Starquakes Might Solve the Mysteries of Stellar Magnetism

Starquakes Might Solve the Mysteries of Stellar Magnetism

That was a surprise—and a possible indication that something crucial was missing in those models: magnetism.

Stellar Symmetry

Last year, Gang Li, an asteroseismologist now at KU Leuven, went digging through Kepler’s giants. He was searching for a mixed-mode signal that recorded the magnetic field in the core of a red giant. “Astonishingly, I actually found a few instances of this phenomenon,” he said.

Typically, mixed-mode oscillations in red giants occur almost rhythmically, producing a symmetric signal. Bugnet and others had predicted that magnetic fields would break that symmetry, but no one was able to make that tricky observation—until Li’s team.

Li and his colleagues found a giant trio that exhibited the predicted asymmetries, and they calculated that each star’s magnetic field was up to “2,000 times the strength of a typical fridge magnet”—strong, but consistent with predictions.

However, one of the three red giants surprised them: Its mixed-mode signal was backward. “We were a bit puzzled,” said Sébastien Deheuvels, a study author and an astrophysicist at Toulouse. Deheuvels thinks this result suggests that the star’s magnetic field is tipped on its side, meaning that the technique could determine the orientation of magnetic fields, which is crucial for updating models of stellar evolution.

A second study, led by Deheuvels, used mixed-mode asteroseismology to detect magnetic fields in the cores of 11 red giants. Here, the team explored how those fields affected the properties of g-modes—which, Deheuvels noted, may provide a way to move beyond red giants and detect magnetic fields in stars that don’t show those rare asymmetries. But first “we want to find the number of red giants that show this behavior and compare them to different scenarios for the formation of these magnetic fields,” Deheuvels said.

Not Just a Number

Using starquakes to investigate the interiors of stars kicked off a “renaissance” in stellar evolution, said Conny Aerts, an astrophysicist at KU Leuven.

The renaissance has far-reaching implications for our understanding of stars and of our place in the cosmos. So far, we know the exact age of just one star—our sun—which scientists determined based on the chemical composition of meteorites that formed during the birth of the solar system. For every other star in the universe, we only have estimated ages based on rotation and mass. Add internal magnetism, and you have a way to estimate stellar ages with more precision.

Stunning James Webb image shows the heart of our Milky Way | Digital Trends

Stunning James Webb image shows the heart of our Milky Way | Digital Trends

A new image from the James Webb Space Telescope shows the heart of our galaxy, in a region close to the supermassive black hole at the center of the Milky Way, Sagittarius A*. The image shows a star-forming region where filaments of dust and gas are clumping together to give birth to new baby stars.

The image was captured using Webb’s NIRCam instrument, a camera that looks in the near-infrared portion of the electromagnetic spectrum with shorter wavelengths shown in blue and cyan and longer wavelengths shown in yellow and red.

The full view of the NASA/ESA/CSA James Webb Space Telescope’s NIRCam (Near-Infrared Camera) instrument reveals a 50 light-years-wide portion of the Milky Way’s dense center. An estimated 500,000 stars shine in this image of the Sagittarius C (Sgr C) region, along with some as-yet-unidentified features. NASA, ESA, CSA, STScI, S. Crowe (UVA)

This region is called Sagittarius C, and is located around 300 light-years away from the supermassive black hole Sagittarius A*. For reference, Earth is located much further away from the galactic center, at a distance of around 26,000 light years from Sagittarius A*.

There are thought to be as many as 500,000 stars in the Sagittarius C region, including many young protostars, some of which will go on to become main-sequence stars like our sun. As stars form, they give off powerful stellar winds which blow away nearby material and prevent more stars from forming very close to them.

These outflows are illuminated in the infrared wavelength, and the cyan-colored patches in the image are created by ionized gas. The young stars give off a great deal of energy, which ionizes the hydrogen gas around them and makes them glow in the infrared.

However, there are actually even more stars in this area than can be seen in the image. The pockets of darkness scattered throughout the image aren’t blank but are dense clouds that are dark in the infrared, including a large dense area in the heart of the region.

There are still some surprises to be found in the image too, with some features that scientists need to study in more depth. “Researchers say they have only begun to dig into the wealth of unprecedented high-resolution data that Webb has provided on this region, and many features bear detailed study,” Webb scientists write. “This includes the rose-colored clouds on the right side of the image, which have never been seen in such detail.”

