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      Signs of a hidden Planet Nine in the solar system may not hold up

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      Planet Nine might be a mirage. What once looked like evidence for a massive planet hiding at the solar system’s edge may be an illusion, a new study suggests.
      “We can’t rule it out,” says Kevin Napier, a physicist at the University of Michigan in Ann Arbor. “But there’s not necessarily a reason to rule it in.”
      Previous work has suggested that a number of far-out objects in the solar system cluster in the sky as if they are being shepherded by an unseen giant planet, at least 10 times the mass of Earth. Astronomers dubbed the invisible world Planet Nine or Planet X.
      Now, a new analysis of 14 of those remote bodies shows no evidence for such clustering, knocking down the primary reason to believe in Planet Nine. Napier and colleagues reported the results February 10 at arXiv.org in a paper to appear in the Planetary Science Journal.

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      The idea of a distant planet lurking far beyond Neptune received a surge in interest in 2014, when astronomers Chad Trujillo of Northern Arizona University and Scott Sheppard of the Carnegie Institution for Science reported a collection of distant solar system bodies called trans-Neptunian objects with strangely bunched-up orbits (SN: 11/14/14).
      In 2016, Caltech planetary scientists Mike Brown and Konstantin Batygin used six trans-Neptunian objects to refine the possible properties of Planet Nine, pinning it to an orbit between 500 and 600 times as far from the sun as Earth’s (SN: 7/5/16).
      But those earlier studies all relied on just a handful of objects that may not have represented everything that’s out there, says Gary Bernstein, an astronomer at the University of Pennsylvania. The objects might have seemed to show up in certain parts of the sky only because that’s where astronomers happened to look.
      “It’s important to know what you couldn’t see, in addition to what you did see,” he says.
      To account for that uncertainty, Napier, Bernstein and colleagues combined observations from three surveys — the Dark Energy Survey, the Outer Solar System Origins Survey and the original survey run by Sheppard and Trujillo — to assess 14 trans-Neptunian objects, more than twice as many as in the 2016 study. These objects all reside between 233 and 1,560 times as far from the sun as Earth.
      The team then ran computer simulations of about 10 billion fake trans-Neptunian objects, distributed randomly all around the sky, and checked to see if their positions matched what the surveys should be able to see. They did.
      “It really looks like we just find things where we look,” Napier says. It’s sort of like if you lost your keys at night and searched for them under a streetlamp, not because you thought they were there, but because that’s where the light was. The new study basically points out the streetlamps.
      “Once you see where the lampposts really are, it becomes more clear that there is some serious selection bias going on with the discovery of these objects,” Napier says. That means the objects are just as likely to be distributed randomly across the sky as they are to be clumped up.
      That doesn’t necessarily mean Planet Nine is done for, he says.
      “On Twitter, people have been very into saying that this kills Planet Nine,” Napier says. “I want to be very careful to mention that this does not kill Planet Nine. But it’s not good for Planet Nine.”
      There are other mysteries of the solar system that Planet Nine would have neatly explained, says astronomer Samantha Lawler of the University of Regina in Canada, who was not involved in the new study. A distant planet could explain why some far-out solar system objects have orbits that are tilted relative to those of the larger planets or where proto-comets called centaurs come from (SN: 8/18/20). That was part of the appeal of the Planet Nine hypothesis.
      “But the entire reason for it was the clustering of these orbits,” she says. “If that clustering is not real, then there’s no reason to believe there is a giant planet in the distant solar system that we haven’t discovered yet.”
      Batygin, one of the authors of the 2016 paper, isn’t ready to give up. “I’m still quite optimistic about Planet Nine,” he says. He compares Napier’s argument to seeing a group of bears in the forest: If you see a bunch of bears to the east, you might think there was a bear cave there. “But Napier is saying the bears are all around us, because we haven’t checked everywhere,” Batygin says. “That logical jump is not one you can make.”
      Evidence for Planet Nine should show up only in the orbits of objects that are stable over billions of years, Batygin adds. But the new study, he says, is “strongly contaminated” by unstable objects — bodies that may have been nudged by Neptune and lost their position in the cluster or could be on their way to leaving the solar system entirely. “If you mix dirt with your ice cream, you’re going to mostly taste dirt,” he says.
      Lawler says there’s not a consensus among people who study trans-Neptunian objects about which ones are stable and which ones are not.
      Everyone agrees, though, that in order to prove Planet Nine’s existence or nonexistence, astronomers need to discover more trans-Neptunian objects. The Vera Rubin Observatory in Chile should find hundreds more after it begins surveying the sky in 2023 (SN: 1/10/20).
      “There always may be some gap in our understanding,” Napier says. “That’s why we keep looking.” More

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      The first black hole ever discovered is more massive than previously thought

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      The first black hole ever discovered still has a few surprises in store.
      New observations of the black hole–star pair called Cygnus X-1 indicate that the black hole weighs about 21 times as much as the sun — nearly 1.5 times heavier than past estimates. The updated mass has astronomers rethinking how some black hole–forming stars evolve. For a star-sized, or stellar, black hole that massive to exist in the Milky Way, its parent star must have shed less mass through stellar winds than expected, researchers report online February 18 in Science.
      Knowing how much mass stars lose through stellar winds over their lifetimes is important for understanding how these stars enrich their surroundings with heavy elements. It’s also key to understanding the masses and compositions of those stars when they explode and leave behind black holes.
      The updated mass measurement of Cygnus X-1 is “a big change to an old favorite,” says Tana Joseph, an astronomer at the University of Amsterdam not involved in the work. Stephen Hawking famously bet physicist Kip Thorne that the Cygnus X-1 system, discovered in 1964, did not include a black hole — and conceded the wager in 1990, when scientists had broadly accepted that Cygnus X-1 contained the first known black hole in the universe (SN: 4/10/19).

