Manoj Joshi, University of East Anglia; Andrew Rushby, Birkbeck, University of London, and Maria Di Paolo, University of East Anglia A team of researchers has recently claimed they have discovered a gas called dimethyl sulphide (DMS) in the atmosphere of K2-18b, a planet orbiting a distant star. The University of Cambridge team’s claims are potentially very exciting because, on Earth at least, the compound is produced by marine bacteria. The presence of this gas may be a sign of life on K2-18b too – but we can’t rush to conclusions just yet. K2-18b has a radius 2.6 times that of Earth, a mass nearly nine times greater and orbits a star that is 124 light years away. We can’t directly tell what kinds of large scale characteristics it has, although one possibility is a world with a global liquid water ocean under a hydrogen-rich atmosphere. Such a world might well be hospitable to life, but different ideas exist about the properties of this planet – and what that might mean for a DMS signature.

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Claims for the detection of life on other planets go back decades. In the 1970s, one of the scientists working on the Viking mission to Mars claimed that his experiment had indicated there could be microorganisms in the Martian soil. However, these conclusions were widely refuted by other researchers. In 1996, a team said that microscopic features resembling bacteria had been found in the Martian meteorite ALH84001. However, subsequent studies cast significant doubt on the discovery. Since the early 2000s there have also been repeated claims for the detection of methane gas in the atmosphere of Mars, both by remote sensing by satellites and by in-situ observations by rovers. Methane can be produced by several mechanisms. One of these potential sources involves production by microorganisms. Such sources are described by scientists as being “biotic”. Other sources of methane, such as volcanoes and hydrothermal vents, don’t require life and are said to be “abiotic”.

Not all of the previous claims for evidence of extraterrestrial life involve the red planet. In 2020, Earth-based observations of Venus’s atmosphere implied the presence of low levels of phosphine gas. Because phosphine gas can be produced by microbes, there was speculation that life might exist in Venus’s clouds. However, the detection of phosphine was later disputed by other scientists. Proposed signs of life on other worlds are known as “biosignatures”. This is defined as “an object, substance, and/or pattern whose origin specifically requires a biological agent”. In other words, any detection requires all possible abiotic production pathways to be considered. In addition to this, scientists face many challenges in the collection, interpretation, and planetary environmental context of possible biosignature gases. Understanding the composition of a planetary atmosphere from limited data, collected from light years away, is very difficult. We also have to understand that these are often exotic environments, with conditions we do not experience on Earth. As such, exotic chemical processes may occur here too. In order to characterise the atmospheres of exoplanets, we obtain what are called spectra. These are the fingerprints of molecules in the atmosphere that absorb light at specific wavelengths. Once the data has been collected, it needs to be interpreted. Astronomers assess which chemicals, or combinations thereof, best fit the observations. It is an involved process and one that requires lots of computer based work. The process is especially challenging when dealing with exoplanets, where available data is at a premium. Once these stages have been carried out, astronomers can then assign a confidence to the likelihood of a particular chemical signature being “real”. In the case of the recent discovery from K2-18b, the authors claim the detection of a feature that can only be explained by DMS with a likelihood of greater than 99.9%. In other words, there’s about a 1 in 1,500 chance that this feature is not actually there. While the team behind the recent result favours a model of K2-18b as an ocean world, another team suggests it could actually have a magma (molten rock) ocean instead. It could also be a Neptune-like “gas dwarf” planet, with a small core shrouded in a thick layer of gas and ices. Both of these options would be much less favourable to the development of life – raising questions as to whether there are abiotic ways that DMS can form.

A higher bar?

