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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

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      NASA’s Ingenuity helicopter made history by flying on Mars

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      Editor’s note: This story will be updated periodically.

      A helicopter just flew on Mars. NASA’s Ingenuity hovered for about 40 seconds above the Red Planet’s surface, marking the first flight of a spacecraft on another planet.

      In the wee hours on April 19, the helicopter spun its carbon fiber rotor blades and lifted itself into the thin Martian air. It rose about three meters above the ground, pivoted to look at NASA’s Perseverance rover, took a picture, and settled back down to the ground.

      “Goosebumps. It looks just the way we had tested it in our test chambers,” Ingenuity project manager MiMi Aung said in a news briefing after the flight. “Absolutely beautiful flight. I don’t think I can ever stop watching it over and over again.”

      As data from the flight started coming in to Ingenuity’s mission control room at NASA’s Jet Propulsion Lab in Pasadena, Calif., at about 6:35 a.m. EDT, a hush fell. And then cheers erupted as Håvard Grip, Ingenuity’s guidance, navigation and control lead, announced: “Confirmed that Ingenuity has performed its first flight, the first flight of a powered aircraft on another planet.”

      NASA’s Ingenuity helicopter took this photo of its own shadow while hovering about three meters in the Martian air on April 19.JPL-Caltech/NASA

      “It’s amazing, brilliant. Everyone is super excited,” said mechanical engineer and team member Taryn Bailey. “I would say it’s a success.”

      The flight, originally scheduled for April 11, was delayed to update the helicopter’s software after a test of the rotor blades showed problems switching from preflight to flight mode. After the reboot, a high-speed spin test April 16 suggested the shift was likely to work, setting the stage for the April 19 flight.

      “I never let you celebrate fully. Every time we hit a major milestone I’m like, not yet, not yet,” Aung told the team moments after the flight was confirmed. Now is the moment to celebrate, she said. “Take that moment and after that, let’s get back to work and more flights. Congratulations.”

      [embedded content]
      Ingenuity lifted into the thin Martian atmosphere for the first time on April 19, proving that flight is possible on another planet. This video was taken by the Perseverance rover, which watched from a safe distance away.

      This first-ever flight was a test of the technology; Ingenuity won’t do any science during its mission, set to last 30 Martian days from the moment it separated from the rover, the equivalent of 31 days on Earth. But its success proves that powered flight is possible in Mars’ thin atmosphere. Future aerial vehicles on Mars could help rovers or human astronauts scout safe paths through unfamiliar landscapes, or reach tricky terrain that a rover can’t traverse.

      “Technology demonstrations are really important for all of us,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate. “It’s taking a tool we haven’t been able to use and putting it in the box of tools we have available for all of our missions at Mars.”

      Ingenuity’s flight was the culmination of more than seven years of imagining, building, testing and hoping for the flight team.

      “That is what building first-of-a-kind systems and flight experiments are all about: design, test, learn from the design, adjust the design, test, repeat until success,” Aung said in a news briefing on April 9.

      Aung and her team began testing early prototypes of a Mars helicopter in a 7.62-meter-wide test chamber at JPL in 2014. It wasn’t a given that flying on Mars would even be possible, Aung said. “It’s challenging for many different reasons.”

      [embedded content]
      Before it hitched a ride to Mars on the rover Perseverance, Ingenuity underwent extensive testing in a Mars simulator on Earth. Its engineers experimented with early prototypes and later with Ingenuity itself. These tests convinced the team that the craft could fly in Mars’ thin atmosphere.

      Even though Mars’ gravity is only about one-third of Earth’s, the air’s density is about 1 percent that at sea level on Earth. It’s difficult for the helicopter’s blades to push against that thin air hard enough to get off the ground.

      Another way to think about it is that the air is thinner on Mars than it is at three times the height of Mount Everest, Ingenuity engineer Amelia Quon of JPL said in the news briefing. “We don’t generally fly things that high,” Quon said. “There were some people who doubted we could generate enough lift to fly in that thin Martian atmosphere.”