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      Astronomers got a new look at Cygnus X-1 using the Very Long Baseline Array, or VLBA. This network of 10 radio dishes stretches across the United States, from Hawaii to the Virgin Islands, collectively forming a continent-sized radio dish. In 2016, the VLBA tracked radio-bright jets of material spewing out of Cygnus X-1’s black hole for six days (the time it took for the black hole and its companion star to orbit each other once). Those observations offered a clear view of how the black hole’s position in space shifted over the course of its orbit. That, in turn, helped researchers refine the estimated distance to Cygnus X-1.
      The new observations suggest that Cygnus X-1 is about 7,200 light-years from Earth, rather than the previous estimate of about 6,000 light-years. This implies that the star in Cygnus X-1 is even brighter, and therefore bigger, than astronomers thought. The star weighs about 40.6 suns, the researchers estimate. The black hole must also be more massive in order to explain its gravitational tug on such a massive star. The black hole weighs about 21.2 suns — much heftier than its previously estimated 14.8 solar masses, the scientists say. 
      The new mass measurement for Cygnus X-1’s black hole is so big that it challenges astronomers’ understanding of the massive stars that collapse to form black holes, says study coauthor Ilya Mandel, an astrophysicist at Monash University in Melbourne, Australia.
      “Sometimes stars are born with quite high masses — there are observations of stars being born with masses of well over 100 solar masses,” Mandel says. But such enormous stars are thought to shed much of their weight through stellar winds before turning into black holes. The bigger the star and the more heavy elements it contains, the stronger its stellar winds. So in heavy element–rich galaxies such as the Milky Way, big stars — no matter their starting mass — are supposed to shrink down to about 15 solar masses before collapsing into black holes.
      Cygnus X-1’s 21-solar-mass black hole undermines that idea.
      The LIGO and Virgo gravitational wave detectors have discovered black holes weighing tens of solar masses in other galaxies (SN: 1/21/21). But that is probably because LIGO peers at distant galaxies that existed earlier in the universe, Joseph says. Back then, fewer heavy elements existed, so stellar winds were weaker. With the new Cygnus X-1 measurement, “now we have to say, hang on, we’re in a [heavy element]–rich environment compared to the early universe … but we still managed to make this really massive black hole,” she says, “so maybe we’re not losing as much mass through stellar winds as we initially thought.” More

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      Here’s how to watch NASA’s Perseverance lander touch down on Mars

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      All eyes are on Mars — and all ears, too. When NASA’s Perseverance rover touches down on the Red Planet on February 18, the landing will be recorded with sight, sound and maybe even touch.
      The rover will cap off a month of Mars arrivals from space agencies around the world (SN: 7/30/20). Perseverance joins Hope, the first interplanetary mission from the United Arab Emirates, which successfully entered Mars orbit on February 9; and Tianwen-1, China’s first Mars mission, which arrived on February 10 and will deploy a rover to the Martian surface in May.
      NASA will broadcast Perseverance’s landing on YouTube starting at 2:15 p.m. EST. The actual moment of touchdown is expected at approximately 3:55 p.m. EST. Perseverance is designed to explore an ancient river delta called Jezero crater, searching for signs of ancient life and collecting rocks for a future mission to return to Earth (SN: 7/28/20).
      The rover will use the landing system pioneered by its predecessor, Curiosity, which has been exploring Mars since 2012 (SN: 8/6/12). But in a first for Mars touchdowns, this rover will record its own landing with dedicated cameras and a microphone.
      As the craft carrying Perseverance zooms through the thin Martian atmosphere, three cameras will look up at the parachute slowing it down from supersonic speeds. When a rocket-powered “sky crane” platform lowers the rover to the ground, a fourth camera on the platform will record the rover’s descent. Another camera on the rover will look back up at the platform, and a sixth camera will look at the ground.
      Perseverance will use the “sky crane” landing system pioneered by its predecessor, Curiosity. The landing involves dangling the rover from a floating platform on cables and touching down directly on its wheels.JPL-Caltech/NASA
      Perseverance will use the “sky crane” landing system pioneered by its predecessor, Curiosity. The landing involves dangling the rover from a floating platform on cables and touching down directly on its wheels.JPL-Caltech/NASA
      “The goal is to see the video and the action of getting from high up in the atmosphere down to the surface,” says engineer David Gruel of NASA’s Jet Propulsion Laboratory in Pasadena, who was the engineering lead for that six-camera system, called EDL-Cam. He hopes every engineer on the team has an image of the rover hanging below the descent stage as their computer desktop background six months from now.
      Because it will take more than 11 minutes for signals to travel between Earth and Mars, the cameras won’t stream the landing movie in real time. And after Perseverance lands, engineers will be focused on making sure the rover is healthy and able to collect science data, so the landing videos won’t be among the first data sent back. Gruel expects to be able to share what the rover saw four days after landing, on February 22.

