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      Saturn has a fuzzy core, spread over more than half the planet’s diameter

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      One of Saturn’s rings has revealed properties of its core, hidden deep beneath the planet’s golden atmosphere.

      That core isn’t the lump of rock and ice that many scientists had envisioned, the new study finds. Instead, the core is diffuse, pervaded by huge amounts of hydrogen and helium and so spread out that it spans 70,000 kilometers, or about 60 percent of the planet’s diameter, researchers report April 28 at arXiv.org.

      The new intel should help planetary scientists better understand not only how giant planets formed in our solar system but also the nature of such worlds orbiting other stars.  

      To ascertain the structure of Saturn’s core, astronomer Christopher Mankovich and astrophysicist Jim Fuller, both at Caltech, examined the giant planet’s rings. Just as earthquakes help seismologists probe Earth’s interior, oscillations inside Saturn can reveal its internal composition. These oscillations alter Saturn’s gravitational forces, inducing waves in the rings —especially the C ring, which is the nearest of the three main rings to the planet (SN: 1/22/19).

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      By analyzing a wave in that ring, along with data on Saturn’s gravity field from the now-defunct Cassini spacecraft (SN: 9/15/17), Mankovich and Fuller found that the core has about 17 Earth masses of rock and ice. But there’s so much hydrogen and helium mixed in, the core encompasses 55 Earth masses altogether — more than half of Saturn’s total, which is equivalent to the mass of 95 Earths. This “ring seismology” work will appear in a future Nature Astronomy.

      “It’s a new way to look at gas giant planets in the solar system,” says Ravit Helled, a planetary scientist at the University of Zurich who was not involved with the work. “This knowledge is important because it reflects on our understanding of giant exoplanets,” and indicates that giant planets in other solar systems probably have more complex structures than many researchers had thought.

      The discovery also illuminates how Saturn formed, says Nadine Nettelmann, a planetary scientist at the German Aerospace Center in Berlin.

      Older theories posited that a gas giant such as Saturn arises when rock and ice orbiting the sun start to conglomerate. Tenuous gaseous envelopes let additional solid materials sink to the center, forming a compact core. Only later, according to this theory, does the core attract lots of hydrogen and helium — the ingredients that make up most of the planet. Although these elements are gases on Earth, Saturn’s great gravity squeezes most of them into a fluid.

      But newer theories say instead that plenty of gas got incorporated into the core of rock and ice when it was taking shape 4.6 billion years ago. As the planet accreted additional mass, the proportion of gas rose. The structure Mankovich and Fuller deduce for Saturn’s core preserves this formation history, Nettelmann says, because the planet’s very center, representing the oldest part of Saturn, has the greatest proportion of rock and ice. The fraction of rock and ice decrease gradually rather than abruptly from the core’s center to its edge, reflecting the core’s development over time.

      “I find the conclusions very important and very exciting and the line of reasoning very convincing,” Nettelmann says. Still, she cautions that additional waves in the rings should be analyzed for confirmation.

      The type of oscillation that Mankovich and Fuller detect inside Saturn also implies that the core is stable rather than bubbling like a pot of water on a hot stove, which is one way a planet can carry heat from its hot interior outward. The core’s stability may help explain a long-standing puzzle: why Saturn emits more energy than it gets from the sun.

      After the planet formed, it was warm with the heat of its birth, but then it cooled off. The core’s stability could have put a lid on some of this cooling, however, which helped the planet retain heat that it still radiates to this day. In contrast, if the core had instead transported heat via the upwelling and downwelling of material, the planet would have cooled off faster and no longer give off so much heat. More

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      A rare glimpse of a star before it went supernova defies expectations

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      A rare glimpse of a star before it exploded in a fiery supernova looks nothing like astronomers expected, a new study suggests.

      Images from the Hubble Space Telescope reveal that a relatively cool, puffy star ended its life in a hydrogen-free supernova. Until now, supernovas without hydrogen were thought to originate only from extremely hot, compact stars.