      So Quon and her team put the helicopter through a battery of tests over the course of five years. “My job … was to make Mars on Earth, and enough of it that we could actually fly our helicopter in it,” Quon said. The Mars simulation chamber could be emptied of Earth air and pumped full of carbon dioxide at Mars-like densities. Some versions of the helicopter were suspended from the ceiling to simulate Mars’ lower gravity. And wind speeds up to 30 meters per second were simulated by a bank of about 900 computer fans blowing at the helicopter.

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      The final version of Ingenuity is light, about 1.8 kilograms. Its blades are longer (about 1.2 meters wingspan) and rotate faster (about 2,400 rotations per minute) than a similar vehicle would need to be able to fly on Earth. By the time the helicopter hitched a ride to Mars with the Perseverance rover in July 2020, the engineers were confident the helicopter could fly and remain in control at Mars (SN: 7/30/20).

      Perseverance landed in a region called Jezero crater on February 18 (SN: 2/22/21). The helicopter was folded up beneath Perseverance’s belly under a protective shield until March 21.

      Over  the next few weeks, Perseverance drove around to find a flat spot for Ingenuity to launch. Then Ingenuity slowly unfolded itself and was finally lowered gently to the ground beneath Perseverance on April 3. The rover drove away quickly to get Ingenuity out of its shadow and allow the helicopter to charge its batteries with its solar panel, giving it enough power to survive the freezing Martian night. 

      Ingenuity arrived on Mars folded up under the Perseverance rover in a protective shield the size of a pizza box. After landing, Perseverance dropped the shield and slowly lowered Ingenuity to the ground, then drove away.JPL-Caltech/NASA

      On April 8 and 9, Ingenuity unfolded its rotor blades and tested their ability to spin in preparation to take to the air. After trouble-shooting the software problem and retesting the rotor blades April 16, the flight got a green light for April 19. It was scheduled for roughly 3:30 a.m. Eastern Daylight Time on April 19, which corresponds to 12:30 p.m. Mars time, in the early afternoon. That gave the craft’s solar panel enough time to charge up its batteries for the flight. It was also a time when Perseverance’s weather sensors, called MEDA, suggested the average wind speed would be about six meters per second.

      NASA’s Ingenuity helicopter tested its spinning rotor blades on April 8, a week and a half before taking flight in the thin Martian air for the first time.JPL-Caltech/NASA

      Ingenuity had to pilot itself through the flight. That’s partly because of the communication delay — Mars is far enough from Earth that light signals take about 15 minutes to travel between the two planets. But it’s also because Mars’ thin air makes the helicopter difficult to steer. “Things happen too quickly for a human pilot to react to it,” Quon said.

      Perseverance filmed the flight from about 65 meters away, at a spot named Van Zyl Overlook. Ingenuity also filmed the flight from its own perspective, with two sets of cameras: Its downward facing navigation cameras capturing the view below it in black and white, and its color cameras scanning the horizon.

      Over several days, NASA’s Perseverance rover gently lowered the Ingenuity helicopter to the ground and then took this selfie with it on April 6 from about four meters away. The rover then drove off to a safe distance of 65 meters to get ready to watch Ingenuity’s first flight.MSSS/JPL-Caltech/NASA

      Now that this first flight went well, the team hopes to take up to four more flights over the course of Ingenuity’s mission, possibly starting as soon as April 22. Each will be a little bit more daring and riskier, Aung said. “We are going to continually push all the way to the limit of this rotorcraft.” And each one will be a nail-biter: Just one bad landing could end things immediately. Ingenuity has no way to right itself after a fall.

      That may be the way the mission ends, Aung admitted in the April 19 news briefing. “Ultimately, we expect the helicopter will meet its limit,” she said. Even if it eventually  wipes out in a crash, the engineering team will learn valuable information from how the helicopter eventually fails.