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      Perseverance will also carry microphones to record first-ever audio of a Mars landing. Unlike the landing cameras, the microphones will continue to work after touchdown, hopefully helping the engineering team keep track of the rover’s health. Motors sound different when they get clogged with dust, for instance, Gruel says. The team will hear the sound of the rover’s wheels crunching across the Martian surface, and maybe the sound of the wind blowing.
      “Are we going to hear a dust devil? What might a dust devil sound like? Could we hear rocks rolling down a hill?” Gruel asks. “You never know what we might stumble onto.”
      Sound will add a way to share Mars with people who have trouble seeing, Gruel notes. “It might appeal to a whole other element of the population who might not have been able to experience past missions the same way,” he says.
      [embedded content]
      Watch NASA’s live coverage of the Perseverance landing here starting at 2:15 p.m. EST.
      Elsewhere on Mars, the InSight lander will be listening to the landing too (SN: 2/24/20). The lander’s seismometer may be able to feel vibrations when two tungsten weights that Perseverance carried to Mars for stability smack into the ground before the rover lands, geophysicist Benjamin Fernando of the University of Oxford and colleagues report in a paper posted December 3 to eartharxiv.org and submitted to JGR Planets.
      “No one’s ever tried to do this before,” Fernando says.
      The ground will move by at most 0.1 nanometers per second, Fernando and colleagues calculated. “It’s incredibly small,” he says. “But the seismometer is also incredibly sensitive.”
      The team may be able to catch that tiny signal because they know exactly when and where the impact will happen. If the lander does pick up the signal, it will tell scientists something about how fast seismic waves travel through the ground, a clue to the details of Mars’ interior structure. And even if they don’t feel anything, that will put limits on the waves’ speed. “It still teaches us something,” Fernando says.
      The InSight team hopes to also feel vibrations from Tianwen-1 when its rover touches down in May. “Detecting one would be great,” Fernando says. “Detecting two would be like, amazing.” More

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      The number of Milky Way nova explosions per year has been pinned down

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      Each year, astronomers discover nova explosions in the Milky Way that cause dim stars to flare up and emit far more light than the sun before they fade again. But our galaxy is so big and dusty that no one knows how many of these eruptions occur throughout its vast domain, where they fling newly minted chemical elements into space.
      Now, by detecting the explosions’ infrared light, which penetrates dust better than visible light does, Caltech astronomer Kishalay De and his colleagues have estimated how often these outbursts occur in the Milky Way. Knowing the nova rate is vital for determining how much these explosions have contributed to the galaxy’s chemical makeup by creating new elements.
      The updated tally puts the rate at 46, give or take 13, a year, the team reports January 11 at arXiv.org. Past estimates of the nova rate have ranged from just 10 a year to 300.
      A nova arises from a binary star — two stars circling each other. One is a white dwarf, a dense star that’s about as small as Earth but approximately as massive as the sun. After the white dwarf receives gas from its companion, the gas explodes, making the dim star shine brilliantly. The nova does not destroy the star, unlike a supernova, which marks a star’s death.
      After observing the sky from Palomar Observatory in California for 17 months, De and colleagues detected 12 nova explosions. Estimating the number of missed outbursts, the astronomers deduced the yearly nova rate. Their rate is similar to, but more precise than, one reported four years ago by Allen Shafter, an astronomer at San Diego State University who pegged the annual nova rate at between 27 and 81.
      “They’re doing a wonderful job,” says Bradley Schaefer, an astrophysicist at Louisiana State University in Baton Rouge, who notes that searching at infrared wavelengths is ideal for finding distant explosions obscured by the galaxy’s dust. “They have an awful lot of really good data.”
      The more precise rate helps firm up estimates for how much these explosions have altered the galaxy’s chemical composition. In this regard, it’s hard for a mere nova to compete with a supernova explosion, which, though rare, releases far more newly produced elements than a nova does. But if the annual nova rate is around 50, then certain scarce isotopes on Earth — such as lithium-7, carbon-13, nitrogen-15 and oxygen-17 — arose partially or mostly in nova explosions, says Sumner Starrfield, an astronomer at Arizona State University in Tempe who was not involved with this study. The blasts then spirited these isotopes away before additional nuclear reactions could destroy them. More

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      Fossil mimics may be more common in ancient rocks than actual fossils

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      When it comes to finding fossils of very ancient microbial life — whether on Earth or on other worlds, such as Mars — the odds are just not in our favor.
      Actual microbial life-forms are much less likely to become safely fossilized in rocks compared with nonbiological structures that happen to mimic their shapes, new research finds. The finding suggests that Earth’s earliest rocks may contain abundant tiny fakers — minuscule objects masquerading as fossilized evidence of early life — researchers report online January 28 in Geology.
      The finding is “at the very least a cautionary tale,” says study author Julie Cosmidis, a geomicrobiologist at the University of Oxford.
      Tiny, often enigmatic structures found in some of Earth’s oldest rocks, dating back to more than 2.5 billion years, can offer tantalizing hints of the planet’s earliest life. And the hunt for ever-more-ancient signs of life on Earth has sparked intense debate — in part because the farther back in time you go, the harder it is to interpret tiny squiggles, filaments and spheres in the rock (SN: 1/3/20). One reason is that the movements of Earth’s tectonic plates over time can squeeze and cook the rocks, deforming and chemically altering tiny fossils, perhaps beyond recognition.