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The Milky Way’s Stars Reveal Its Turbulent Past

The Milky Way’s Stars Reveal Its Turbulent Past

To make maps of these structures, astronomers turn to individual stars. Each star’s composition records its birthplace, age, and natal ingredients, so studying starlight enables a form of galactic cartography—as well as genealogy. By situating stars in time and place, astronomers can retrace history and infer how the Milky Way was built, piece by piece, over billions of years.

The first major effort to study the primordial Milky Way’s formation began in the 1960s, when Olin Eggen, Donald Lynden-Bell and Alan Sandage, who was Edwin Hubble’s former graduate student, argued that the galaxy collapsed from a spinning gas cloud. For a long time after that, astronomers thought that the first structure to emerge in our galaxy was the halo, followed by a bright, dense disk of stars. As more powerful telescopes came online, astronomers built increasingly precise maps and started refining their ideas about how the galaxy came together.

Everything changed in 2016, when the first data from the European Space Agency’s Gaia satellite came back to Earth. Gaia precisely measures the paths of millions of stars throughout the galaxy, allowing astronomers to learn where those stars are located, how they move through space, and how fast they are going. With Gaia, astronomers could paint a sharper picture of the Milky Way—one that revealed many surprises.

The bulge is not spherical but peanut-shaped, and it’s part of a larger bar spanning the middle of our galaxy. The galaxy itself is warped like the brim of a beat-up cowboy hat. The thick disk is also flared, growing thicker toward its edges, and it may have formed before the halo. Astronomers aren’t even sure how many spiral arms the galaxy really has.

The map of our island universe is not as neat as it once seemed. Nor as calm.

“If you look at a traditional picture of the Milky Way, you have this nice spherical halo and a nice regular-looking disk, and everything is kind of settled and stationary. But what we know now is that this galaxy is in a state of disequilibrium,” said Charlie Conroy, an astronomer at the Harvard-Smithsonian Center for Astrophysics. “This picture of it being simple and well ordered has been really tossed out in the past couple of years.”

A New Map of the Milky Way

Three years after Edwin Hubble realized Andromeda was a galaxy unto itself, he and other astronomers were busy imaging and classifying hundreds of island universes. Those galaxies seemed to exist in a few prevailing shapes and sizes, so Hubble developed a basic classification scheme known as the tuning fork diagram: It divides galaxies into two categories, ellipticals and spirals.

Astronomers still use this scheme to categorize galaxies, including ours. For now, the Milky Way is a spiral, with arms that are the main nurseries for stars (and therefore planets). For a half-century, astronomers thought there were four main arms—the Sagittarius, Orion, Perseus, and Cygnus arms (we live in a smaller offshoot, unimaginatively called the Local Arm). But new measurements of supergiant stars and other objects are drawing a different picture, and astronomers no longer agree on the number of arms or their sizes, or even whether our galaxy is an oddball among islands.

“Strikingly, almost no external galaxies present four spirals extending from their centers to their outer regions,” Xu Ye, an astronomer with China’s Purple Mountain Observatory, said in an email.

Astronomers discover how tiny dwarf galaxies form fossils | Digital Trends

Astronomers discover how tiny dwarf galaxies form fossils | Digital Trends

Galaxies come in many different shapes and sizes, including those considerably smaller than our Milky Way. These smaller galaxies, called dwarf galaxies, can have as few as 1,000 stars, compared to the several hundred billion in our galaxy. And when these dwarf galaxies age and begin to erode away, they can transform into an even smaller and more dense shape, called an ultra-compact dwarf galaxy.

The Gemini North telescope has recently been studying more than 100 of these eroding dwarf galaxies, seeing how they lose their outer stars and gas to become ultra-compact dwarf galaxies or UCDs.

This illustration shows a dwarf galaxy in the throes of transitioning to an ultra-compact dwarf galaxy as it’s stripped of its outer layers of stars and gas by a nearby larger galaxy. Ultra-compact dwarf galaxies are among the densest stellar groupings in the Universe. Being more compact than other galaxies with similar mass, but larger than star clusters — the objects they most closely resemble — these mystifying objects have defied classification. The missing piece to this puzzle has been a lack of sufficient transitional, or intermediate objects to study. A new galaxy survey, however, fills in these missing pieces to show that many of these enigmatic objects are likely formed from the destruction of dwarf galaxies. NOIRLab/NSF/AURA/M. Zamani

“Our results provide the most complete picture of the origin of this mysterious class of galaxy that was discovered nearly 25 years ago,” said one of the researchers, NOIRLab astronomer Eric Peng in a statement. “Here we show that 106 small galaxies in the Virgo cluster have sizes between normal dwarf galaxies and UCDs, revealing a continuum that fills the ‘size gap’ between star clusters and galaxies.”

