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Erin Wayman is the managing editor for print and longform content at Science News. She has a master’s degree in biological anthropology from the University of California, Davis and a master’s degree in science writing from Johns Hopkins University. More
The biography of our home galaxy may be due for some revisions. That’s because a bar-shaped collection of stars at the center of the Milky Way appears to be much younger than expected.
The bar is a prominent feature of our galaxy (SN: 6/25/21). It spans thousands of light-years and links the galaxy’s spiraling arms of stars, making them resemble streams of water coming from a spinning lawn sprinkler. In computer simulations of the Milky Way’s evolution, the bar tends to form early in the galaxy’s roughly 13-billion-year lifetime. But the ages and locations of metal-rich stars suggest the bar finished forming just a few billion years ago, researchers report. The study, submitted November 28 to arXiv.org, is in press at Astronomy & Astrophysics Letters.
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“These metal-rich stars are basically like fossil records of ancient stars that are telling the story of our home galaxy,” says Samir Nepal, an astrophysicist at the Leibniz Institute for Astrophysics Potsdam in Germany.
Stars with large proportions of metal elements are built from the remnants of stars that have since exploded, ejecting the metals they forged from lighter elements. Those spewed metals enrich the materials in the core of galaxies like the Milky Way, which is why a new generation of metal-rich stars can form only deep inside galaxies. The spinning bar at the center of the Milky Way then scattered some of those stars throughout our galaxy.
Using data from the European Space Agency’s Gaia space telescope, Nepal and colleagues reconstructed the development of the Milky Way bar through its influence on the distribution of metal-rich stars (SN: 5/9/18). They inferred the bar’s history, just as you might deduce where the batters stand in a baseball game by looking at the flight of the balls they hit, even if you can’t see home plate.
In tracking the ages of the metal-rich stars, the researchers identified a burst of star formation in the central part of the galaxy that petered out about 3 billion years ago. The downturn seems to mark the end of the Milky Way bar’s developmental phase, the researchers report. After that burst, they say, the inflow of new material into the bar probably dropped off substantially. That suggests the bar we see today is a stable feature that’s about 10 billion years younger than the galaxy as a whole.
The new insights about the metal-rich stars “are like the tip of the iceberg” of data coming from the Gaia telescope, says astrophysicist Cristina Chiappini, also with the Leibniz Institute for Astrophysics Potsdam. Assuming the revised age estimate of the bar is confirmed, future models of the galaxy’s evolution will have to account for why the bar developed so late.
The study has broader implications than correcting the history of our galaxy, says Ortwin Gerhard, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who was not involved in the research. “The possibility of detailed observations of the motions and chemical abundances of stars in the Milky Way, particularly based on [data] from the Gaia satellite,” he says, means we can “expect to learn about the evolution of bars [in other galaxies] generally by studying the bar in the Milky Way.” More
A survey of small, cool stars is helping to narrow in on the conditions that might set the stage for life beyond our solar system.
A look at about 200 ultracool dwarf stars shows that they lack sufficient ultraviolet light intensity to have the potential to jump-start life, researchers report December 1 in the Monthly Notices of the Royal Astronomical Society. That may initially seem to be bad news for finding signs of alien life on distant planets. But the diminutive stars could instead serve as test beds to determine what other conditions can create the chemical foundations of life.
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Compared with our sun, the dwarf stars in the new study are minuscule, roughly the size of Jupiter and weighing about a tenth as much as the sun. They’re also among the most common types of stars. And because they’re cool and comparatively dim, it’s often easier to spot planets orbiting them than it is in the glare of large bright stars. Astronomers studying the tiny red star TRAPPIST-1, for example, found that it hosts seven Earth-sized planets, including three that may be within the star’s habitable zone, where conditions are amenable to life (SN: 2/22/17).
For life to exist on a habitable planet, though, it must start somehow. One possibility is that UV starlight provides the energy needed to link together the hydrogen, oxygen, carbon, nitrogen, sulfur and other atoms that make up the compounds that are precursors to life.