      The discovery “is a very important test case for stellar evolution,” says Sung-Chul Yoon, an astrophysicist at Seoul National University in South Korea, who was not involved in the work. Theorists have some ideas about how massive stars behave right before they blow up, but such hefty stars are scant in the local universe and many are nowhere near ready to go supernova, Yoon says. Retroactively identifying the star responsible for a supernova provides an opportunity to test scenarios of how stars evolve right before exploding.

      Finding those stars, however, is difficult, explains Charlie Kilpatrick, an astronomer at Northwestern University in Evanston, Ill. A telescope must have looked at that exact region of the sky in the years leading up to the supernova. And the explosion must have happened close enough for light from its much fainter source star to have reached a telescope.

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      Although both conditions are tricky to meet, Kilpatrick is undaunted by the hunt. After scientists discovered a supernova in December 2019, in a galaxy called NGC 4666 about 46 million light-years away, he and colleagues rushed to check old Hubble observations from the same region of the sky. They wanted to find the star behind the explosion, dubbed SN 2019yvr.

      After pouring over images and cross-checking observations with those from ground-based telescopes, the team found their quarry: a star at the same spot as the supernova, observed about 2.6 years before the explosion. It appeared to be a yellow star about 6,500° Celsius and about 320 times wider than the sun.

      “I was kind of puzzled by all that,” Kilpatrick says. The supernova SN 2019yvr lacked hydrogen, so its progenitor was expected to be hydrogen-deficient, too. But “if a star lacks a hydrogen envelope, then you expect to be seeing deeper inside of the star to the hotter layers,” Kilpatrick says. That is, the star should have looked extremely hot and blue and compact — maybe 10,0000 to 50,000° C, and no more than 50 times wider than the sun. The cool, large, yellow progenitor of SN 2019yvr, on the other hand, appeared to be padded with lots of hydrogen. The researchers report the results May 5 in the Monthly Notices of the Royal Astronomical Society.

      For this kind of star to have produced a supernova like SN 2019yvr, it must have shed much of its hydrogen before blowing up, Kilpatrick says. But how?

      He and colleagues have come up with a couple scenarios. The star could have expelled much of its hydrogen into space through violent eruptions, possibly caused by some instability in the star’s core or interference from another star nearby. Or perhaps the star’s hydrogen could have been stripped off by another star that was in orbit around it.

      To whittle these possibilities down, Jan Eldridge, an astrophysicist at the University of Auckland in New Zealand, suggests turning the Hubble telescope back on that area of the sky. Astronomers should first make sure that the star seen 2.6 years before SN 2019yvr really is gone now, says Eldridge, who was not involved in the work. Researchers could also check whether a star that once orbited SN 2019yvr’s progenitor still remains.

      “They’ve found a mystery, and they’ve got some solutions,” Eldridge notes. Trying to figure out how such an unlikely star pulled off this particular supernova, she says, “is going to be fun.” More

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      NASA’s Ingenuity helicopter’s mission with Perseverance has been extended

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      The Ingenuity helicopter proved it could fly on Mars. Now it has loftier goals. Having passed all its original engineering tests, the tiny spacecraft will now begin a new job, supporting the Perseverance rover in its science mission.

      “It’s like Ingenuity is graduating,” said Ingenuity project manager MiMi Aung of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., in a news briefing on April 30.

      The helicopter arrived at Mars with two main goals: demonstrate that flight was possible on the Red Planet and show that it could return critical flight data to Earth. Those were both achieved in Ingenuity’s first flight on April 19 and then surpassed as the helicopter flew farther, higher and faster on April 22 and April 25 (SN: 4/19/21).

      The original plan was for Ingenuity to take up to six flights total, then ground itself forever as Perseverance drove away to do science. That was partly because the Perseverance team expected to drive far from the rover’s landing site in search of rocks that might preserve signs of past Martian life (SN: 2/22/21).