      At the end of Ingenuity’s mission, Perseverance will drive off, leaving the little helicopter that could behind, and continue its own mission: to search for signs of past life in Jezero crater, and to store rocks for a future mission to return to Earth. More

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      How the laws of physics constrain the size of alien raindrops

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      Whether they’re made of methane on Saturn’s moon Titan or iron on the exoplanet WASP 76b, alien raindrops behave similarly across the Milky Way. They are always close to the same size, regardless of the liquid they’re made of or the atmosphere they fall in, according to the first generalized physical model of alien rain.

      “You can get raindrops out of lots of things,” says planetary scientist Kaitlyn Loftus of Harvard University, who published new equations for what happens to a falling raindrop after it has left a cloud in the April Journal of Geophysical Research: Planets. Previous studies have looked at rain in specific cases, like the water cycle on Earth or methane rain on Saturn’s moon Titan (SN: 3/12/15). But this is the first study to consider rain made from any liquid.

      “They are proposing something that can be applied to any planet,” says astronomer Tristan Guillot of the Observatory of the Côte d’Azur in Nice, France. “That’s really cool, because this is something that’s needed, really, to understand what’s going on” in the atmospheres of other worlds.

      Comprehending how clouds and precipitation form are important for grasping another world’s climate. Cloud cover can either heat or cool a planet’s surface, and raindrops help transport chemical elements and energy around the atmosphere.

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      Clouds are complicated (SN: 3/5/21). Despite lots of data on earthly clouds, scientists don’t really understand how they grow and evolve.

      Raindrops, though, are governed by a few simple physical laws. Falling droplets of liquid tend to default to similar shapes, regardless of the properties of the liquid. The rate at which that droplet evaporates is set by its surface area.

      “This is basically fluid mechanics and thermodynamics, which we understand very well,” Loftus says.

      She and Harvard planetary scientist Robin Wordsworth considered rain in a variety of different forms, including water on early Earth, ancient Mars and a gaseous exoplanet called K2 18b that may host clouds of water vapor (SN: 9/11/19). The pair also considered Titan’s methane rain, ammonia “mushballs” on Jupiter and iron rain on the ultrahot gas giant exoplanet WASP 76b (SN: 3/11/20). “All these different condensables behave similarly, [because] they’re governed by similar equations,” she says.

      The team found that worlds with higher gravity tend to produce smaller raindrops. Still, all the raindrops studied fall within a fairly narrow size range, from about a tenth of a millimeter to a few millimeters in radius. Much bigger than that, and raindrops break apart as they fall, Loftus and Wordsworth found. Much smaller, and they’ll evaporate before hitting the ground (for planets that have a solid surface), keeping their moisture in the atmosphere.

      Eventually the researchers would like to extend the study to solid precipitation like snowflakes and hail, although the math there will be more complicated. “That adage that every snowflake is unique is true,” Loftus says.

      The work is a first step toward understanding precipitation in general, says astronomer Björn Benneke of the University of Montreal, who discovered water vapor in the atmosphere of K2 18b but was not involved in the new study. “That’s what we are all striving for,” he says. “To develop a kind of global understanding of how atmospheres and planets work, and not just be completely Earth-centric.” More

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      Earth sweeps up 5,200 tons of extraterrestrial dust each year

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      As our planet orbits the sun, it swoops through clouds of extraterrestrial dust — and several thousand metric tons of that material actually reaches Earth’s surface every year, new research suggests.

      During three summers in Antarctica over the past two decades, researchers collected more than 2,000 micrometeorites from three snow pits that they’d dug. Extrapolating from this meager sample to the rest of the world, tiny pebbles from space account for a whopping 5,200 metric tons of weight gain each year, researchers report in the April 15 Earth and Planetary Science Letters.

