Space & Astronomy

Clouds of sand can condense, grow and disappear in some extraterrestrial atmospheres. A new look at old data shows that clouds made of hot silicate minerals are common in celestial objects known as brown dwarfs.

“This is the first full contextual understanding of any cloud outside the solar system,” says astronomer Stanimir Metchev of the University of Western Ontario in London, Canada. Metchev’s colleague Genaro Suárez presented the new work July 4 at the Cool Stars meeting in Toulouse, France.

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Clouds come in many flavors in our solar system, from Earth’s puffs of water vapor to Jupiter’s bands of ammonia. Astronomers have also inferred the presence of “extrasolar clouds” on planets outside the solar system (SN: 9/11/19).

But the only extrasolar clouds that have been directly detected were in the skies of brown dwarfs — dim, ruddy orbs that are too large to be planets but too small and cool to be stars. In 2004, astronomers used NASA’s Spitzer Space Telescope to observe brown dwarfs and spotted spectral signatures of sand — more specifically, grains of silicate minerals such as quartz and olivine. A few more tentative examples of sand clouds were spotted in 2006 and 2008.

Floating in one of these clouds would feel like being in a sandstorm, says planetary scientist Mark Marley of the University of Arizona in Tucson, who was involved in one of those early discoveries. “If you could take a scoop out of it and bring it home, you would have hot sand.”

Astronomers at the time found six examples of these silicate clouds. “I kind of thought that was it,” Marley says. Theoretically, there should be a lot more than six brown dwarfs with sandy skies. But part of the Spitzer telescope ran out of coolant in 2009 and was no longer able to measure similar clouds’ chemistry.

While Suárez was looking into archived Spitzer data for a different project, he realized there were unpublished or unanalyzed data on dozens of brown dwarfs. So he analyzed all of the low-mass stars and brown dwarfs that Spitzer had ever observed, 113 objects in total, 68 of which had never been published before, the team reports in the July Monthly Notices of the Royal Astronomical Society.

“It’s very impressive to me that this was hiding in plain sight,” Marley says.

Not every brown dwarf in the sample showed strong signs of silicate clouds. But together, the brown dwarfs followed a clear trend. For dwarfs and low-mass stars hotter than about 1700˚ Celsius, silicates exist as a vapor, and the objects show no signs of clouds. But below that temperature, signs of clouds start to appear, becoming thickest around 1300˚ C. Then the signal disappears for brown dwarfs that are cooler than about 1000˚ C, as the clouds sink deep into the atmospheres.

The finding confirms previous suspicions that silicate clouds are widespread and reveals the conditions under which they form. Because brown dwarfs are born hot and cool down over time, most of them should see each phase of sand cloud evolution as they age. “We are learning how these brown dwarfs live,” Suárez says. Future research can extrapolate the results to better understand atmospheres in planets like Jupiter, he notes.

The recently launched James Webb Space Telescope will also study atmospheric chemistry in exoplanets and brown dwarfs and will specifically look for clouds (SN: 10/6/21). Marley looks forward to combining the trends from this study with future results from JWST. “It’s really going to be a renaissance in brown dwarf science,” he says. More

PASADENA, Calif. — A lucky celestial alignment has given astronomers a rare look at a galaxy in the early universe that is seeding its surroundings with the elements needed to forge subsequent generations of stars and galaxies.

Seen as it was just 700 million years after the Big Bang, the distant galaxy has gas flowing over its edges. It is the earliest-known run-of-the-mill galaxy, one that could have grown into something like the Milky Way, to show such complex behavior, astronomer Hollis Akins said June 14 during a news conference at the American Astronomical Society meeting.

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“These results also tell us that this outflow activity seems to be able to shape galaxy evolution, even in this very early part of the universe,” said Akins, an incoming graduate student at the University of Texas at Austin. He and colleagues also submitted their findings June 14 to arXiv.org.

The galaxy, called A1689-zD1,­ shows up in light magnified by Abell 1689, a large galaxy cluster that can bend and intensify, or gravitationally lens, light from the universe’s earliest galaxies (SN: 2/13/08; SN: 10/6/15). Compared with other observed galaxies in the early universe, A1689-zD1 doesn’t make a lot of stars — only about 30 suns each year — meaning the galaxy isn’t very bright to our telescopes. But the intervening cluster magnified A1689-zD1’s light by nearly 10 times.

Akins and colleagues studied the lensed light with the Atacama Large Millimeter/submillimeter Array, or ALMA, a large network of radio telescopes in Chile. The team mapped the intensities of a specific spectral line of oxygen, a tracer for hot ionized gas, and a spectral line of carbon, a tracer for cold neutral gas. Hot gas shows up where the bright stars are, but the cold gas extends four times as far, which the team did not expect.

“There has to be some mechanism [to get] carbon out into the circumgalactic medium,” the space outside of the galaxy, Akins says.