While astronomers did predict that dwarf galaxies could become UCDs, they hadn’t observed many cases of one transforming into the other. So this study looked for these “missing links” to see how this transition occurred. They found that these in-between galaxies were most often located near larger galaxies, which stripped away stars and gas from the small dwarf galaxies to leave a UCD behind.

“Once we analyzed the Gemini observations and eliminated all the background contamination, we could see that these transition galaxies existed almost exclusively near the largest galaxies. We immediately knew that environmental transformation had to be important,” explained lead author Kaixiang Wang of Peking University.

These objects were spotted using data from sky surveys, which was followed up using observations from Gemini North. That allowed the researchers to pick out the small dwarf galaxies from the many background galaxies visible in the sky.

“It’s exciting that we can finally see this transformation in action,” said Peng. “It tells us that many of these UCDs are visible fossil remnants of ancient dwarf galaxies in galaxy clusters, and our results suggest that there are likely many more low-mass remnants to be found.”

The research is published in the journal Nature.

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James Webb observes merging stars creating heavy elements | Digital Trends

James Webb observes merging stars creating heavy elements | Digital Trends

In its earliest stages, the universe was composed mostly of hydrogen and helium. All of the other, heavier elements that make up the universe around us today were created over time, and it is thought that they were created primarily within stars. Stars create heavy elements within them in the process of fusion, and when these stars reach the ends of their lives they may explode in supernovas, spreading these elements in the environment around them.

That’s how heavier elements like those up to iron are created. But for the heaviest elements, the process is thought to be different. These are created not within stellar cores, but in extreme environments such as the merging of stars, when massive forces create exceedingly dense environments that forge new elements.

Now, the James Webb Space Telescope has detected some of these heavy elements being created in a star merger for the first time. Researchers used the telescope to observe the effects of a kilonova, a huge outpouring of energy that occurs when two neutron stars merge. The event created a particularly bright gamma-ray burst which allowed the researchers to zero in and identify the location of the merger.

A team of scientists has used the NASA/ESA/CSA James Webb Space Telescope to observe an exceptionally bright gamma-ray burst, GRB 230307A, and its associated kilonova. Kilonovas—an explosion produced by a neutron star merging with either a black hole or with another neutron star—are extremely rare, making it difficult to observe these events. The highly sensitive infrared capabilities of Webb helped scientists identify the home address of the two neutron stars that created the kilonova. This image from Webb’s NIRCam (Near-Infrared Camera) instrument highlights GRB 230307A’s kilonova and its former home galaxy among their local environment of other galaxies and foreground stars. The neutron stars were kicked out of their home galaxy and traveled a distance of about 120,000 light-years, approximately the diameter of the Milky Way galaxy, before finally merging several hundred million years later. NASA, ESA, CSA, STScI, A. Levan (IMAPP, Warw), A. Pagan (STScI)

Webb observed the element tellurium being ejected by the kilonova, which was likely created in the merger. Although scientists have long theorized that this is how heavy elements could be created, this is the first time such direct evidence has been observed as kilonovas are rare and brief events. The particular brightness of the gamma-ray burst GRB 230307A was key to helping to locate this event.

“Webb provides a phenomenal boost and may find even heavier elements,” said Ben Gompertz, a co-author of the study at the University of Birmingham in the United Kingdom. “As we get more frequent observations, the models will improve and the spectrum may evolve more in time. Webb has certainly opened the door to do a lot more, and its abilities will be completely transformative for our understanding of the Universe.”

The research is published in the journal Nature.

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This peculiar galaxy has two supermassive black holes | Digital Trends

This peculiar galaxy has two supermassive black holes | Digital Trends

As hard as it is to picture, with billions or even trillions of galaxies in the universe, entire galaxies can collide with each other. When that happens, one galaxy can be destroyed or the two can merge into one. But even in the case of galaxy mergers, the effects of the collision are often visible for billions of years afterward.

That’s shown in a recent image taken by the Gemini South observatory, which shows the chaotic result of a merger between two spiral galaxies 1 billion years ago.