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      But an even more pernicious and contentious problem is that such tiny filaments or spheres may not be biological in origin at all. Increasingly, scientists have found that nonbiological chemical processes can create similar shapes, suggesting the possibility of “false positives” in the biological record.
      One such discovery led to the new study, Cosmidis says. A few years ago, she and others were trying to grow bacteria and make them produce sulfur. “We were mixing sulfides with organic matter, and we started forming these objects,” she says. “We thought they were formed by the bacteria, because they looked so biological. But then we realized they were forming in laboratory tubes that happened to have no bacteria in them at all.”
      That led her to wonder about such processes happening in the rocks themselves. So she and others decided to examine what would happen if they tried to re-create the early formation stages of chert, a kind of compact, silica-rich rock common on the early Earth. “Microfossils are often found in chert formations,” says study coauthor Christine Nims, a geobiologist now at the University of Michigan in Ann Arbor. “Anything hosted in [chert] will be well-preserved.”
      Chert forms out of silica-rich water; the silica precipitates out of the water and accumulates, eventually hardening into rock. Cosmidis, Nims and colleagues added sulfur-containing bacteria called Thiothrix to solidifying chert to see what might happen during actual fossilization. To other chert samples, they added sulfur-containing “biomorphs,” spheres and filaments made of tiny crystals but shaped like bacteria.
      At first, nanoparticles of silica encrusted the bacteria and the biomorphs, Nims says. But after a week or so, the bacteria started to deform, their cells deflating from cylinders into flattened, unrecognizable ribbons as the sulfur inside the cells diffused out and reacted with the silica outside the cells, forming new minerals.
      The biomorphs, on the other hand, “had this impressive resiliency,” she says. Although they, too, lost sulfur to the surrounding solution, they kept their silica crust. As a result, “they kept their shape and showed very little change over time.” That endurance suggests that enigmatic structures found in the early rock record have a better chance of being pseudofossils, rather than actual fossils, the team says.
      In a new study, researchers produced twisted filament-shaped biomorphs (top) from the reactions of sulfide with prebiotic organic compounds. The biomorphs resemble possible microbial fossils (bottom, filaments indicated by red arrows) found in rocks dating to 3.5 billion years ago.From top: C. Nims; R.J. Baumgartner et al/Geology 2019
      The idea that once-living creatures are harder to preserve makes sense, says Sean McMahon, an astrobiologist at the University of Edinburgh who was not involved in the new study. “It’s not totally surprising,” he says. “We know that biomass does tend to break down quite quickly.”
      In fact, scientists have known for centuries that certain chemical reactions can act as “gardens” that “grow” strange-shaped mineral objects, twisting into tubes or sprouting branches or otherwise mimicking the weirdness of life. “There’s a complacency about it, a misconception that we kind of know all this and it’s already been dealt with,” McMahon says.
      Strategies to deal with this conundrum have included looking for particular structures — such as mound-shaped stromatolites — or chemical compounds in a potential fossil that are thought to be uniquely formed or modified by the presence of life (SN: 10/17/18). Those criteria are the product of decades of field studies, through which scientists have amassed a vast reference dataset of fossil structures, against which researchers can compare and evaluate any new discoveries.
      “Anything we find, we can look at through that lens,” McMahon says. But what’s lacking is a similarly rich dataset for how such structures might form in the absence of life. This study, he says, highlights that attempts “to define criteria for recognizing true fossils in very ancient rocks are premature, because we don’t yet know enough about how nonbiological processes mimic true fossils.”
      It’s an increasingly urgent problem with rising stakes, as NASA’s Perseverance rover is about to set down on Mars to begin a new search for traces of life in ancient rocks (SN: 7/28/20), he adds. “Paleontologists and Mars exploration scientists should take [this study] very seriously.” More

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      Two exoplanet families redefine what planetary systems can look like

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      Two tightly packed families of exoplanets are pushing the boundaries of what a planetary system can look like. New studies of the makeup of worlds orbiting two different stars show a wide range of planetary possibilities, all of them different from our solar system.
      “When we study multiplanet systems, there’s simply more information kept in these systems” than any single planet by itself, says geophysicist Caroline Dorn of the University of Zurich. Studying the planets together “tells us about the diversity within a system that we can’t get from looking at individual planets.”
      Dorn and colleagues studied an old favorite planetary system called TRAPPIST-1, which hosts seven Earth-sized planets orbiting a small dim star about 40 light-years away. Another team studied a recently identified system called TOI-178, which has at least six planets — three already known and three newly found — circling a bright, hot star roughly 200 light-years away.
      Both systems offer planetary scientists an advantage over the more than 3,000 other exoplanet families spotted to date: All seven planets in TRAPPIST-1 and all six in TOI-178 have well-known masses and radii. That means planetary scientists can figure out their densities, a clue to the planets’ composition (SN: 5/11/18).