Only a few scenarios could explain that outflowing gas. Perhaps small galaxies are merging with A1689-zD1 and flinging gas farther out where it cools, Akins said. Or maybe the heat from star formation is pushing the gas out. The latter would be a surprise considering the relatively low rate of star formation in this galaxy. While astronomers have seen outflowing gas in other early-universe galaxies, those galaxies are bustling with activity, including converting thousands of solar masses of gas into stars per year.

Galaxy A169-zD1 (pictured, in radio waves) exists in the universe’s first 700 million years.ALMA/ESO, NAOJ and NRAO; H. Akins/Grinnell College; B. Saxton/NRAO/AUI/NSF

The researchers again used the ALMA data to measure the motions of both the cold neutral and hot ionized gas. The hot gas showed a larger overall movement than the cold gas, which implies it’s being pushed from A1689-zD1’s center to its outer regions, Akins said at the news conference.

Despite the galaxy’s relatively low rate of star formation, Akins and his colleagues still think the 30-solar-masses of stars a year heat the gas enough to push it out from the center of the galaxy. The observations suggest a more orderly bulk flow of gas, which implies outflows, however the researchers are analyzing the movement of the gas in more detail and cannot yet rule out alternate scenarios.

They think when the hot gas flows out, it expands and eventually cools, Akins said, which is why they see the colder gas flowing over the galaxy’s edge. That heavy-element-rich gas enriches the circumgalactic medium and will eventually be incorporated into later generations of stars (SN: 6/17/15). Due to gravity’s pull, cool gas, often with fewer heavy elements, around the galaxy also falls toward its center so A1689-zD1 can continue making stars.

These observations of A1689-zD1 show this flow of gas happens not only in the superbright, extreme galaxies, but even in normal ones in the early universe. “Knowing how this cycle is working helps us to understand how these galaxies are forming stars, and how they grow,” says Caltech astrophysicist Andreas Faisst, who was not involved in the study.

Astronomers aren’t done learning about A1689-zD1, either. “It’s a great target for follow-up observations,” Faisst says. Several of Akins’s colleagues plan to do just that with the James Webb Space Telescope (SN: 10/6/21). More

It sounds like the setup for a joke: If radio waves give you radar and sound gives you sonar, what do gravitational waves get you?

The answer might be “GRADAR” — gravitational wave “radar” — a potential future technology that could use reflections of gravitational waves to map the unseen universe, say researchers in a paper accepted to Physical Review Letters. By looking for these signals, scientists may be able to find dark matter or dim, exotic stars and learn about their deep insides.

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Astronomers routinely use gravitational waves — traveling ripples in the fabric of space and time itself, first detected in 2015 — to watch cataclysmic events that are hard to study with light alone, such as the merging of two black holes (SN: 2/11/2016).

But physicists have also known about a seemingly useless property of gravitational waves: They can change course. Einstein’s theory of gravity says that spacetime gets warped by matter, and any wave passing through these distortions will change course. The upshot is that when something emits gravitational waves, part of the signal comes straight at Earth, but some might arrive later — like an echo — after taking longer paths that bend around a star or anything else heavy.

Scientists have always thought these later signals, called “gravitational glints,” should be too weak to detect. But physicists Craig Copi and Glenn Starkman of Case Western Reserve University in Cleveland, Ohio, took a leap: Working off Einstein’s theory, they calculated how strong the signal would be when waves scatter through the gravitational field inside a star itself.

“The shocking thing is that you seem to get a much larger result than you would have expected,” Copi says. “It’s something we’re still trying to understand, where that comes from — whether it’s believable, even, because it just seems too good to be true.”

If gravitational glints can be so strong, astronomers could possibly use them to trace the insides of stars, the team says. Researchers could even look for massive bodies in space that would otherwise be impossible to detect, like globs of dark matter or lone neutron stars on the other side of the observable universe.“That would be a very exciting probe,” says Maya Fishbach, an astrophysicist at Northwestern University in Evanston, Ill., who was not involved in the study.

There are still reasons to be cautious, though. If this phenomenon stands up to more detailed scrutiny, Fishbach says, scientists would have to understand it better before they could use it — and that will probably be difficult.

“It’s a very hard calculation,” Copi says.

But similar challenges have been overcome before. “The whole story of gravitational wave detection has been like that,” Fishbach says. It was a struggle to do all the math needed to understand their measurements, she says, but now the field is taking off (SN: 1/21/21). “This is the time to really be creative with gravitational waves.” More

PASADENA, Calif. — The faint dwarf galaxies in a nearby galaxy group seem to have missed the memo. Instead of being dispersed evenly around the group’s most massive galaxy, which is what happens in our own galaxy group, these newly found dwarfs cluster in one region. And astronomers don’t know why.

“This satellite distribution is just weird,” astronomer Eric Bell said June 13 at the American Astronomical Society meeting.