But is the bar higher for claims of extraterrestrial life than for other areas of science? In a study claiming the detection of a biosignature, the usual level of scientific rigour expected for all research should apply to the collection and processing of the data, along with the interpretation of the results. However, even when these standards have been met, claims that indicate the presence of life have in the past still been meet with high levels of scepticism. The reasons for this are probably best summed up by the phrase “extraordinary claims require extraordinary evidence”. This is attributed to the American planetary scientist, author and science communicator Carl Sagan. While on Earth there are no known means of producing DMS without life, the chemical has been detected on a comet called 67/P, which was studied up close by the European Space Agency’s Rosetta spacecraft. DMS has even been detected in the interstellar medium, the space between stars, suggesting that it can be produced by non-biological, or abiotic, mechanisms. Given the uncertainties about the nature of K2-18b, we cannot be sure if the presence of this gas might simply be a sign of non-biological processes we don’t yet understand. The claimed discovery of DMS on K2-18b is interesting, exciting, and reflects huge advances in astronomy, planetary science and astrobiology. However, its possible implications mean that we have to consider the results very cautiously. We must also entertain alternative explanations before supporting such a profound conclusion as the presence of extraterrestrial life. Manoj Joshi, Professor of Climate Dynamics, University of East Anglia; Andrew Rushby, Lecturer, School of Natural Sciences, Birkbeck, University of London, and Maria Di Paolo, PhD Candidate, School of Engineering, Mathematics and Physics, University of East Anglia This article is republished from The Conversation under a Creative Commons license. Read the original article.

David Garofalo, Kennesaw State University One of the most powerful objects in the universe is a radio quasar – a spinning black hole spraying out highly energetic particles. Come too close to one, and you’d get sucked in by its gravitational pull, or burn up from the intense heat surrounding it. But ironically, studying black holes and their jets can give researchers insight into where potentially habitable worlds might be in the universe. As an astrophysicist, I’ve spent two decades modeling how black holes spin, how that creates jets, and how they affect the environment of space around them.

What are black holes?

Black holes are massive, astrophysical objects that use gravity to pull surrounding objects into them. Active black holes have a pancake-shaped structure around them called an accretion disk, which contains hot, electrically charged gas. The plasma that makes up the accretion disk comes from farther out in the galaxy. When two galaxies collide and merge, gas is funneled into the central region of that merger. Some of that gas ends up getting close to the newly merged black hole and forms the accretion disk. There is one supermassive black hole at the heart of every massive galaxy. Black holes and their disks can rotate, and when they do, they drag space and time with them – a concept that’s mind-boggling and very hard to grasp conceptually. But black holes are important to study because they produce enormous amounts of energy that can influence galaxies. How energetic a black hole is depends on different factors, such as the mass of the black hole, whether it rotates rapidly, and whether lots of material falls onto it. Mergers fuel the most energetic black holes, but not all black holes are fed by gas from a merger. In spiral galaxies, for example, less gas tends to fall into the center, and the central black hole tends to have less energy. One of the ways they generate energy is through what scientists call “jets” of highly energetic particles. A black hole can pull in magnetic fields and energetic particles surrounding it, and then as the black hole rotates, the magnetic fields twist into a jet that sprays out highly energetic particles. Magnetic fields twist around the black hole as it rotates to store energy – kind of like when you pull and twist a rubber band. When you release the rubber band, it snaps forward. Similarly, the magnetic fields release their energy by producing these jets.

These jets can speed up or suppress the formation of stars in a galaxy, depending on how the energy is released into the black hole’s host galaxy.