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      “We thought we would be doing an intensive drive campaign in which the helicopter would not be able to keep up,” said Perseverance project scientist Ken Farley of Caltech in the briefing. “But based on the rocks we have seen in the area, we really wish to spend a considerable amount of time where we are.”

      Ingenuity has also been performing surprisingly well, Aung said. The rover and the helicopter might be able to communicate from more than a kilometer apart, giving them both more flexibility.

      Ingenuity took its fourth flight on April 30 to scout for a new launch pad. The fifth flight, to be scheduled after the team has examined the data, will be a one-way journey to that new home.

      After that, Ingenuity will switch into support mode. Up until now, the Perseverance team has generously supported the helicopter, Aung said. “The rover is primary going forward,” she said.

      The helicopter will have future flights in support mode. The team says Ingenuity will scout potential scientific observations and rover routes from the sky, make 3-D digital elevation maps and take a look at places a rover can’t go. “The lessons learned from that exercise will benefit future missions with aerial platforms tremendously,” Aung said.

      The team isn’t sure how the helicopter’s mission will end. Ingenuity was designed to last just 30 Martian days. The new support phase will extend its mission by another 30 days, unless something goes wrong before then. “We don’t know how many freeze and thaw cycles it can go through before something breaks,” said Ingenuity chief engineer Bob Balaram. More

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      Stars made of antimatter could lurk in the Milky Way

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      Fourteen pinpricks of light on a gamma-ray map of the sky could fit the bill for antistars, stars made of antimatter, a new study suggests.

      These antistar candidates seem to give off the kind of gamma rays that are produced when antimatter — matter’s oppositely charged counterpart — meets normal matter and annihilates. This could happen on the surfaces of antistars as their gravity draws in normal matter from interstellar space, researchers report online April 20 in Physical Review D.

      “If, by any chance, one can prove the existence of the antistars … that would be a major blow for the standard cosmological model,” says Pierre Salati, a theoretical astrophysicist at the Annecy-le-Vieux Laboratory of Theoretical Physics in France not involved in the work. It “would really imply a significant change in our understanding of what happened in the early universe.”

      It’s generally thought that although the universe was born with equal amounts of matter and antimatter, the modern universe contains almost no antimatter (SN: 3/24/20). Physicists typically think that as the universe evolved, some process led to matter particles vastly outnumbering their antimatter alter egos (SN: 11/25/19). But an instrument on the International Space Station recently cast doubt on this assumption by detecting hints of a few antihelium nuclei. If those observations are confirmed, such stray antimatter could have been shed by antistars.

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      Intrigued by the possibility that some of the universe’s antimatter may have survived in the form of stars, a team of researchers examined 10 years of observations from the Fermi Gamma-ray Space Telescope. Among nearly 5,800 gamma-ray sources in the catalog, 14 points of light gave off gamma rays with energies expected of matter-antimatter annihilation, but did not look like any other known type of gamma-ray source, such as a pulsar or black hole.

      Based on the number of observed candidates and the sensitivity of the Fermi telescope, the team calculated how many antistars could exist in the solar neighborhood. If antistars existed within the plane of the Milky Way, where they could accrete lots of gas and dust made of ordinary matter, they could emit lots of gamma rays and be easy to spot. As a result, the handful of detected candidates would imply that only one antistar exists for every 400,000 normal stars.

      If, on the other hand, antistars tended to exist outside the plane of the galaxy, they would have much less opportunity to accrete normal matter and be much harder to find. In that scenario, there could be up to one antistar lurking among every 10 normal stars.

      But proving that any celestial object is an antistar would be extremely difficult, because besides the gamma rays that could arise from matter-antimatter annihilation, the light given off by antistars is expected to look just like the light from normal stars. “It would be practically impossible to say that [the candidates] are actually antistars,” says study coauthor Simon Dupourqué, an astrophysicist at the Institute of Research in Astrophysics and Planetology in Toulouse, France. “It would be much easier to disprove.”