Gemini South, one half of the International Gemini Observatory operated by NSF’s NOIRLab, captures the billion-year-old aftermath of a double spiral galaxy collision. At the heart of this chaotic interaction, entwined and caught in the midst of the chaos, is a pair of supermassive black holes — the closest such pair ever recorded from Earth. International Gemini Observatory/NOIRLab/NSF/AURA; Image processing: T.A. Rector (University of Alaska Anchorage/NSF’s NOIRLab), J. Miller (International Gemini Observatory/NSF’s NOIRLab), M. Rodriguez (International Gemini Observatory/NSF’s NOIRLab), M. Zamani (NSF’s NOIRLab)

The resulting galaxy, called NGC 7727 and located 90 million light-years away, shows the cloudy blobs of dust and gas that now swirl around the galactic core. The stretching arms of the spiral galaxies have been pulled apart by the gravitational forces of the merger, leaving behind an unstructured shape which leads this to be classified as a “peculiar galaxy.” Despite its messy appearance, parts of the newly formed galaxy are ideal locations for the formation of stars as pockets of dust and gas and pulled around and pushed together.

At the heart of almost every galaxy is an enormous supermassive black hole, but this galaxy is a little different. It has not one but two supermassive black holes, one from each of the original galaxies. One of these is 154 million times the mass of the sun, and the other just 6.3 million times the mass of the sun, and the two are located 1,600 light-years apart in their own galactic nuclei.

This galaxy won’t remain in this unusual state forever though. Eventually, the huge gravitational forces of the two supermassive black holes will pull them closer and closer together, and scientists estimate that the two will merge in around 250 million years’ time. This monumental event will send out ripples in spacetime called gravitational waves and will create an even larger supermassive black hole.

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The Mystery of Cosmic Radio Bursts Gets Bright New Clues

The Mystery of Cosmic Radio Bursts Gets Bright New Clues

A second study, published the same day in Science, shone more light on the mysterious bursts and their variety. This group of researchers, mostly at Australian institutions, spotted the farthest and brightest fast radio burst ever seen. In less than a millisecond, it blazed out as much energy as the sun emits in more than 16 years, and it did this some 10 billion light-years away. That exceeds the previous record-holder’s distance by about 4 billion light-years, and it’s five times more energetic, too. This suggests that the bursts don’t just come from the nearby universe.

An international team led by astronomer Ryan Shannon of the Swinburne University of Technology used Australia’s Square Kilometre Array Pathfinder to glimpse this fast radio burst, which originated when the universe was less than half its current age. “That you can have these millisecond signals that—though not perfectly undisturbed—travel 8 billion years just to get to Earth is pretty astounding,” Shannon says.

That signal, known as FRB 20220610A, is the brightest, or most energetic, fast radio burst ever detected. Shannon likens the energy to a microwave oven, since its frequency range is similar: The energy from that single burst would be enough to microwave a bowl of popcorn twice the size of the sun, he says.

A fast radio burst doesn’t travel straight through space, because space isn’t exactly a vacuum. The signal passes through gas, which might be turbulent or clumpy, dense or diffuse. It slightly distorts the signal, spreading it out or making it noisier. The gravitational pull of a massive celestial body can also deflect the radio waves, a process called gravitational lensing. These distortions embed the burst’s signal with information about the stuff it passed through on its way to Earth.

A distortion like this gave Shannon and his colleagues their clue that FRB 20220610A probably came from far away. They noticed that the radio signal was a bit off, thanks to a frequency-dependent time delay caused by the gas the burst traveled through between its host galaxy and ours.

These distortions also mean that ultrafast flashes could also be used as astrophysical probes to study the clouds of gas and dust that a radio burst passes through between its source and the Earth, says Jason Hessels, a colleague of Snelders’ at the University of Amsterdam. These gases are too faint to see, but we can tell where they are—or how abundant or clumpy they are—by how they bend the radio signals. “Because these bursts are so short, it only takes a tiny little bit of gas between stars and galaxies to distort the radio signal. It can be broadened or scattered or gravitationally lensed,” Hessels says. He calls fast radio bursts “unique tools for studying otherwise invisible material.”

“The shorter they are, the more precisely you can do that,” he says.

Altogether, the broad range of fast radio bursts cataloged in the two studies implies that there could be many kinds of sources—they might not all be blasting from pulsating magnetars. Some could come from bright pulsars, whose beams are powered by their rotation rather than magnetic fields. Others might come from black holes feeding on stars while emitting jets of gas that create shock waves generating radio flashes. This diversity could explain why some bursts last a million times longer, or are thousands of times brighter, than others. It could also explain why it’s been so hard to pin down a single type of source—because there probably isn’t just one.