      Much of Antarctica is the perfect repository for micrometeorites because there’s no liquid water to dissolve or otherwise destroy them, says Jean Duprat, a cosmochemist at Sorbonne University in Paris (SN: 5/29/20). Nevertheless, collecting the samples was no easy chore.

      First, Duprat and colleagues had to dig down two meters or more to reach layers of snow deposited before 1995, the year when researchers set up a field station at an inland site dubbed Dome C. Then they used ultraclean tools to collect hundreds of kilograms of snow, melt it and sieve the tiny treasures from the frigid water.

      To hunt for micrometeorites that have fallen to Antarctica in recent decades, researchers dig trenches (pictured) to collect snow that is later melted and then sieved for the space dust.J. Duprat, C. Engrand, CNRS Photothèque

      In all, the team found 808 spherules that had partially melted as they blazed through Earth’s atmosphere and another 1,280 micrometeorites that showed no such damage. The particles ranged in size from 30 to 350 micrometers across and all together weigh mere fractions of a gram. But the micrometeorites were all found within three areas totaling just a few square meters, the merest fraction of Earth’s surface. Assuming that particles of space dust are just as likely to fall in Antarctica as anywhere else let the team estimate how much dust fell over the entire planet.

      The team’s findings “are a wonderful complement to previous studies,” says Susan Taylor, a geologist at the Cold Regions Research and Engineering Laboratory in Hanover, N.H., who was not involved in the new study. That’s because Duprat and colleagues found a lot of the small stuff that would have dissolved elsewhere, she notes.

      About 80 percent of the micrometeorites originate from comets that spend much of their orbits closer to the sun than Jupiter, the researchers estimate. Much of the rest probably derive from collisions of objects in the asteroid belt. All together, these tiny particles deliver somewhere between 20 and 100 metric tons of carbon to Earth each year, Duprat and colleagues suggest, and could have been an important source of carbon-rich compounds such as amino acids early in Earth’s history (SN: 12/4/20). More

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      A record-breaking, oxygen-starved galaxy may be full of gigantic stars’ shrapnel

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      The most oxygen-poor star-forming galaxy ever found hints that the first galaxies to arise after the universe’s birth glittered with supermassive stars that left behind big black holes.

      Such galaxies are rare now because almost as soon as a galaxy initiates star formation, massive stars produce huge amounts of oxygen, which is the most abundant element in the cosmos after hydrogen and helium. Astronomers prize the few such galaxies found close to home because they offer a glimpse of what conditions were like in the very early universe, before stars had made much oxygen (SN: 8/7/19).

      The new galaxy’s oxygen-to-hydrogen ratio — a standard measure of relative oxygen abundance in the cosmos — is well under 2 percent of the sun’s, researchers report in a paper to appear in the Astrophysical Journal and posted online March 22 at arXiv.org.

      “It is quite difficult to pick up such a rare object,” says astrophysicist Takashi Kojima, who, along with colleagues, made the discovery while he was at the University of Tokyo.

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      Named HSC J1631+4426, the record-breaking galaxy, found by using the Subaru Telescope in Hawaii, is 430 million light-years from Earth in the constellation Hercules. The galaxy is a dwarf, with far fewer stars to create oxygen than the Milky Way has. Those relatively few stars have given the runt just a pinch of oxygen: one oxygen atom for every 126,000 hydrogen atoms. That’s only 1.2 to 1.6 percent of the oxygen level in the sun.

      “Any new galaxy is good,” says Trinh Thuan, an astronomer at the University of Virginia in Charlottesville who helped find the previous champion four years ago. “We’re counting the number of [very oxygen-poor galaxies] in the palm of our hand.” The new galaxy’s oxygen-to-hydrogen ratio is 83 percent that of the previous record holder, J0811+4730, which is 620 million light-years away in the constellation Lynx.