Rotating black holes

Some black holes, however, rotate in a different direction than the accretion disk around them. This phenomenon is called counterrotation, and some studies my colleagues and I have conducted suggest that it’s a key feature governing the behavior of one of the most powerful kinds of objects in the universe: the radio quasar. Radio quasars are the subclass of black holes that produce the most powerful energy and jets. You can imagine the black hole as a rotating sphere, and the accretion disk as a disk with a hole in the center. The black hole sits in that center hole and rotates one way, while the accretion disk rotates the other way. This counterrotation forces the black hole to spin down and eventually up again in the other direction, called corotation. Imagine a basketball that spins one way, but you keep tapping it to rotate in the other. The tapping will spin the basketball down. If you continue to tap in the opposite direction, it will eventually spin up and rotate in the other direction. The accretion disk does the same thing. Since the jets tap into the black hole’s rotational energy, they are powerful only when the black hole is spinning rapidly. The change from counterrotation to corotation takes at least 100 million years. Many initially counterrotating black holes take billions of years to become rapidly spinning corotating black holes. So, these black holes would produce powerful jets both early and later in their lifetimes, with an interlude in the middle where the jets are either weak or nonexistent. When the black hole spins in counterrotation with respect to its accretion disk, that motion produces strong jets that push molecules in the surrounding gas close together, which leads to the formation of stars. But later, in corotation, the jet tilts. This tilt makes it so that the jet impinges directly on the gas, heating it up and inhibiting star formation. In addition to that, the jet also sprays X-rays across the galaxy. Cosmic X-rays are bad for life because they can harm organic tissue. For life to thrive, it most likely needs a planet with a habitable ecosystem, and clouds of hot gas saturated with X-rays don’t contain such planets. So, astronomers can instead look for galaxies without a tilted jet coming from its black hole. This idea is key to understanding where intelligence could potentially have emerged and matured in the universe.

Black holes as a guide

By early 2022, I had built a black hole model to use as a guide. It could point out environments with the right kind of black holes to produce the greatest number of planets without spraying them with X-rays. Life in such environments could emerge to its full potential.

Where are such conditions present? The answer is low-density environments where galaxies had merged about 11 billion years ago. These environments had black holes whose powerful jets enhanced the rate of star formation, but they never experienced a bout of tilted jets in corotation. In short, my model suggested that theoretically, the most advanced extraterrestrial civilization would have likely emerged on the cosmic scene far away and billions of years ago. David Garofalo, Professor of Physics, Kennesaw State University This article is republished from The Conversation under a Creative Commons license. Read the original article.

Chris Impey, University of Arizona The detection of life beyond Earth would be one of the most profound discoveries in the history of science. The Milky Way galaxy alone hosts hundreds of millions of potentially habitable planets. Astronomers are using powerful space telescopes to look for molecular indicators of biology in the atmospheres of the most Earth-like of these planets. But so far, no solid evidence of life has ever been found beyond the Earth. A paper published in April 2025 claimed to detect a signature of life in the atmosphere of the planet K2-18b. And while this discovery is intriguing, most astronomers – including the paper’s authors – aren’t ready to claim that it means extraterrestrial life exists. A detection of life would be a remarkable development. The astronomer Carl Sagan used the phrase, “Extraordinary claims require extraordinary evidence,” in regard to searching for alien life. It conveys the idea that there should be a high bar for evidence to support a remarkable claim. I’m an astronomer who has written a book about astrobiology. Over my career, I’ve seen some compelling scientific discoveries. But to reach this threshold of finding life beyond Earth, a result needs to fit several important criteria.

When is a result important and reliable?

There are three criteria for a scientific result to represent a true discovery and not be subject to uncertainty and doubt. How does the claim of life on K2-18b measure up? First, the experiment needs to measure a meaningful and important quantity. Researchers observed K2-18b’s atmosphere with the James Webb Space Telescope and saw a spectral feature that they identified as dimethyl sulfide. On Earth, dimethyl sulfide is associated with biology, in particular bacteria and plankton in the oceans. However, it can also arise by other means, so this single molecule is not conclusive proof of life. Second, the detection needs to be strong. Every detector has some noise from the random motion of electrons. The signal should be strong enough to have a low probability of arising by chance from this noise. The K2-18b detection has a significance of 3-sigma, which means it has a 0.3% probability of arising by chance. That sounds low, but most scientists would consider that a weak detection. There are many molecules that could create a feature in the same spectral range. The “gold standard” for scientific detection is 5-sigma, which means the probability of the finding happening by chance is less than 0.00006%. For example, physicists at CERN gathered data patiently for two years until they had a 5-sigma detection of the Higgs boson particle, leading to a Nobel Prize one year later in 2013.