“The types of bursts we’re finding and the places we’re finding these sources are becoming more and more diverse,” Hessels says. “It suggests that there’s more than one explanation. That would make the theorists happy, because there are dozens and dozens of theories.”

Hubble observes mysterious bright explosion called The Finch | Digital Trends

Hubble observes mysterious bright explosion called The Finch | Digital Trends

The Hubble Space Telescope recently observed something strange: an extremely bright, extremely fast flash of light that popped up in the middle of nowhere. Technically known as a Luminous Fast Blue Optical Transient (LFBOT), the odd thing about this rare event was that it occurred outside of a galaxy.

These flashes have been observed only a few times since they were discovered in 2018, and this particular event was named The Finch. Hubble was used to track the flash’s origin point, which was in between two galaxies: 50,000 light-years away from a larger spiral galaxy and around 15,000 light-years away from a smaller galaxy. This has astronomers puzzled, as these events were thought to issue from inside galaxies where stars are forming — but this event happened far away from any star-forming region.

This is an artist’s concept of one of the brightest explosions ever seen in space. Called a Luminous Fast Blue Optical Transient (LFBOT), it shines intensely in blue light and evolves rapidly, reaching peak brightness and fading again in a matter of days, unlike supernovae which take weeks or months to dim. Only a handful of previous LFBOTs have been discovered since 2018. And they all happen inside galaxies where stars are being born. But as this illustration shows, the LFBOT flash discovered in 2023 by Hubble was seen between galaxies. This only compounds the mystery of what these transient events are. Because astronomers don’t know the underlying process behind LFBOTs, the explosion shown here is purely conjecture based on some known transient phenomenon. NASA, ESA, NSF’s NOIRLab, M. Garlick , M. Zamani

“The Hubble observations were really the crucial thing. They made us realize that this was unusual compared to the other ones like that, because without the Hubble data we would not have known,” said lead researcher Ashley Chrimes in a statement.

The flash was also observed using other instruments like the ground-based Gemini South observatory, which found that the temperature of the Finch was an incredible 20,000 degrees Celsius (approximately 36,000 Fahrenheit).

These flashes brighten and dim in just a few days, compared to other brief astronomical events like supernovas which tend to brighten and dim over a period of months. Scientists thought that LFBOTs might be created by a particular and rare type of supernova that happens to very large and short-lived stars. As these stars don’t last for long, they tend to be found close to stellar nurseries where they are born. But this new flash challenges that concept.

“The more we learn about LFBOTs, the more they surprise us,” said Chrimes. “We’ve now shown that LFBOTs can occur a long way from the center of the nearest galaxy, and the location of the Finch is not what we expect for any kind of supernova.”

It could be that the flashes are not in fact caused by supernovas but are instead caused by stars being ripped apart by black holes. Or it could be that a fast-moving star was passing between the two galaxies and exploded during its journey. As the events are so rare, it’s hard to tell at this stage of research.

“The discovery poses many more questions than it answers,” said Chrimes. “More work is needed to figure out which of the many possible explanations is the right one.”

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Zoom into detailed James Webb image of the Orion nebula | Digital Trends

Zoom into detailed James Webb image of the Orion nebula | Digital Trends

A new image from the James Webb Space Telescope shows the majesty of the gorgeous Orion nebula in tremendous detail. The European Space Agency (ESA) has shared an extremely high-resolution version of the image that you can zoom into to see the details of this stunning cloud of dust and gas which hosts sites of star formation where new stars are being born.

The full image is available to view in the ESASky application, where you can zoom in a compare images of the same target taken in different wavelengths. There’s also a very large version of the image if you want to download and pursue it at your leisure.

This image shows a short-wavelength NIRCam mosaic of the inner Orion Nebula and Trapezium Cluster. It shows a region 4 light-years across, slightly less than the distance between the Sun and our nearest neighbor, Proxima Centauri. The full image on ESASky measures 21,000 x 14,351 pixels. NASA, ESA, CSA; Science leads and image processing: M. McCaughrean, S. Pearson

Also known as Messier 42, the Orion nebula is located just to the south of the Orion’s belt constellation and is one of the brightest nebulae in the sky, making it a key target for scientists studying star formation. As new stars are born, those which are young and very hot give off ultraviolet radiation which illuminates the clouds of dust and gas around them. At the heart of this nebula is a group of stars called the Trapezium Cluster, which are young and bright, some of which are up to 30 times the mass of our sun.