      A newly discovered galaxy has only about half the oxygen-to-hydrogen ratio of I Zwicky 18 (pictured), which once held the record for the most oxygen-poor star-forming galaxy known.NASA, ESA, A. Aloisi/Space Telescope Science Institute and European Space Agency.

      In HSC J1631+4426, Kojima and his colleagues also find odd abundances of another chemical element: iron. While the overall amount of iron in the galaxy is low, “we discovered that the iron-to-oxygen abundance ratio is surprisingly high,” he says.

      The same pattern also appears in the oxygen-poor galaxy in Lynx. In contrast, ancient stars in the Milky Way usually have little iron relative to oxygen. That’s because newborn stars get most of their iron from the explosions of long-lived stars. Those explosions had not occurred by the time the Milky Way’s oldest stars formed. But in the two nearly pristine galaxies, the amount of iron relative to oxygen is as high as that of the sun, which acquired large amounts of both elements from previous generations of stars.

      “This is a very unusual pattern, and it’s not obvious how to explain that,” says Volker Bromm, an astrophysicist at the University of Texas at Austin who was not involved with the discovery.

      Just before Kojima earned his Ph.D. in 2020, he hit upon a possible explanation: High-mass stars in dense star clusters merged together to make stellar goliaths more than 300 times as massive as the sun. These superstars then exploded and showered their galactic homes with both iron and oxygen, leading to high iron-to-oxygen ratios in the two primitive galaxies as well as a source of what little oxygen exists there.

      No stars this massive are known to exist in the modern Milky Way. But Kojima says their presence in the two most oxygen-poor star-making galaxies suggests that primordial galaxies had them too.

      When the superstars died, they should have left behind intermediate-mass black holes, which are more than 100 times as massive as the sun (SN: 9/2/20). That’s about 10 times as massive as typical black holes, which can form when bright stars die.

      Kojima’s team sees evidence for these big black holes in the newly discovered galaxy. Gas swirling around such large black holes should get so hot it emits high-energy photons, or particles of light. Because of their high energy, these photons would tear electrons even from helium atoms, which cling tightly to their electrons, and turn the atoms into positively charged ions. Sure enough, the galaxy in Hercules emits a wavelength of blue light that comes from just such helium ions.

      The record-breaking galaxy is “an exciting preview of things to come,” Bromm says. In coming years, he says, enormous telescopes will open that will find even more extreme galaxies (SN: 1/10/20). “Then we will have a wonderfully complementary way to learn about the early universe.” More

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      A meteor may have exploded over Antarctica 430,000 years ago

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      Seventeen tiny particles recovered from a flat-topped mountain in eastern Antarctica suggest that a space rock shattered low in the atmosphere over the ice-smothered continent about 430,000 years ago.

      The nickel- and magnesium-rich bits were sifted from more than 6 kilograms of loose sediments collected atop the 2,500-meter-tall summit of Walnumfjellet, says Matthias van Ginneken, a cosmochemist at the University of Kent in England. Their exotic chemistry doesn’t match Earth rocks, but it does match the proportions of elements seen in a type of meteorite called a carbonaceous chondrite, van Ginneken and his colleagues report March 31 in Science Advances.

      Most of the particles range in size from 0.1 to 0.3 millimeters across, and more than half consist of spherules that are fused together into odd-shaped globs. The elemental mix in the spherules closely matches that of particles found at two other far-flung sites in Antarctica— one more than 2,750 kilometers away — which suggests that all of the materials originated in the same event. Because the other particles were found in ice cores and dated to about 430,000 years ago, the team presumes that the newly found particles from Walnumfjellet fell then too.  