Bell, of the University of Michigan in Ann Arbor, and colleagues used the Subaru telescope in Hawaii to hunt for faint clumps of stars, indicating dwarf galaxies, around the galaxy M81. This Milky Way–like galaxy is the most prominent member in a relatively nearby group of galaxies, all about 12 million light-years from Earth. The team found one definite dwarf galaxy and six possible fainter ones.

Most of the known satellite galaxies (circled in red) in the M81 galaxy group, along with seven newfound candidates (yellow), seem to cluster toward one side of the galaxy M81 (center).Sloan Digital Sky Survey

“The part that’s just bananas,” Bell said, is that the newfound satellite galaxies all sit on one side of M81.

Computer simulations of galaxy evolution suggest that the largest galaxies have many faint, small galaxies sprinkled uniformly throughout the outer part of the dominant galaxy’s diffuse cloudlike halo. Observations in our galaxy group back this up: The dozens of dwarf galaxies known to orbit in the Milky Way’s outskirts are distributed evenly around the galaxy, as are most of the dwarf galaxies seen around our nearest large neighbor, the Andromeda Galaxy (SN: 3/11/15; SN: 8/19/15).

But in the M81 group, the seven newly identified star clumps appear to surround a smaller member of that group, NGC 3077, which is about one-tenth the mass of M81. “The fact that the bigger thing doesn’t have more satellites,” Bell says, “nobody expects that.” More

After two decades of debate, scientists are getting closer to figuring out exactly what the sun — and thus the whole universe — is made of.

The sun is mostly composed of hydrogen and helium. There are also heavier elements such as oxygen and carbon, but just how much is controversial. New observations of ghostly subatomic particles known as neutrinos suggest that the sun has an ample supply of “metals,” the term astronomers use for all elements heavier than hydrogen and helium, researchers report May 31 at arXiv.org.

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The results “are fully compatible with [a] high metallicity” for the sun, says Livia Ludhova, a physicist at Research Center Jülich in Germany.

Elements heavier than hydrogen and helium are crucial for creating rock-iron planets like Earth and sustaining life-forms like humans. By far the most abundant of these elements in the universe is oxygen, followed by carbon, neon and nitrogen.

But astronomers don’t know exactly how much of these elements exist relative to hydrogen, the most common element in the cosmos. That’s because astronomers typically use the sun as a reference point to gauge elemental abundances in other stars and galaxies, and two techniques imply very different chemical compositions for our star.

One technique exploits vibrations inside the sun to deduce its internal structure and favors a high metal content. The second technique determines the sun’s composition from how atoms on its surface absorb certain wavelengths of light. Two decades ago, a use of this second technique suggested that oxygen, carbon, neon and nitrogen levels in the sun were 26 to 42 percent lower than an earlier determination found, creating the current conflict.

Another technique has now emerged that could decide the long-standing debate: using solar neutrinos.

These particles arise from nuclear reactions in the sun’s core that turn hydrogen into helium. About 1 percent of the sun’s energy comes from reactions involving carbon, nitrogen and oxygen, which convert hydrogen into helium but do not get used up in the process. So the more carbon, nitrogen and oxygen the sun actually has, the more neutrinos this CNO cycle should emit.

In 2020, scientists announced that Borexino, an underground detector in Italy, had spotted these CNO neutrinos (SN: 6/24/20). Now Ludhova and her colleagues have recorded enough neutrinos to calculate that carbon and nitrogen atoms together are about 0.06 percent as abundant as hydrogen atoms in the sun — the first use of neutrinos to determine the sun’s makeup.

And though that number sounds small, it’s even higher than the one favored by astronomers who support a high-metal sun. And it’s 70 percent greater than the number a low-metal sun should have.

“This is a great result,” says Marc Pinsonneault, an astronomer at Ohio State University in Columbus who has long advocated for a high-metal sun. “They’ve been able to demonstrate robustly that the current low-metallicity solution is inconsistent with the data.”

Still, because of uncertainties in both the observed and predicted neutrino numbers, Borexino can’t fully rule out a low-metal sun, Ludhova says.

The new work is “a significant improvement,” says Gaël Buldgen, an astrophysicist at Geneva University in Switzerland who favors a low-metal sun. But the predicted numbers of CNO neutrinos come from models of the sun that he criticizes as too simplified. Those models neglect the sun’s spin, which could induce mixing of chemical elements over its life and change the amount of carbon, nitrogen and oxygen near the sun’s center, thereby changing the predicted number of CNO neutrinos, Buldgen says.

Additional neutrino observations are needed for a final verdict, Ludhova says. Borexino shut down in 2021, but future experiments could fill the void.

The stakes are high. “We’re arguing about what the universe is made of,” Pinsonneault says, because “the sun is the benchmark for all of our studies.”

So if the sun has much more carbon, nitrogen and oxygen than currently thought, so does the whole universe. “That changes our understanding about how the chemical elements are made. It changes our understanding of how stars evolve and how they live and die,” Pinsonneault says. And, he adds, it’s a reminder that even the best-studied star — our sun — still has secrets. More