There are lots of “ifs” and “buts” at this stage, as Prof Madhusudhan’s team freely admits.

Firstly, this latest detection is not at the standard required to claim a discovery.

For that, the researchers need to be about 99.99999% sure that their results are correct and not a fluke reading. In scientific jargon, that is a five sigma result.

These latest results are only three sigma, or 99.7%. Which sounds like a lot, but it is not enough to convince the scientific community. However it is much more than the one sigma result 0f 68% the team obtained 18 months ago. which was greeted with much skepticism at the time.

But even if the Cambridge team obtains a five sigma result, that won’t be conclusive proof that life exists on the planet, according to Prof Catherine Heymans of Edinburgh University and Scotland’s Astronomer Royal, who is independent of the research team.

“Even with that certainty, there is still the question of what is the origin of this gas,” she told BBC News.

“On Earth it is produced by microorganisms in the ocean, but even with perfect data we can’t say for sure that this is of a biological origin on an alien world because loads of strange things happen in the Universe and we don’t know what other geological activity could be happening on this planet that might produce the molecules.”

That view is one the Cambridge team agree with. They are working with other groups to see if DMS and DMDS can be produced by non-living means in the lab.

“There is still a 0.3% chance that it might be a statistical fluke,” Prof Madhusudhan said.

Suggesting life may exist on another planet was “a big claim if true”, he told BBC Radio 4’s Today programme, adding: “So we want to be really, really thorough, and make more observations, and get the evidence to the level that there is less than a one-in-a-million chance of it being a fluke.”

He said this should be possible in “maybe one or two years”.

Other research groups have put forward alternative, lifeless, explanations for the data obtained from K2-18b. There is a strong scientific debate not only about whether DMS and DMDS are present but also the planet’s composition.

The reason many researchers infer that the planet has a vast liquid ocean is the absence of the gas ammonia in K2-18b’s atmosphere. Their theory is that the ammonia is absorbed by a vast body of water below.

But it could equally be explained by an ocean of molten rock, which would preclude life, according to Prof Oliver Shorttle of Cambridge University.

“Everything we know about planets orbiting other stars comes from the tiny amounts of light that glance off their atmospheres. So it is an incredibly tenuous signal that we are having to read, not only for signs of life, but everything else,” he said.

“With K2-18b part of the scientific debate is still about the structure of the planet.”

Dr Nicolas Wogan at Nasa’s Ames Research Center has yet another interpretation of the data. He published research suggesting that K2-18b is a mini gas giant with no surface.

Both these alternative interpretations have also been challenged by other groups on the grounds that they are inconsistent with the data from JWST, compounding the strong scientific debate surrounding K2-18b.

Prof Chris Lintott, presenter of the BBC’s The Sky at Night, said he had “great admiration” for Prof Madhusudhan’s team, but was treating the research with caution.

“I think we’ve got to be very careful about claiming that this is ‘a moment’ on the search to life. We’ve [had] such moments before,” he told Today.

He said the research should be seen instead as “part of a huge effort to try and understand what’s out there in the cosmos”.

Prof Madhusudhan acknowledges that there is still a scientific mountain to climb if he is to answer one of the biggest questions in science. But he believes he and his team are on the right track.

“Decades from now, we may look back at this point in time and recognise it was when the living universe came within reach,” he said.

“This could be the tipping point, where suddenly the fundamental question of whether we’re alone in the universe is one we’re capable of answering.”

The research has been published in The Astrophysical Journal Letters.

[BBC]