      The chemistry of nickel- and magnesium-rich spherules (pictured) found on a mountaintop in Antarctica match that of a certain type of stony meteorites.Scott Peterson/micro-meteorites.com

      The chemistry of nickel- and magnesium-rich spherules (pictured) found on a mountaintop in Antarctica match that of a certain type of stony meteorites.Scott Peterson/micro-meteorites.com

      The meteor that broke up over Antarctica was between 100 to 150 meters across, the team’s simulations suggest, and probably burst at low altitude. Blast waves may have pummeled a 100,000-square-kilometer area of the ice sheet, the team estimates. The explosion left no crater, but peak temperatures where the plume of hot gases reached Earth’s surface would have hit 5,000° Celsius and may have melted up to a few centimeters of ice. A similar airburst over a densely populated area today would result in millions of casualties and severely damage an area hundreds of kilometers across (SN: 5/2/17). More

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      Physicists’ devotion to symmetry has led them astray before

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      Second of two parts

      Physicists have a lot in common with Ponce de León and U2’s Bono. After decades of searching, they aren’t getting any younger. And they still haven’t found what they’re looking for.

      In this case, the object of the physicists’ quest is SUSY. SUSY is not a real person or even a fountain relevant to aging in any way. It’s a mathematical framework based on principles of symmetry that could help physicists better explain the mysteries of the universe. Many experts believe that particles predicted by SUSY are the weakly interacting massive particles, or WIMPs, that supposedly make up the invisible “dark matter” lurking throughout the cosmos.

      So far, though, SUSY has been something of a disappointment. Despite multiple heroic searches, SUSY has remained concealed from view. Maybe it is a mathematical mirage.

      If SUSY does turn out to be a myth, it won’t be the first time that symmetry has led science on a wild WIMP chase. Reasoning from the symmetry of circular motion originally suggested the existence of a new form of matter out in space more than two millennia ago. Devotion to that symmetry blinded science to the true nature of the solar system and planetary motion for the next 19 centuries.

      You can blame Plato and Aristotle. In their day, ordinary matter supposedly consisted of four elements: earth, air, fire and water. Aristotle built an elaborate theory of motion based on those elements. He insisted that they naturally moved in straight lines; earth and water moving straight down (toward the center of the world), air and fire moving straight up. In the heavens, though, Aristotle noticed that motion appeared to be circular, as the stars rotated around the nighttime sky. “Our eyes tell us that the heavens revolve in a circle,” he wrote in On the Heavens. Since the known four elements all moved in a straight line, Aristotle deduced that the heavens must consist of a fifth element, called aether — absent on Earth but predominant in space.

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      Plato, on theoretical rather than observational grounds, had already insisted that circularity’s symmetry signified perfection, and therefore circular motion should be required in the heavens. And so for centuries, the assumption that celestial motion must be circular held a stranglehold on natural philosophers attempting to understand of the universe. As late as the 16th century, Copernicus was willing to depose Aristotle’s Earth from the middle of everything but still believed that the Earth and other planets revolved around the sun with a combination of circular motions. Another half century passed before Kepler established that planetary orbits are elliptical, not circular.

      Aristotle’s belief in an exotic form of matter in space is not so different from the picture scientists paint of the heavens today, albeit in a rather more rigorous and sophisticated theoretical way. Dark matter predominates in space, astronomers believe; it is inferred to exist from gravitational effects altering the motions of stars and galaxies. And physicists have determined that the dark matter cannot (for various noncircular reasons) be made of the same ordinary matter found on Earth.

      SUSY particles have long been one of the most popular proposals for the identity of this cosmic dark matter, based on more complicated notions of symmetry than those available to Plato and Aristotle. And since the onset of the 20th century, symmetry math has generated an astounding string of scientific successes. From Einstein’s relativity to the theory of elementary particles and forces, symmetry considerations now form the core of science’s understanding of nature.  

      These mathematical forms of symmetry are more elaborate examples of symmetry as commonly understood: a change that leaves things looking like they did before. A perfectly symmetric face looks the same when a mirror swaps left with right. A perfect sphere’s appearance is not altered when you rotate it to see the other side. Rotate a snowflake by any multiple of 60 degrees and you see the same snowflake.