      Astronomers could watch how gamma rays or radio signals from the candidates change over time to double-check that these objects aren’t really pulsars. Researchers could also look for optical or infrared signals that might indicate the candidates are actually black holes.

      “Obviously this is still preliminary … but it’s interesting,” says Julian Heeck, a physicist at the University of Virginia in Charlottesville not involved in the work.

      The existence of antistars would imply that substantial amounts of antimatter somehow managed to survive in isolated pockets of space. But Heeck doubts that antistars, if they exist, would be abundant enough to account for all the universe’s missing antimatter. “You would still need an explanation for why matter overall dominates over antimatter.” More

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      NASA’s Perseverance rover split CO2 to make breathable air on Mars

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      NASA’s Perseverance rover just created a breath of fresh air on Mars. An experimental device on the rover split carbon dioxide molecules into their component parts, creating about 10 minutes’ worth of breathable oxygen. It was also enough oxygen to make tiny amounts of rocket fuel.

      The instrument, called MOXIE (Mars Oxygen In-Situ Resource Utilization Experiment), is about the size of a toaster (SN: 7/28/20). Its job is to break oxygen atoms off carbon dioxide, the primary component of Mars’ atmosphere. It’s like “an electrical tree,” says principal investigator Michael Hecht of MIT. “We breathe in CO2 and breathe out oxygen.”

      MOXIE flew to Mars with Perseverance, which arrived on the Red Planet on February 18 (SN: 2/22/21). On April 20, the instrument warmed up to about 800° Celsius and ran for long enough to produce five grams of oxygen. That’s not enough to breathe for very long. But the main reason to make oxygen on Mars isn’t for breathing, Hecht says. It’s for making fuel for the return journey to Earth.

      “When we burn anything, gas in the car or a log in the fireplace, most of what we’re burning is oxygen,” Hecht says. On Earth, we take all that oxygen for granted. “It’s free here. We don’t think about it.”

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      Future astronauts will have to either bring oxygen with them or make it on Mars. A rocket powerful enough to lift a few astronauts off the Red Planet’s surface would need about 25 metric tons of oxygen — too much to pack for the journey.

      MOXIE is a prototype for the device astronauts could use to make rocket fuel in the future. When running at full power, the instrument can create about 10 grams of oxygen per hour. The instrument, powered by Perseverance, will run for about one Martian day at a time. A scaled-up version could run continuously for 26 months before astronauts arrive, Hecht says.

      MOXIE can’t run continuously because Perseverance needs to divert its power back to its other instruments to continue its science mission of searching for signs of past life on Mars (SN: 1/10/18). MOXIE will get a chance to run at least nine more times over the next Martian year (about two Earth years).

      The success of the technology could set the stage for a permanent research station on Mars, like the McMurdo station in Antarctica, something Hecht would like to someday see. “That’s not something I expect to see in my lifetime, but something I expect to see progress towards in my lifetime,” he says. “MOXIE brings it closer by a decade.” More

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      Mysterious ‘yellowballs’ littering the Milky Way are clusters of newborn stars

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      Scientists have cracked the case of mysterious cosmic objects dubbed “yellowballs.” The celestial specks mark the birthplaces of many kinds of stars with a wide range of masses, rather than single supermassive stars, researchers report April 13 in the Astrophysical Journal.

      The stars in the clusters are relatively young, only about 100,000 years old. “I think of these as stars in utero,” says Grace Wolf-Chase, an astronomer at the Planetary Science Institute who is based in Naperville, Ill. For comparison, the massive stars forming in the Orion nebula are about 3 million years old, and the middle-aged sun is 4.6 billion years old.

      Volunteers with the Milky Way Project first identified the objects while scouring pictures of the galaxy taken by the Spitzer Space Telescope. The now-defunct observatory saw the cosmos in infrared light, which let astronomers take a sort of stellar ultrasound “to probe what’s going on in these cold environments before the stars are actually born,” says Wolf-Chase.