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      The two systems also offer another advantage: The planets are packed in so close to their stars that most are engaged in a delicate orbital dance called a resonance chain. Every time an outer planet completes an orbit around its star, some of its closer-in sibling planets complete multiple orbits.
      Resonance chains are fragile arrangements, and knocking a planet even slightly out of its orbit can destroy them. That means the TRAPPIST-1 and TOI-178 systems must have formed slowly and gently, says astronomer Adrien Leleu of the University of Geneva.
      [embedded content]
      TOI-178’s planets are engaged in a delicate orbital dance called a resonance chain that suggests the system formed gently. This video illustrates this rhythmic dance: as an outer planet completes one full orbit, the inner planets complete multiple orbits. Each full and half orbit is assigned a musical note. When planets align, the notes harmonize.
      “We don’t think there could have been giant impacts, or strong interactions where one planet ejected another planet,” Leleu says. That gentle evolution gives astronomers a unique opportunity to use TRAPPIST-1 and TOI-178 as testbeds for planetary theory.
      In a pair of papers, two teams describe these systems in unprecedented detail. Both buck the trend astronomers expected from theories of how planetary systems form.
      In the TOI-178 system, the planets’ densities are all jumbled up, Leleu and colleagues report January 25 in Astronomy & Astrophysics.
      “In the most vanilla scenario, we expect that planets farther from the star…would have larger components of hydrogen and helium gas than the planets closer in,” says astrophysicist Leslie Rogers of the University of Chicago, who was not involved in either study. The closer to the star, the denser a planet should be. That’s because farther-out planets probably formed where it’s cold, and there was more low-density material like frozen water, rather than rock, to begin with. Plus, starlight can strip atmospheres from close-in planets more easily than far-out ones, leaving the inner planets with thinner atmospheres — or no atmospheres at all (SN: 7/1/20).
      TOI-178 flouts that trend entirely. The innermost planets seem to be rocky, with densities similar to Earth’s. The third one is “very fluffy,” Leleu says, with a density like Jupiter’s, but in a much smaller planet. The next planet out has a density like Neptune’s, about one-third Earth’s density. Then, there’s one with about 60 percent Earth’s density, still fluffy enough to float if you could put it in a tub of water, and the final planet is Jupiter-like.
      “The orbits seem to point out that there was no strong evolution from [the system’s] formation,” Leleu says. “But the compositions are not what we would have expected from a gentle formation in the disk.”
      TRAPPIST-1’s planet septet, on the other hand, has an eerie self-similarity. Each world is roughly the same size as Earth, between 0.76 and 1.13 times Earth’s radius, astrophysicist Eric Agol of the University of Washington in Seattle and colleagues reported in 2017 (SN: 2/22/17). Plus, at least three of them appear to be in the star’s habitable zone, the region where temperatures might be right for liquid water.
      Now, Agol, Dorn and colleagues have made the most precise measurements of the TRAPPIST-1 masses yet. All seven worlds are almost identical to each other but slightly less dense than Earth, the team reports in the February Planetary Science Journal. That means the planets could be rocky yet have a lower proportion of heavy elements such as iron compared with Earth. Or it could mean they have more oxygen bound to the iron in their rocks, “basically rusting it,” Agol says.
      TRAPPIST-1’s seven planets seem to have similar compositions to each other, but different from Earth. They could have an Earthlike makeup but with a smaller iron-rich core (center), or have no core at all (left). They could also have deep oceans (right), but the inner three planets are probably too hot for that much water to last.JPL-Caltech/NASA
      TRAPPIST-1’s seven planets seem to have similar compositions to each other, but different from Earth. They could have an Earthlike makeup but with a smaller iron-rich core (center), or have no core at all (left). They could also have deep oceans (right), but the inner three planets are probably too hot for that much water to last.JPL-Caltech/NASA
      Oxidized iron wouldn’t form a planetary core, which could be bad news for life, Rogers says. No core might mean no magnetic field to protect the planets from the star’s damaging flares (SN: 3/5/18).
      However, it’s not clear how to form coreless planets. “There are propositions for how to form such planets, but we don’t actually have one candidate in the solar system where we see this,” Dorn says. The analogs in the solar system are all asteroid-sized bodies much less massive than Earth.
      Astronomers may soon get a better handle on the compositions of TRAPPIST-1’s planets. The James Webb Space Telescope, set to launch in October, will probe the planets’ atmospheres (if they have any) for signs of chemical elements that would reveal in more detail what they’re made of.
      The TRAPPIST-1 planets’ similarities to each other are not as surprising as the differences among TOI-178’s planets, Rogers says. But they’re still unexpected. If all the planets have identical compositions, then any formation model needs to explain that, she says.
      While these systems challenge astronomers’ views of what sorts of planets are possible, Dorn says, it will take discovering more multiplanet systems to tell how weird they truly are. More

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      Einstein’s theory of general relativity unveiled a dynamic and bizarre cosmos

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      Albert Einstein’s mind reinvented space and time, foretelling a universe so bizarre and grand that it has challenged the limits of human imagination. An idea born in a Swiss patent office that evolved into a mature theory in Berlin set forth a radical new picture of the cosmos, rooted in a new, deeper understanding of gravity.
      Out was Newton’s idea, which had reigned for nearly two centuries, of masses that appeared to tug on one another. Instead, Einstein presented space and time as a unified fabric distorted by mass and energy. Objects warp the fabric of spacetime like a weight resting on a trampoline, and the fabric’s curvature guides their movements. With this insight, gravity was explained.
      Einstein presented his general theory of relativity at the end of 1915 in a series of lectures in Berlin. But it wasn’t until a solar eclipse in 1919 that everyone took notice. His theory predicted that a massive object — say, the sun — could distort spacetime nearby enough to bend light from its straight-line course. Distant stars would thus appear not exactly where expected. Photographs taken during the eclipse verified that the position shift matched Einstein’s prediction. “Lights all askew in the heavens; men of science more or less agog,” declared a New York Times headline.
      Even a decade later, a story in Science News Letter, the predecessor of Science News, wrote of “Riots to understand Einstein theory” (SN: 2/1/30, p. 79). Apparently extra police had to be called in to control a crowd of 4,500 who “broke down iron gates and mauled each other” at the American Museum of Natural History in New York City to hear an explanation of general relativity.