      In a similar way, more sophisticated mathematical frameworks, known as symmetry groups, describe aspects of the physical world, such as time and space or the families of subatomic particles that make up matter or transmit forces. Symmetries in the equations of such math can even predict previously unknown phenomena. Symmetry in the equations describing subatomic particles, for instance, revealed that for each particle nature allowed an antimatter particle, with opposite electric charge.

      In fact, all the known ordinary matter and force particles fit neatly into the mathematical patterns described by symmetry groups. But none of those particles can explain the dark matter.

      SUSY particles as a dark matter possibility emerged in the 1970s and 1980s, when theorists proposed an even more advanced symmetry system. That math, called supersymmetry (hence SUSY), suggested the existence of a “super” partner particle for each known particle: a force-particle partner for every matter particle, and a matter-particle partner for every force particle. It was an elegant concept mathematically, and it solved (or at least ameliorated) some other vexing theoretical problems. Plus, of the super partner particles it predicted, the lightest one (whichever one that was) seemed likely to be a perfect dark matter WIMP.

      Alas, efforts to detect WIMPs (which should be hitting the Earth all the time) have almost all failed to find any. One experiment that did claim a WIMP detection seems to be on shaky ground — a new experiment, using the same method and materials, reports no such WIMP evidence. And attempts to produce SUSY particles in the world’s most powerful particle accelerator, the Large Hadron Collider, have also come up empty.

      Some physicists have therefore given up on SUSY. And perhaps supersymmetry has been as misleading as the Greek infatuation with circular motion. But the truth is that SUSY is not a theory that can be slain by a single experiment. It is a more nebulous mathematical notion, a framework within which many specific theories can be constructed.   

      “You can’t really kill SUSY because it’s not a thing,” physicist Patrick Stengel of the International Higher School of Advanced Studies in Trieste, Italy, said at a conference in Washington, D.C., in 2019. “It’s not an idea that you can kill. It’s basically just a framework for a bunch of ideas.”

      At the same conference, University of Texas at Austin physicist Katherine Freese pointed out that there was never any guarantee that the Large Hadron Collider would discover SUSY. “Even before the LHC got built, there were a lot of people who said, well, it might not go to a high enough energy,” she said.

      So SUSY may yet turn out to be an example of symmetry that leads physics to success. But just in case, physicists have pursued other dark matter possibilities. One old suggestion that has recently received renewed interest is a lightweight hypothetical particle called an axion (SN: 3/24/20).

      Of course, if axions do exist, symmetry fans could still rejoice — the motivation for proposing the axion to begin with was resolving an issue with yet another form of symmetry. More

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      Uranium ‘snowflakes’ could set off thermonuclear explosions of dead stars

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      Tiny crystals of uranium could set off massive explosions within a dead star, physicists propose, making for a cosmic version of a thermonuclear bomb.

      Expired stars called white dwarfs slowly cool as they age. In the process, heavy elements such as uranium begin to crystalize, forming “snowflakes” in the stars’ cores. If enough uranium clumps together — about the mass of a grain of sand — it could initiate a chain of nuclear fission reactions, or the splitting of atomic nuclei.

      Those reactions could raise temperatures within the star, setting off nuclear fusion — the merging of atomic nuclei — and generating an enormous explosion that destroys the star, two physicists calculate in a paper published March 29 in Physical Review Letters. The effect is akin to a hydrogen bomb, a powerful thermonuclear weapon in which fission reactions trigger fusion, says Matt Caplan of Illinois State University in Normal. The scenario is still hypothetical, Caplan admits — more research is needed to determine if uranium snowflakes could really spur a stellar detonation.

      White dwarfs are already known to be explosion-prone: They’re the source of blasts called type 1a supernovas. Typically, these explosions happen when a white dwarf pulls matter off a companion star (SN: 3/23/16). The researchers’ uranium snowflake proposal is an entirely new mechanism that might explain a small fraction of type 1a supernovas, without the need for another star. More