With that in mind, space scientist Antígona Segura and colleagues used the Transiting Exoplanet Survey Satellite to measure the amount of UV radiation — among other things — emitted from 208 comparatively nearby ultracool dwarfs within 130 light-years from Earth (SN: 4/12/18). The stars they studied do emit UV light, as our sun does, and many produce bursts of UV when they let off flares. But overall, the UV energy the small stars release is too low to forge the chemicals needed to kick-start life, the team found.
The dearth of UV light doesn’t necessarily quash hopes of finding life around such stars. “UV is an energy source for prebiotic chemistry that we can measure, and that is why we focused on it,” says Segura, of the National Autonomous University of Mexico in Mexico City. “But there are many other energy sources, like cosmic and stellar particles, and particles, radiation and heat produced by radioactive decay, to name a few.”
Ultracool stars might be useful to discover whether something other than UV light can get life going. “We should search for life on the planets that have the least [UV] activity, where we can know with confidence that UV-driven prebiotic chemistry cannot happen,” says Paul Rimmer, a University of Cambridge astrophysicist who was not involved in the study. “If we find evidence of life on these [types of] planets, this will show that there are other paths to life.”
Also, Segura notes, sources of energy that could start life can also make planets less habitable for more complex life-forms. “We cannot currently say which [effects] would prevail. The best approach now is to study case by case and wait for more observational constraints, like for the case of the TRAPPIST-1 system.”
If astronomers eventually find life on planets orbiting ultracool dwarfs, it will confirm the potential for UV-free origins of life. But, Rimmer says, if it turns out that there are no signs of habitability on planets around the many small, dim dwarf stars we see, that would lower estimates of the likelihood of finding life outside our solar system. Either way, future surveys of ultracool dwarf stars could give researchers a better handle on the possible prevalence of extraterrestrial life in the universe. More
Enormous polygon patterns in rock lie dozens of meters below Mars’ surface, ground-penetrating radar data suggest.
Similar patterns develop on the surface in Earth’s polar regions when icy sediments cool and contract. A comparable process long ago may have created the shapes on Mars, found near the planet’s dry equator, researchers report November 23 in Nature Astronomy.
If so, the finding hints that the Red Planet’s equator was much wetter and icier, more like a polar region, when the polygons formed 2 billion to 3 billion years ago.
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“Buried possible polygons at that depth have yet to be reported” on Mars, says planetary scientist Richard Soare of Dawson College in Montreal, who was not involved in the study. Searching for ancient polygonal terrain on Mars using ground-penetrating radar is a new idea that “could be powerful,” he adds, and could help scientists understand how Mars’ climate has changed in the past.
On Earth, polygonal terrain forms in chilly climes when sharp temperature drops cause icy ground to contract and crack open. These thermal fractures are small at first. But the little cracks can fill with ice, sand, or a bit of both, forming “wedges” that prevent the cracks from healing and gradually pry open the earth as they grow. Because this wedging process requires multiple cycles of freezing and thawing, polygonal ground is a good hint that the terrain was icy when the patterns formed.
Polygonal surface patterns on Mars — like these in western Utopia Planitia, glimpsed from orbit by the Mars Global Surveyor satellite —might have been carved by “ice wedges” that form over many cycles of freezing and thawing.JPL/NASA, Malin Space Science Systems
But the Chinese Zhurong rover’s landing site, on a part of Mars called Utopia Planitia, is not the kind of place one would expect to find the terrain on Earth — at least not today (SN: 5/19/21). Polygons have been spotted at higher latitudes on the Martian surface from orbit, but the landing site sits near the Martian equator in a dry, sandy dune field (SN: 8/24/04).