      Citizen scientists had been looking through these images for baby stars thought to be at least 10 times the mass of the sun that were blowing giant bubbles of ionized gas. A year or two into the project, some users began labeling certain objects with the tag #yellowballs¸ because that’s what they looked like in the false-color images. Between 2010 and 2015, the volunteers found 928 yellowballs.

      Wolf-Chase’s team initially thought the balls represented early stage gas bubbles. But because yellowballs were a serendipitous discovery, the researchers knew they probably hadn’t caught enough of them to definitively ID the objects. In 2016, the team asked Milky Way Project volunteers to find more. By the following year, the group had spotted more than 6,000 yellowballs.

      Astronomers first thought ‘yellowballs’ (circled left) were precursors to gas bubbles blown around massive, young stars (right). But a new study suggests yellowballs are actually clusters of less massive stars.JPL-Caltech/NASA

      Wolf-Chase and colleagues compared about 500 of those balls to existing catalogs of star clusters and other structures to try to figure out what they were. “Now we have a good answer: They’re infant star clusters,” Wolf-Chase says. The clusters blow ionized bubbles of their own, similar to the stellar bubbles blown by single young, big stars.

      Wolf-Chase hopes researchers will be able to use the work to pick out yellowballs with telescopes like the James Webb Space Telescope, which is due to launch in October, and figure out more about the balls’ physical properties. More

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      Fast radio bursts could help solve the mystery of the universe’s expansion

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      Astronomers have been arguing about the rate of the universe’s expansion for nearly a century. A new independent method to measure that rate could help cast the deciding vote.

      For the first time, astronomers calculated the Hubble constant — the rate at which the universe is expanding — from observations of cosmic flashes called fast radio bursts, or FRBs. While the results are preliminary and the uncertainties are large, the technique could mature into a powerful tool for nailing down the elusive Hubble constant, researchers report April 12 at arXiv.org.

      Ultimately, if the uncertainties in the new method can be reduced, it could help settle the longstanding debate that holds our understanding of the universe’s physics in the balance (SN: 7/30/19).

      “I see great promises in this measurement in the future, especially with the growing number of detected repeated FRBs,” says Stanford University astronomer Simon Birrer, who was not involved with the new work.

      Astronomers typically measure the Hubble constant in two ways. One uses the cosmic microwave background, the light released shortly after the Big Bang, in the distant universe. The other uses supernovas and other stars in the nearby universe. These approaches currently disagree by a few percent. The new value from FRBs comes in at an expansion rate of about 62.3 kilometers per second for every megaparsec (about 3.3 million light-years). While lower than the other methods, it’s tentatively closer to the value from the cosmic microwave background, or CMB.

      “Our data agrees a little bit more with the CMB side of things compared to the supernova side, but the error bar is really big, so you can’t really say anything,” says Steffen Hagstotz, an astronomer at Stockholm University. Nonetheless, he says, “I think fast radio bursts have the potential to be as accurate as the other methods.”

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      No one knows exactly what causes FRBs, though eruptions from highly magnetic neutron stars are one possible explanation (SN: 6/4/20). During the few milliseconds when FRBs blast out radio waves, their extreme brightness makes them visible across large cosmic distances, giving astronomers a way to probe the space between galaxies (SN: 5/27/20).

      As an FRB signal travels through the dust and gas separating galaxies, it becomes scattered in a predictable way that causes some frequencies to arrive slightly later than others. The farther away the FRB, the more dispersed the signal. Comparing this delay with distance estimates to nine known FRBs, Hagstotz and colleagues measured the Hubble constant.

      The largest error in the new method comes from not knowing precisely how the FRB signal disperses as it exits its home galaxy before entering intergalactic space, where the gas and dust content is better understood. With a few hundred FRBs, the team estimates that it could reduce the uncertainties and match the accuracy of other methods such as supernovas.

      “It’s a first measurement, so not too surprising that the current results are not as constraining as other more matured probes,” says Birrer.