      By 1931, physicist Albert A. Michelson, the first American to win a Nobel Prize in the sciences, called the theory “a revolution in scientific thought unprecedented in the history of science.”
      But for all the powers of divination we credit to Einstein today, he was a reluctant soothsayer. We now know that general relativity offered much more than Einstein was willing or able to see. “It was a profoundly different way of looking at the universe,” says astrophysicist David Spergel of the Simons Foundation’s Flatiron Institute in New York City, “and it had some wild implications that Einstein himself didn’t want to accept.” What’s more, says Spergel (a member of the Honorary Board of the Society for Science, publisher of Science News), “the wildest aspects of general relativity have all turned out to be true.”
      What had been masquerading as a quiet, static, finite place is instead a dynamic, ever-expanding arena filled with its own riot of space-bending beasts. Galaxies congregate in superclusters on scales vastly greater than anything experts had considered before the 20th century. Within those galaxies reside not only stars and planets, but also a zoo of exotic objects illustrating general relativity’s propensity for weirdness, including neutron stars, which pack a fat star’s worth of mass into the size of a city, and black holes, which pervert spacetime so strongly that no light can escape. And when these behemoths collide, they shake spacetime, blasting out ginormous amounts of energy. Our cosmos is violent, evolving and filled with science fiction–like possibilities that actually come straight out of general relativity.
      “General relativity opened up a huge stage of stuff for us to look at and try out and play with,” says astrophysicist Saul Perlmutter of the University of California, Berkeley. He points to the idea that the universe changes dramatically over its lifetime — “the idea of a lifetime of a universe at all is a bizarre concept” — and the idea that the cosmos is expanding, plus the thought that it could collapse and come to an end, and even that there might be other universes. “You get to realize that the world could be much more interesting even than we already ever imagined it could possibly be.”