Third, a result needs to be repeatable. Results are considered reliable when they’ve been repeated – ideally corroborated by other investigators or confirmed using a different instrument. For K2-18b, this might mean detecting other molecules that indicate biology, such as oxygen in the planet’s atmosphere. Without more and better data, most researchers are viewing the claim of life on K2-18b with skepticism.

Claims of life on Mars

In the past, some scientists have claimed to have found life much closer to home, on the planet Mars. Over a century ago, retired Boston merchant turned astronomer Percival Lowell claimed that linear features he saw on the surface of Mars were canals, constructed by a dying civilization to transport water from the poles to the equator. Artificial waterways on Mars would certainly have been a major discovery, but this example failed the other two criteria: strong evidence and repeatability. Lowell was misled by his visual observations, and he was engaging in wishful thinking. No other astronomers could confirm his findings.

In 1996, NASA held a press conference where a team of scientists presented evidence for biology in the Martian meteorite ALH 84001. Their evidence included an evocative image that seemed to show microfossils in the meteorite. However, scientists have come up with explanations for the meteorite’s unusual features that do not involve biology. That extraordinary claim has dissipated. More recently, astronomers detected low levels of methane in the atmosphere of Mars. Like dimethyl sulfide and oxygen, methane on Earth is made primarily – but not exclusively – by life. Different spacecraft and rovers on the Martian surface have returned conflicting results, where a detection with one spacecraft was not confirmed by another. The low level and variability of methane on Mars is still a mystery. And in the absence of definitive evidence that this very low level of methane has a biological origin, nobody is claiming definitive evidence of life on Mars.

Claims of advanced civilizations

Detecting microbial life on Mars or an exoplanet would be dramatic, but the discovery of extraterrestrial civilizations would be truly spectacular. The search for extraterrestrial intelligence, or SETI, has been underway for 75 years. No messages have ever been received, but in 1977 a radio telescope in Ohio detected a strong signal that lasted only for a minute. This signal was so unusual that an astronomer working at the telescope wrote “Wow!” on the printout, giving the signal its name. Unfortunately, nothing like it has since been detected from that region of the sky, so the Wow! Signal fails the test of repeatability.

In 2017, a rocky, cigar-shaped object called ‘Oumuamua was the first known interstellar object to visit the solar system. ‘Oumuamua’s strange shape and trajectory led Harvard astronomer Avi Loeb to argue that it was an alien artifact. However, the object has already left the solar system, so there’s no chance for astronomers to observe it again. And some researchers have gathered evidence suggesting that it’s just a comet. While many scientists think we aren’t alone, given the enormous amount of habitable real estate beyond Earth, no detection has cleared the threshold enunciated by Carl Sagan.

Claims about the universe

These same criteria apply to research about the entire universe. One particular concern in cosmology is the fact that, unlike the case of planets, there is only one universe to study. A cautionary tale comes from attempts to show that the universe went through a period of extremely rapid expansion a fraction of a second after the Big Bang. Cosmologists call this event inflation, and it is invoked to explain why the universe is now smooth and flat. In 2014, astronomers claimed to have found evidence for inflation in a subtle signal from microwaves left over after the Big Bang. Within a year, however, the team retracted the result because the signal had a mundane explanation: They had confused dust in our galaxy with a signature of inflation. On the other hand, the discovery of the universe’s acceleration shows the success of the scientific method. In 1929, astronomer Edwin Hubble found that the universe was expanding. Then, in 1998, evidence emerged that this cosmic expansion is accelerating. Physicists were startled by this result. Two research groups used supernovae to separately trace the expansion. In a friendly rivalry, they used different sets of supernovae but got the same result. Independent corroboration increased their confidence that the universe was accelerating. They called the force behind this accelerating expansion dark energy and received a Nobel Prize in 2011 for its discovery. On scales large and small, astronomers try to set a high bar of evidence before claiming a discovery. Chris Impey, University Distinguished Professor of Astronomy, University of Arizona This article is republished from The Conversation under a Creative Commons license. Read the original article.