The polygons appear to be roughly 70 meters across and are bordered by wedges nearly 30 meters wide and tens of meters deep — about 10 times as large as typical polygons and wedges on Earth. So it’s possible, Soare says, the structures here formed a bit differently than ice-wedge polygons on Earth.Forming polygons near the Martian equator wouldn’t be possible today, says study coauthor Ross Mitchell, a geoscientist also at the Institute of Geology and Geophysics. To form polygons, the region must have been colder and wetter in the past, he says — much like a polar region.
Changes in the tilt of Mars’ axis could explain such a shift in climate. Simulations of Mars’ orbit have suggested that the planet’s spin axis has at times been so extremely tilted that the planet essentially lay halfway on its side. This would cause the poles to receive more direct sunlight while equatorial regions froze. Finding potential polygons buried near the Martian equator, Mitchell says, is “smoking gun evidence” supporting the idea that the tilt of Mars’ axis has varied so substantially in the past.
“We think of every planet other than Earth as dead,” Mitchell says. But if Mars’s axis does swing around often, he says, our neighboring planet’s climate would be far more dynamic than currently believed. More
The “Oh-My-God” particle has a new companion.
In 1991, physicists spotted a particle from space that crashed into Earth with so much energy that it warranted an “OMG!” With 320 quintillion electron volts, or exaelectron volts, it had the kinetic energy of a baseball zipping along at about 100 kilometers per hour.
Now, a new particle of comparable energy has been found, researchers report in the Nov. 24 Science. Detected in 2021 by the Telescope Array experiment near Delta, Utah, the particle had an energy of about 240 exaelectron volts. And mysteriously, scientists are unable to pinpoint any cosmic source for the particle.
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“It’s a huge, huge amount of energy but in a tiny, tiny, tiny object,” says astroparticle physicist John Matthews of the University of Utah in Salt Lake City, co-spokesperson of the Telescope Array collaboration.
Cosmic rays consist of protons and atomic nuclei that zip through space at wide range of energies. Particles with energies over 100 exaelectron volts are exceedingly rare: On average, scientists estimate, one such particle falls on a square kilometer of Earth’s surface each century. And particles over 200 exaelectron volts are even rarer — only a few such particles have previously been detected.
When a cosmic ray hits Earth, it collides with a nucleus of an atom in the atmosphere, creating a cascade of other particles that can be detected on Earth’s surface.
To catch the rarest, highest-energy particles, scientists build giant arrays of detectors. The Telescope Array monitors an area of 700 square kilometers using more than 500 detectors made of plastic scintillator, material that emits light when hit by a charged particle. Additional detectors measure ultraviolet light produced in the sky by the shower of particles (although those detectors weren’t operating during the newly reported particle’s arrival). Based on the times that individual scintillator detectors were hit by the cascade of particles, scientists can determine the direction of the incoming cosmic ray and use that information to trace it back to its origins.
Extremely high-energy cosmic rays come from outside the Milky Way, but their exact sources are unknown (SN: 9/21/17). Most scientists think they are accelerated in violent cosmic environments, such as the jets of radiation that blast out of the areas around certain supermassive black holes, or starburst galaxies that form stars at a frenetic pace.
Whatever their origins, the particles must come from the relatively nearby cosmic neighborhood. That’s because the highest-energy cosmic rays lose energy as they travel, by interacting with the cosmic microwave background, the afterglow of the Big Bang (SN: 7/24/18).
Tracing back the particle’s location is complicated. “The issue is that when you detect a high-energy cosmic ray at Earth, the arrival direction that you get will not point to the source because it will be deflected by … any magnetic field that would be in the way,” says Telescope Array collaborator Noémie Globus, an astroparticle physicist at the University of California, Santa Cruz and the RIKEN research institute in Japan.
The magnetic fields present in the Milky Way and its environs scatter the cosmic rays like fog scatters light. To trace the particle to its home, scientists must take that scattering into account. But that backtracking pinpointed a cosmic void, a region of space with few galaxies at all, much less ones with violent processes going on.