This image reveals some cosmic oddities as well. Scientists told the New York Times that the observations included 150 free-floating objects, some of which are in pairs. They are similar to rogue planets that don’t orbit a star, but it’s not clear how they formed within the nebula. “There’s something wrong with either our understanding of planet formation, star formation — or both,” ESA scientist Samuel Pearson told the Times, puzzling over the presence of these objects. “They shouldn’t exist.”

The unusual objects have been named Jupiter Mass Binary Objects, or JuMBOs, and can be smaller than Jupiter but reach temperatures of over 1,000 degrees Fahrenheit. The unexpected discovery suggests there may be aspects of planetary formation that we don’t yet understand.

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What Will Plants Be Like on Alien Worlds?

What Will Plants Be Like on Alien Worlds?

Duffy and Haworth speculate that on remote planets, communities of purple bacteria could swell in black sulfurous oceans, or spread in films around local sources of hydrogen sulfide. If they evolved into plants that could survive on land, like Earth plants they would still angle their light-absorbing surfaces toward their star, but they might be purple, red, or orange, depending on the wavelengths of light they are attuned to. They’d still have clumps of cells that coax nutrients from the ground, but they would be seeking different nutrients. (For plants on Earth, nitrates and phosphates are critical.)

If these scientists are correct that botanical life could arise in red dwarf systems, astronomers then need to figure out where to point their telescopes to find it. To start, scientists typically focus on the habitable zone around each star, also sometimes called a “Goldilocks” region because it’s neither too hot nor too cold for liquid water on a planet’s surface. (Too hot and water will evaporate away. Too cold and it will permanently turn to ice.) Since water is likely necessary for most kinds of life, it’s an exciting development when astronomers find a rocky world in this zone—or in the case of the TRAPPIST-1 system, multiple worlds.

But University of Georgia astrophysicist Cassandra Hall says perhaps it’s time to rethink the habitable zone in a way that emphasizes not just water but also light. In a study earlier this year, Hall’s group focused on factors like starlight intensity, the planet’s surface temperature, the density of its atmosphere, and how much energy organisms would need to expend for mere survival, rather than growth. Considering these together, they estimated a “photosynthetic habitable zone” that lies a bit closer to a planet’s star than the traditional habitable zone for water. Think of an orbit more like Earth’s and less like Mars’.

Hall highlights five promising worlds that have already been discovered: Kepler-452 b, Kepler-1638 b, Kepler-1544 b, Kepler-62 e and Kepler-62 f. They’re rocky planets in the Milky Way, mostly a bit larger than Earth but not gas giants like “mini-Neptunes,” and they spend a significant fraction of their orbits, if not the entire orbit, within their star’s photosynthetic habitable zone. (Astronomers found them all within the past decade using NASA’s Kepler Space Telescope.) 

Of course, the hard part is trying to spot clear signs of life from more than 1,000 light-years away. Astrobiologists look for particular chemical signatures lurking in exoplanets’ atmospheres. “Generally, you’re looking for signs of chemical disequilibrium, large amounts of gases that are incompatible with each other because they react with each other to form different things,” Hall says. These could indicate life processes like respiration or decay. 

A combination of carbon dioxide and methane would be a prime example, since both can be given off by life forms, and methane doesn’t last long unless it’s constantly being produced, such as from the decomposition of plant matter by bacteria. But that’s no smoking gun: Carbon and methane could just as well be produced by a lifeless, volcanically active world. 

Other signatures could include oxygen, or its spin-off, ozone, which is generated when stellar radiation splits oxygen molecules. Or perhaps sulfide gases could indicate the presence of photosynthesis without the presence of oxygen. Yet all of these can come from abiotic sources, such as ozone from water vapor in the atmosphere, or sulfides from volcanoes.

While Earth is a natural reference point, scientists shouldn’t limit their perspective to only life as we know it, argues Nathalie Cabrol, an astrobiologist and director of the SETI Institute’s Carl Sagan Center. Seeking just the right conditions for oxygenic photosynthesis could mean narrowing the search too much. It’s possible life isn’t that rare in the universe. “Right now, we have no clue if we have the only biochemistry,” she says.

If alien plants can survive or even thrive without oxygenic photosynthesis, that ultimately could mean expanding, rather than tapering, the habitable zone, Cabrol says. “We need to keep our minds open.”