Alien life, dimethyl sulfide, and exoplanets: what’s really been found?

On K2-18b, an exoplanet 8.6 times Earth’s size, a shortage of ammonia paired with the discovery of dimethyl sulfide and dimethyl disulfide in the atmosphere has sparked debate about whether the planet houses biological activity - or life. These molecules are significant because they contain carbon and here, on Earth, they are produced exclusively by life forms - primarily by phytoplankton. They rise to our atmosphere from the oceans, and are quickly reacted with to form other compounds, yet on K2-18b, these molecules survive for longer, and could point to something scientists have looked for proof of for centuries.

Cambridge University astrophysicist Nikku Madhusudhan initally found evidence if dimethyl sulfide on the planet in 2023, but last week presented that a second instrument of the James Webb Space Telescope found evidence of an abundance of the gas on the planet. This ultimately means that, on this planet, which orbits its dwarf star in the habitable zone, there could be an oceanic surface under the hydrogen-rich atmosphere: it could, just maybe support life. Now, this is not conclusive evidence that there is life here, that there is water here, or that that water would even be a temperature that could support life. However, it does encourage astronomers to look beyond “smaller rocky planets” like ours, to ‘sub-Neptune’ ‘Hycean worlds’ in the search for life in space.

Nora Hänni, a chemist at the University of Bern, says that dimethyl sulfide could be an “unambiguous biosignature”, a sign that life exists on planets other than Earth. However, her research has found the molecule on a frozen comet, and so she emphasises that caution must be taken, and that our knowledge of K2-18b is too limited to know whether dimethyl sulfide was produced abiotically in a way we simply do not see on Earth. While the Cambridge team seem convinced by the Occam’s razor explanation that dimethyl sulfide is clear evidence for “biological activity outside the Solar System”, other researchers are unconvinced, for reasons ranging from the planet even having water or a habitable surface to the gases not even being dimethyl sulfide or dimethyl disulfide.

So, while the finding is promising, it is not concrete proof that we are not alone. In 2023, when the team first reported their findings, they were “not robust”, but the team recognise the gravity of their research and how they report on it. It's a step in the right direction, and an important reminder any life we find may be microbial, on a planet completely unlike our own, and entirely not what we expect.