The essential guide to proving we’ve found alien life

From mudstones on Mars to strange gases in exoplanet atmospheres, tentative evidence for extraterrestrial life is starting to come thick and fast. But when we've found it, how will we know for sure?

Space

The essential guide to proving we’ve found alien life

From mudstones on Mars to strange gases in exoplanet atmospheres, tentative evidence for extraterrestrial life is starting to come thick and fast. But when we've found it, how will we know for sure?

By Miriam Frankel

22 December 2025

[New Scientist. Science news and long reads from expert journalists, covering developments in science, technology, health and the environment on the website and the magazine.]

Giulia Calistro

The afternoon of 7 August 1996 isn’t a time that sticks in many people’s minds. But if things had worked out differently, it might have been etched into our collective memory. At 1.15pm, US President Bill Clinton stepped onto the White House’s verdant South Lawn to speak about the possible detection of life in a Martian meteorite. “If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered,” he said.

But it wasn’t: it joined a list of inconclusive claims about alien life. We don’t, of course, have to go all the way back to the 1990s to find other such claims – just a few years ago, the discovery of phosphine gas in the atmosphere of Venus got scientists excited. And in 2017, Avi Loeb at Harvard University said the interstellar object ʻOumuamua was a piece of alien technology.

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With a slew of new missions poised to return data from alien worlds, the pace of these potential discoveries is likely to accelerate. So, what questions should we ask ourselves when apparent evidence of alien life inevitably arrives? In this guide, we’ll walk through the most plausible ways it might first show up, from faint chemical signatures to fossilised microbes. Think of it as a scientific gut-check – a sliding scale of how close different scenarios come to proving alien life exists next time headlines proclaim we’re not alone.

Scenario 1: We detect biosignatures in the atmosphere of a distant exoplanet

News breaks that a planet light years away from Earth has an atmosphere seemingly laced with gases we associate with life. Headlines trumpet a “breathable world”, and the internet goes wild. But what should we make of such a claim?

This is one of the most plausible ways we might first see signs of alien life – not through little green men, but via telltale molecules in the air. On Earth, life has radically altered the atmosphere: microbes, plants and people all leave chemical traces. If the same is true elsewhere, then telescopes trained on exoplanets could pick out radiation absorbed and emitted by gas molecules exhaled by alien organisms that’s present in the exoplanets’ atmospheres.

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Some gases are more suggestive than others. Carbon dioxide and water vapour, for instance, can come from both biological and geological processes. Others, known as biosignatures, are harder to explain without the presence of life. But identifying them – and proving they truly point to biology – is trickier than it sounds.

Take K2-18b, a planet about 120 light years away from us. In 2023, Nikku Madhusudhan at the University of Cambridge and his team discovered tentative signs of dimethyl sulphide (DMS) in data from the James Webb Space Telescope (JWST). On Earth, DMS is produced only by marine plankton and bacteria.

Madhusudhan also found a second, similar gas, dimethyl disulphide (DMDS), in data from a different instrument, supporting his team’s original findings. This year, a paper found similar, though slightly weaker, evidence of DMS, also via the JWST. “It is still not at a level that we want for claiming a robust detection,” says Madhusudhan. “But the signal has increased, so we’re going in the right direction. We need more data.”

[Groups dressed as aliens ride through downtown Roswell, New Mexico July 1, 2000 as they participate in the annual UFO Encounter.]

The first signs of alien life are unlikely to look like little green men arriving in saucers, but may instead be more subtle clues, like trace gases in a planet’s atmosphere

Joe Raedle/Newsmakers/Getty Images

Madhusudhan, for one, is optimistic. “We are seeing tentative signs of molecules that have been predicted to be biomarkers well before the observations, and that’s the important bit,” he says.

Others argue that data like this is too flimsy to support a claim of life. Andrew Rushby at Birkbeck, University of London, builds atmospheric models to test how reliable such detections really are. One of his students, Ruohan Liu, is currently investigatingalternative explanations for Madhusudhan’s findings. For example, many scientists assume that K2-18b has liquid water, which makes biological processes more likely but which may ultimately fail to be true. “Maybe it is more like a mini Neptune,” says Rushby, pointing out that a gas-rich planet could also produce the same spectra.

Rushby and his colleagues have even proposed a framework for evaluating potential signs of life based on probability. Instead of just asking “Could life make this?”, it asks “What else could?”, weighing different explanations by likelihood and context. That includes the planet’s temperature, chemical balance and the type of star it orbits.

[Alien plants: The search for photosynthesis on other worlds]

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Madhusudhan acknowledges the need for caution, but pushes back against the idea that gas signatures are always insufficient for proving alien life exists. Treating life as an extraordinary claim requiring extraordinary evidence stems from human bias, he says. After all, how are claims of life different from those of astrophysical objects that we can’t see directly? “How do we know there is a black hole at the centre of our galaxy?” he asks.

Ultimately, no single observation is likely to clinch the case for life. Proof, if it exists, will come in stages, and we do have some guidelines for how such a process would work (see “Seven steps to proving alien life exists”, below).

But to make a truly convincing case out of hot air, Rushby says the detection of more biologically produced gases from the same exoplanet could strengthen the findings, highlighting oxygen and methane as crucial. And we will need several observations, he says: “The more independent lines of evidence we can have, the better we can rule out systematics like problems with a specific telescope that might be fooling us.”

So if, in the not-too-distant future, an exoplanet’s atmosphere sets the internet ablaze, keep your cool. A few whiffs of something fishy in the air don’t yet mean life is out there. But they do mean it’s worth looking harder.

Potential for detecting alien life: 1/10.

Scenario 2: Water samples from the ocean of an icy moon contain biological molecules

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