That makes this particle particularly interesting, says astrophysicist Vasiliki Pavlidou of the University of Crete in Heraklion, Greece. “It’s actually pointing towards nothing at all, absolutely in the middle of nowhere.”
That might hint that scientist are missing something. For example, researchers may need to better understand the magnetic fields of the galaxy, says Pavlidou, who was not involved with the research.
“Every time you have one of these very high-energy events, just because they are so rare, it’s a big deal.” More
In May, a star in the Pinwheel galaxy exploded in just the right place at nearly the right time for astronomers to unravel details of its death. It was the closest supernova to go off in the last five years.
Astronomers have long been fascinated with these stellar explosions, which signal the end of life for the universe’s biggest stars. Not only are they dramatic — a supernova can shine 5 billion times as bright as our sun — but the explosions seed the universe with gold, silver, zinc and other elements forged only in a dying star’s last moments.
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A long-ago reshuffling of the giant planets in our solar system may have been instrumental in giving Earth its moon.
For decades, planetary scientists have hypothesized that Jupiter, Saturn, Uranus and Neptune were born much closer to the sun and that gravitational interactions among those planets jolted them into their contemporary trajectories (SN: 5/10/22). But the timing of that “giant planet orbital instability” has been tricky to nail down.
Now, an analysis of meteorite data suggests that the instability took place between 60 million and 100 million years after the solar system started forming, planetary scientist Alessandro Morbidelli reported October 5 in San Antonio at a meeting of the American Astronomical Society’s Division for Planetary Sciences. That timing also roughly coincides with when Earth’s moon is thought to have formed in the wake of a Mars-sized planet running into our own.
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The instability of the giant planets “is related to a complete reshaping of the solar system, the formation of the cometary reservoirs, the sculpting of the asteroid belt,” says Morbidelli, of the Observatoire de la Côte d’Azur in Nice, France. “Understanding when it occurred means fixing a milestone in the history of the solar system.”
The giant planet migration, first proposed by Morbidelli and colleagues in 2005, is a widely accepted hypothesis for explaining much about the solar system. In particular, the giant planets travel along slightly elongated orbits that are askew to one another. But observations of other planetary systems and computer simulations of planet formation suggest that giant planets, in general, form on closer-in orbits that are circular and coplanar. Using simulations, Morbidelli and others showed that if the giant planets in our solar system formed like that, they wouldn’t stay that way. Gravitational interactions would eventually knock the planets into the orbits they have today.
At first, the team thought this instability occurred about 600 million years after the birth of the solar system. That timing meant the migration of the giant planets could also explain an apparent asteroid bombardment of the terrestrial planets — Mercury, Venus, Earth and Mars — as evidenced by craters on the moon and lunar rocks brought back by Apollo astronauts. More recent work, however, has cast doubt on the reality of this “lunar cataclysm.”
Now, Morbidelli suspects the orbital instability came a lot earlier. His reasoning starts with a rare type of meteorite called EL enstatite chondrites.
The mix of elements in these meteorites suggests they must be the remnants of a large rocky body, a few hundred kilometers across, born near the terrestrial planets in the dusty disk that once swirled around our sun.
Then, in 2022, Morbidelli and colleagues showed that any of these enstatites that land on Earth today must come from a collection of bits and pieces in the asteroid belt between Mars and Jupiter. Those bits all broke off Athor, one of the many asteroids there, after it collided long ago with some other object in the belt. Altogether, that family of material adds up to an asteroid roughly 60 kilometers across — much smaller than the one thought to have birthed the enstatites in the first place. That means Athor must be only one piece of that larger rocky body, left over from a collision that destroyed it much closer to the sun.
“The question is, which dynamical mechanism can allow the implantation of Athor in the asteroid belt?” Morbidelli said at the meeting.
He tried computer simulations of several possibilities, but so far, only the giant planet instability seems capable of sending Athor to a stable orbit in the asteroid belt.
