth day and with mysterious origins — known to be smaller and lighter than Earth.

      Using those measurements, the team then calculated the planet’s density: about 8.1 grams per cubic centimeter, or slightly denser than iron. A computer analysis of the planet’s interior structure suggests that 86 percent of it could comprise an iron core, with only a sliver of rock left on top.

      Mercury has a similarly large core, Lam notes (SN: 4/22/19). Scientists think that’s a result of a giant impact with another planet that stripped away most of its outer layers. GJ 367b could have formed after a similar collision. It could also have once been a gaseous planet whose atmosphere was blasted off by radiation from its star (SN: 7/1/20).

      Whatever its origins, GJ 367b is so close to its star that it’s almost certainly covered in melted metallic lava now. “At 1400° Celsius, I don’t think it would be very nice to stand on it,” Lam says. More

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      Astronomers have found the Milky Way’s first known ‘feather’

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      The Milky Way has a “feather” in its cap.A long, thin filament of cold, dense gas extends jauntily from the galactic center, connecting two of the galaxy’s spiral arms, astronomers report November 11 in the Astrophysical Journal Letters. This is the first time that such a structure, which looks like the barb of a feather fanning off the central quill, has been spotted in the Milky Way.

      The team that discovered our galaxy’s feather named it the Gangotri wave, after the glacier that is the source of India’s longest river, the Ganges. In Hindi and other Indian languages, the Milky Way is called Akasha Ganga, “the river Ganga in the sky,” says astrophysicist Veena V.S. of the University of Cologne in Germany.She and colleagues found the Gangotri wave by looking for clouds of cold carbon monoxide gas, which is dense and easy to trace, in data from the APEX telescope in San Pedro de Atacama, Chile. The structure stretches 6,000 to 13,000 light-years from the Norma arm of the Milky Way to a minor arm near the galactic center called the 3-kiloparsec arm. So far, all other known gas tendrils in the Milky Way align with the spiral arms (SN: 12/30/15).

      The Gangotri wave has another unusual feature: waviness. The filament appears to wobble up and down like a sine wave over the course of thousands of light-years. Astronomers aren’t sure what could cause that, Veena says.

      Other galaxies have gaseous plumage, but when it comes to the Milky Way, “it’s very, very difficult” to map the galaxy’s structure from the inside out, she says. She hopes to find more galactic feathers and other bits of our galaxy’s structure. “One by one, we’ll be able to map the Milky Way.” More