      General relativity has become the foundation for today’s understanding of the cosmos. But the current picture is far from complete. Plenty of questions remain about mysterious matter and forces, about the beginnings and the end of the universe, about how the science of the big meshes with quantum mechanics, the science of the very small. Some astronomers believe a promising route to answering some of those unknowns is another of general relativity’s initially underappreciated features — the power of bent light to magnify features of the cosmos.
      Today’s scientists continue to poke and prod at general relativity to find clues to what they might be missing. General relativity is now being tested to a level of precision previously impossible, says astrophysicist Priyamvada Natarajan of Yale ​University. “General relativity expanded our cosmic view, then gave us sharper focus on the cosmos, and then turned the tables on it and said, ‘now we can test it much more strongly.’ ” It’s this testing that will perhaps uncover problems with the theory that might point the way to a fuller picture.
      And so, more than a century after general relativity debuted, there’s plenty left to foretell. The universe may turn out to be even wilder yet.
      Ravenous beasts
      Just over a century after Einstein unveiled general relativity, scientists obtained visual confirmation of one of its most impressive beasts. In 2019, a global network of telescopes revealed a mass warping spacetime with such fervor that nothing, not even light, could escape its snare. The Event Horizon Telescope released the first image of a black hole, at the center of galaxy M87 (SN: 4/27/19, p. 6).
      In 2019, the Event Horizon Telescope Collaboration released this first-ever image of a black hole, at the heart of galaxy M87. The image shows the shadow of the monster surrounded by a bright disk of gas.Event Horizon Telescope Collaboration
      “The power of an image is strong,” says Kazunori Akiyama, an astrophysicist at the MIT Haystack Observatory in Westford, Mass., who led one of the teams that created the image. “I somewhat expected that we might see something exotic,” Akiyama says. But after looking at the first image, “Oh my God,” he recalls thinking, “it’s just perfectly matching with our expectation of general relativity.”
      For a long time, black holes were mere mathematical curiosities. Evidence that they actually reside out in space didn’t start coming in until the second half of the 20th century. It’s a common story in the annals of physics. An oddity in some theorist’s equation points to a previously unknown phenomenon, which kicks off a search for evidence. Once the data are attainable, and if physicists get a little lucky, the search gives way to discovery.
      In the case of black holes, German physicist Karl Schwarzschild came up with a solution to Einstein’s equations near a single spherical mass, such as a planet or a star, in 1916, shortly after Einstein proposed general relativity. Schwarzschild’s math revealed how the curvature of spacetime would differ around stars of the same mass but increasingly smaller sizes — in other words, stars that were more and more compact. Out of the math came a limit to how small a mass could be squeezed. Then in the 1930s, J. Robert Oppenheimer and Hartland Snyder described what would happen if a massive star collapsing under the weight of its own gravity shrank past that critical size — today known as the “Schwarzschild radius” — reaching a point from which its light could never reach us. Still, Einstein — and most others — doubted that what we now call black holes were plausible in reality.
      The term “black hole” first appeared in print in Science News Letter. It was in a 1964 story by Ann Ewing, who was covering a meeting in Cleveland of the American Association for the Advancement of Science (SN: 1/18/64, p. 39). That’s also about the time that hints in favor of the reality of black holes started coming in.
      Just a few months later, Ewing reported the discovery of quasars — describing them in Science News Letter as “the most distant, brightest, most violent, heaviest and most puzzling sources of light and radio waves” (SN: 8/15/64, p. 106). Though not linked to black holes at the time, quasars hinted at some cosmic powerhouses needed to provide such energy. The use of X-ray astronomy in the 1960s revealed new features of the cosmos, including bright beacons that could come from a black hole scarfing down a companion star. And the motions of stars and gas clouds near the centers of galaxies pointed to something exceedingly dense lurking within.  
      Quasars (one illustrated) are so bright that they can outshine their home galaxies. Though baffling when first discovered, these outbursts are powered by massive, feeding black holes.Mark Garlick/Science Source
      Black holes stand out among other cosmic beasts for how extreme they are. The largest are many billion times the mass of the sun, and when they rip a star apart, they can spit out particles with 200 trillion electron volts of energy. That’s some 30 times the energy of the protons that race around the world’s largest and most powerful particle accelerator, the Large Hadron Collider.
      As evidence built into the 1990s and up to today, scientists realized these great beasts not only exist, but also help shape the cosmos. “These objects that general relativity predicted, that were mathematical curiosities, became real, then they were marginal. Now they’ve become central,” says Natarajan.
      We now know supermassive black holes reside at the centers of most if not all galaxies, where they generate outflows of energy that affect how and where stars form. “At the center of the galaxy, they define everything,” she says.
      Though visual confirmation is recent, it feels as though black holes have long been familiar. They are a go-to metaphor for any unknowable space, any deep abyss, any endeavor that consumes all our efforts while giving little in return.
      Real black holes, of course, have given plenty back: answers about our cosmos plus new questions to ponder, wonder and entertainment for space fanatics, a lost album from Weezer, numerous episodes of Doctor Who, the Hollywood blockbuster Interstellar.
      For physicist Nicolas Yunes of the University of Illinois at Urbana-Champaign, black holes and other cosmic behemoths continue to amaze. “Just thinking about the dimensions of these objects, how large they are, how heavy they are, how dense they are,” he says, “it’s really breathtaking.”
      [embedded content]
      In 2019, scientists gave us the first real picture of the supermassive black hole at the center of galaxy M87. How? We explain.
      Spacetime waves
      When general relativity’s behemoths collide, they disrupt the cosmic fabric. Ripples in spacetime called gravitational waves emanate outward, a calling card of a tumultuous and most energetic tango.
      Einstein’s math predicted such waves could be created, not only by gigantic collisions but also by explosions and other accelerating bodies. But for a long time, spotting any kind of spacetime ripple was a dream beyond measure. Only the most dramatic cosmic doings would create signals that were large enough for direct detection. Einstein, who called the waves gravitationswellen, was unaware that any such big events existed in the cosmos.
      Gravitational waves ripple away from two black holes that orbit each other before merging (shown in this simulation). The merging black holes created a new black hole that’s much larger than those found in previous collisions.Deborah Ferguson, Karan Jani, Deirdre Shoemaker and Pablo Laguna/Georgia Tech, Maya Collaboration
      Beginning in the 1950s, when others were still arguing whether gravitational waves existed in reality, physicist Joseph Weber sunk his career into trying to detect them. After a decade-plus effort, he claimed detection in 1969, identifying an apparent signal perhaps from a supernova or from a newly discovered type of rapidly spinning star called a pulsar. In the few years after reporting the initial find, Science News published more than a dozen stories on what it began calling the “Weber problem” (SN: 6/21/69, p. 593). Study after study could not confirm the results. What’s more, no sources of the waves could be found. A 1973 headline read, “The deepening doubt about Weber’s waves” (SN: 5/26/73, p. 338).
      Weber stuck by his claim until his death in 2000, but his waves were never verified. Nonetheless, scientists increasingly believed gravitational waves would be found. In 1974, radio astronomers Russell Hulse and Joseph Taylor spotted a neutron star orbiting a dense companion. Over the following years, the neutron star and its companion appeared to be getting closer together by the distance that would be expected if they were losing energy to gravitational waves. Scientists soon spoke not of the Weber problem, but of what equipment could possibly pick up the waves. “Now, although they have not yet seen, physicists believe,” Dietrick E. Thomsen wrote in Science News in 1984 (SN: 8/4/84, p. 76).
      It was a different detection strategy, decades in the making, that would provide the needed sensitivity. The Advanced Laser Interferometry Gravitational-wave Observatory, or LIGO, which reported the first confirmed gravitational waves in 2016, relies on two detectors, one in Hanford, Wash., and one in Livingston, La. Each detector splits the beam of a powerful laser in two, with each beam traveling down one of the detector’s two arms. In the absence of gravitational waves, the two beams recombine and cancel each other out. But if gravitational waves stretch one arm of the detector while squeezing the other, the laser light no longer matches up.