This couldn’t have happened earlier than 60 million years after the birth of the solar system, Morbidelli said. Radioactive elements in the enstatite chondrites indicate their parent body was slowly cooling until then, which means it was still large. It hadn’t yet collided with one of the many other planetesimals stirred around by the forming terrestrial planets.
On the other hand, simulations suggest the instability can’t have occurred later than about 100 million years after the solar system began. A study in 2018 concluded that if the giant planets migrated later, a pair of asteroids dubbed Patroclus-Menoetius, trailing Jupiter around the sun while orbiting each other, would have been pulled apart.
That 60-million- to 100-million-year window makes the instability a prime suspect in the diversion of a hypothesized planet that hit Earth, creating the moon (SN: 3/15/23). The timing “seems right,” says Matthew Clement, an astrophysicist at the Johns Hopkins Applied Physics Laboratory in Laurel, Md. “Lots of things were happening in the solar system’s early history. However, dynamically speaking, we don’t have a whole lot of reason to believe that things changed much after the moon-forming impact.”
But he cautions that Morbidelli’s estimate is based on “one data point, of the breakup of one asteroid, pieces of which serendipitously happened to get to Earth.”
Still, “it’s nice that [the new result] is actually based on some real data, even if it’s indirectly, rather than just computer models,” says planetary scientist John Chambers. He has questions, though. “They suggest this happened when the formation of the terrestrial planets was more or less complete, apart maybe from the giant impact that formed the moon,” says Chambers, of the Carnegie Institution for Science in Washington, D.C. “But then there’s a good chance it would have messed up the orbits of the terrestrial planets and possibly led to some of them colliding,” which the current lineup of planets suggests did not happen.
Both Chambers and Matthews have worked on scenarios in which the orbital instability occurred even earlier, just a few million years after the solar system began. That earlier time for the instability would help explain one of the outstanding riddles of the solar system: the relatively small size of Mars compared with Earth and Venus. That’s because the instability would have removed many objects from near the orbit of Mars before it could grow to the size of Earth or larger. The new result from Morbidelli’s team seems to exclude that solution.
“I’m prepared to rely on the evidence,” Chambers says. But he isn’t convinced yet because many facets of the solar system’s present structure must be reconciled with any date for the giant planet instability.
Clement agrees. “There’s problems if the instability happened at 500 million years. There’s still problems that we have to resolve if it happened when they say it happened,” he says. “And there’s still problems if it happened immediately after the planets formed, in the first few million years. This story is not done being told yet.” More
Venus has long been depicted as having a hellish atmosphere. Despite that reputation, evidence increasingly suggests that our neighbor’s cloudy shroud isn’t riddled with lightning.
Last month, planetary scientists proposed that most if not all of the bright flashes observed in Venus’ atmosphere could be explained by meteors (SN: 9/11/23). Now, a separate team suggests that whistling electromagnetic waves often created by lightning on Earth and other planets are not sparked by lightning on Venus.
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width: 150px;
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In 2021, the Parker Solar Probe swung past the nighttime side of Venus on its way toward the sun (SN: 12/15/21). As it did, its instruments picked up radio waves that scientists have dubbed whistler waves. Such disturbances got their name because their frequency quickly changes from high to low before they disappear, says Harriet George, a space physicist at the University of Colorado Boulder.
Oddly, Venus’ whistler waves were traveling downward toward the planet, not up and away from it, as they would have been if they’d been caused by lightning, George and her team report in the Oct. 16 Geophysical Research Letters.
An alternate source of energy for these whistler waves could be disturbances in Venus’ weak magnetic fields, the researchers suggest. As magnetic field lines shift and reconnect, they can release prodigious amounts of energy, George says. In other milieus, magnetic reconnections accelerate the solar wind, help heat the sun’s outer atmosphere and help trigger Earth’s auroras (SN: 6/7/23; SN: 11/14/19; SN: 7/24/08). They’ve got the right characteristics to create whistler waves on Venus too. More