      New FRB data might be coming soon. Many new radio observatories are coming online and larger surveys, such as ones proposed for the Square Kilometer Array, could discover tens to thousands of FRBs every night. Hagstotz expects there will sufficient FRBs with distance estimates in the next year or two to accurately determine the Hubble constant. Such FRB data could also help astronomers understand what’s causing the bright outbursts.

      “I am very excited about the new possibilities that we will have soon,” Hagstotz says. “It’s really just beginning.” More

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      Neutron stars may not be as squishy as some scientists thought

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      Like a dried-up lemon from the back of the fridge, neutron stars are less squeezable than expected, physicists report.

      New measurements of the most massive known neutron star find that it has a surprisingly large diameter, suggesting that the matter within isn’t as squishy as some theories predicted, physicists with the Neutron star Interior Composition Explorer, or NICER, reported April 17 at a virtual meeting of the American Physical Society.

      When a dying star explodes, it can leave behind a memento: a remnant crammed with neutrons. These neutron stars are extraordinarily dense — like compressing Mount Everest into a teaspoon, said NICER astrophysicist Zaven Arzoumanian of NASA’s Goddard Space Flight Center in Greenbelt, Md. “We don’t know what happens to matter when it’s crushed to this extreme point.”

      The more massive the neutron star, the more extreme the conditions in its core. Jammed together at tremendous densities, particles may form unusual states of matter. For example, particles known as quarks — usually contained within protons and neutrons — may roam freely in a neutron star’s center.

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      The core’s composition determines its squeezability. For example, if quarks are free agents within the most massive neutron stars, the immense pressure will compress the neutron star’s core more than if quarks remain within neutrons. Because of that compressibility, for neutron stars, more mass doesn’t necessarily translate to a larger diameter. If neutron star matter is squishy, the objects could counterintuitively shrink as they become more massive (SN: 8/12/20).

      To understand how neutron star innards respond to being put through the cosmic wringer, scientists used the X-ray telescope NICER aboard the International Space Station to estimate the diameters of rapidly spinning neutron stars called pulsars. In 2020, NICER sized up a pulsar with a mass about 1.4 times the sun’s: It was about 26 kilometers wide (SN: 1/3/20).

      Researchers have now gauged the girth of the heftiest confirmed neutron star, with about 2.1 times the mass of the sun. But the beefy neutron star’s radius is about the same as its more lightweight compatriot’s, according to two independent teams within the NICER collaboration. Combining NICER data with measurements from the European Space Agency’s XMM-Newton satellite, one team found a diameter of around 25 kilometers while the other estimated 27 kilometers, physicists reported in a news conference and in two talks at the meeting.

      Many theories predict that the more massive neutron star should have a radius that is smaller. “That it is not tells us that, in some sense, the matter inside neutron stars is not as squeezable as many people had predicted,” said astrophysicist Cole Miller of the University of Maryland in College Park, who presented the second result.

      “This is a bit puzzling,” said astrophysicist Sanjay Reddy of the University of Washington in Seattle, who was not involved in the research. The finding suggests that inside a neutron star, quarks are not confined within neutrons, but they still interact with one another strongly, rather than being free to roam about unencumbered, Reddy said.

      The measurements reveal another neutron star enigma. Pulsars emit beams of X-rays from two hot spots associated with the magnetic poles of the pulsar. According to the textbook picture, those beams should be emitted from opposite sides. But for both of the neutron stars measured by NICER, the hot spots were in the same hemisphere.

      “It implies that we have a somewhat complex magnetic field,” said NICER astrophysicist Anna Watts of the University of Amsterdam, who presented the first team’s result. “Your beautiful cartoon of a pulsar … is for these two stars completely wrong. And that’s brilliant.”

      Beams of radiation are emitted from the magnetic poles of spinning neutron stars called pulsars. Scientists typically envision pulsars with two beams on opposite sides, like a lighthouse. But the beams of a newly measured pulsar (illustrated) come from the same hemisphere.NASA’s Goddard Space Flight Center More