Does the Planet K2-18b Really Have a 99.7% Chance of Life?

If you have seen 99.7% attached to the planet K2-18b, it does not mean the planet has a 99.7% chance of life. The figure comes from an April 2025 result, and it measures something narrower.

It is a confidence level: how sure researchers are that a chemical signature in the planet's atmosphere is real, rather than a statistical fluke. That is a claim about data, not about biology.

The result behind that figure is dated April 2025. A team led by Nikku Madhusudhan at the University of Cambridge's Institute of Astronomy reported tentative chemical evidence in K2-18b's atmosphere.

The evidence came from NASA's James Webb Space Telescope, and the team linked it to chemicals that could be associated with life. The research was published in The Astrophysical Journal Letters, with news reports on 17 and 18 April 2025.

The team was careful about its language: it described a possible biosignature, an indicator of a biological process, not the discovery of living organisms.

What Webb Actually Detected

This is not a photograph of the atmosphere. The measurement relies on transit spectroscopy: as K2-18b passes in front of its host star from Earth's viewpoint, a small fraction of starlight filters through its atmosphere.

Webb measures the star's slight dimming and that filtered light. From those measurements, researchers infer which gases the atmosphere contains.

The gases at the centre of the story are dimethyl sulphide (DMS) and dimethyl disulphide (DMDS). On Earth, both are produced by marine phytoplankton, bacteria and marine algae. Webb's data show at least one of the two, possibly both, at 99.7% confidence.

The team estimated the gas concentration at more than 10 parts per million by volume — thousands of times higher than in Earth's atmosphere. Madhusudhan said that, based on existing knowledge, it cannot be explained without biological activity.

The gas was apparently detected during a single observation window. Madhusudhan said he was surprised by how much gas was estimated, and added that if the association with life is real, the planet will be teeming with life.

This was not the first notable signal from K2-18b. Earlier Webb observations had identified methane and carbon dioxide in its atmosphere — the first time carbon-based molecules were found in an exoplanet in a star's habitable zone.

Why 99.7% Is Not Enough

A 99.7% confidence level corresponds to three sigma, and it comes with a mirror image: a 0.3% chance that the observation is a statistical fluke.

So the figure is not a 99.7% probability that K2-18b has life. It is the confidence that the signature of those molecules is real rather than a fluke.

Researchers require a five-sigma result before calling something a discovery. BBC described five sigma as about 99.99999% confidence, or a less-than-one-in-a-million chance of being a fluke.

The April 2025 result was the second, more promising JWST detection of life-associated chemicals in K2-18b's atmosphere. About 18 months earlier, the team had reported a one-sigma result at 68% confidence, which was greeted with scepticism.

The evidence had strengthened. It still fell short of the five-sigma bar, which is why the team described a possibility rather than a confirmed result.

What Five Sigma Would Not Prove

Even a five-sigma result would not settle the biology question. Scientists would still need to work out whether the molecules came from living things or from non-biological geological or chemical processes.

Harvard astrophysicist Avi Loeb noted one reason for caution: researchers also see these molecules on comets, and comets do not host life.

Independent astronomer Catherine Heymans made a related point. Even with perfect data, she said, nobody could be sure of a biological origin on an alien world, because unknown geological activity might produce the same molecules.

The Cambridge team agrees that non-biological production has to be ruled out. It is working with other groups to test whether DMS and DMDS can be produced without living things in the laboratory.

Debate Over K2-18b's Structure

Underneath the chemistry sits a more basic uncertainty: what kind of planet K2-18b actually is. Cambridge's Oliver Shorttle cautioned that the signal is an incredibly tenuous reading of tiny amounts of light, and part of the debate concerns the planet's structure.

Many researchers infer a liquid ocean from the absence of ammonia in the atmosphere, since water could absorb it. Shorttle points to another explanation: an ocean of molten rock, which would rule out life.

NASA Ames researcher Nicolas Wogan published an interpretation that K2-18b is a mini gas giant with no surface at all. Other groups have challenged that and other alternatives as inconsistent with Webb's data, so the planet's nature remains debated.

The planet orbits a red dwarf smaller and less luminous than the Sun, about 124 light-years away in the constellation Leo. It is about 2.6 times Earth's diameter and roughly 8.6 times as massive as Earth, and it orbits in its star's habitable zone.

What the Team Says Next

In April 2025, Madhusudhan said he hoped to obtain confirming evidence within one to two years, aiming to reduce the chance of a fluke to less than one in a million.

Asked whether the planet might host multicellular or intelligent life, he said the team cannot answer at this stage. Its baseline assumption is simple microbial life.

The team is not claiming to have found organisms. It describes a possible biosignature and says more observations are needed.

The 99.7% figure is not a probability of life. It is a confidence score for one chemical signal — a promising lead from April 2025, not a discovery.