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      The machines are an incredible feat of engineering. Even spacetime ripples detected from colliding black holes might stretch an arm of the LIGO detector by as little as one ten-thousandth of the width of a proton.
      When the first detection, from two colliding black holes, was announced, the discovery was heralded as the beginning of a new era in astronomy. It was Science News’ story of the year in 2016, and such a big hit that the pioneers of the LIGO detector won the Nobel Prize in physics the following year.
      Scientists with LIGO and another gravitational wave detector, Virgo, based in Italy, have by now logged dozens more detections (SN: 1/30/21, p. 30). Most of the waves have emanated from mergers of black holes, though a few events have featured neutron stars. Smashups so far have revealed the previously unknown birthplaces of some heavy elements and pointed to a bright jet of charged subatomic particles that could offer clues to mysterious flashes of high-energy light known as gamma-ray bursts. The waves also have revealed that midsize black holes, between 100 and 100,000 times the sun’s mass, do in fact exist — along with reconfirming that Einstein was right, at least so far.
      Researchers at two gravitational wave observatories, LIGO in the United States and Virgo in Italy (shown), have reported dozens of detections of black hole smashups, as well as neutron star mergers, in the last five years.The Virgo Collaboration
      Just five years in, some scientists are already eager for something even more exotic. In a Science News article about detecting black holes orbiting wormholes via gravitational waves, physicist Vítor Cardoso of Instituto Superior Técnico in Lisbon, Portugal, suggested a coming shift to more unusual phenomena: “We need to look for strange but exciting signals,” he said (SN: 8/29/20, p. 12).
      Gravitational wave astronomy is truly only at its beginnings. Improved sensitivity at existing Earth-based detectors will turn up the volume on gravitational waves, allowing detections from less energetic and more distant sources. Future detectors, including the space-based LISA, planned for launch in the 2030s, will get around the troublesome noise that interferes when Earth’s surface shakes.
      “Perhaps the most exciting thing would be to observe a small black hole falling into a big black hole, an extreme mass ratio inspiraling,” Yunes says. In such an event, the small black hole would zoom back and forth, back and forth, swirling in different directions as it followed wildly eccentric orbits, perhaps for years. That could offer the ultimate test of Einstein’s equations, revealing whether we truly understand how spacetime is warped in the extreme. More

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      The Milky Way’s newfound high-energy glow hints at the secrets of cosmic rays

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      The Milky Way glows with a gamma ray haze, with energies vastly exceeding anything physicists can produce on Earth, according to a new paper. Gamma rays detected in the study, to be published in Physical Review Letters, came from throughout the galaxy’s disk, and reached nearly a quadrillion (1015) electron volts, known as a petaelectron volt or PeV.
      These diffuse gamma rays hint at the existence of powerful cosmic particle accelerators within the Milky Way. Physicists believe such accelerators are the source of mysterious, highly energetic cosmic rays, charged particles that careen through the galaxy, sometimes crash-landing on Earth. When cosmic rays — which mainly consist of protons — slam into interstellar debris, they can produce gamma rays, a form of high-energy light.  
      Certain galactic environments could rev up cosmic ray particles to more than a PeV, scientists suspect. In comparison, the Large Hadron Collider, the premier particle accelerator crafted by humans, accelerates protons to 6.5 trillion electron volts. But physicists haven’t definitively identified any natural cosmic accelerators capable of reaching a PeV, known as PeVatrons. One possibility is that supernova remnants, the remains of exploded stars, host shock waves that can accelerate cosmic rays to such energies (SN: 11/12/20).
      If PeVatrons exist, the cosmic rays they emit would permeate the galaxy, producing a diffuse glow of gamma rays of extreme energies. That’s just what researchers with the Tibet AS-gamma experiment have found. “It’s nice to see things fitting together,” says physicist David Hanna of McGill University in Montreal, who was not involved with the study.

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      After cosmic rays are spewed out from their birthplaces, scientists believe, they roam the galaxy, twisted about by its magnetic fields. “We live in a bubble of cosmic rays,” says astrophysicist Paolo Lipari of the National Institute for Nuclear Physics in Rome, who was not involved with the research. Because they are not deflected by magnetic fields, gamma rays point back to their sources, revealing the whereabouts of the itinerant cosmic rays. The new study “gives you information about how these particles fill the galaxy.”
      Lower-energy gamma rays also permeate the galaxy. But it takes higher-energy gamma rays to understand the highest-energy cosmic rays. “In general, the higher the energy of the gamma rays, the higher the energy of the cosmic rays,” says astrophysicist Elena Orlando of Stanford University, who was not involved with the research. “Hence, the detection … tells us that PeV cosmic rays originate and propagate in the galactic disk.”
      Scientists with the Tibet AS-gamma experiment in China observed gamma rays with energies between about 100 trillion and a quadrillion electron volts coming from the region of the sky covered by the disk of the Milky Way. A search for possible sources of the 38 highest-energy gamma rays, above 398 trillion electron volts, came up empty, supporting the idea that the gamma rays came from cosmic rays that had wandered about the galaxy. The highest-energy gamma ray carried about 957 trillion electron volts.
      Tibet AS-gamma researchers declined to comment on the study.
      Scientists have previously seen extremely energetic gamma rays from individual sources within the Milky Way, such as the Crab Nebula, a supernova remnant (SN: 6/24/19). Those gamma rays are probably produced in a different manner, by electrons radiating gamma rays while circulating within the cosmic accelerator. More