K2-18 b, which is approximately 2.6 times the radius of Earth, sits 124 light years away in the constellation of Leo. If you’re keen to spot it, you can. “If you locate the Big Dipper, there are two stars that form the far side of the pan. If you follow those downwards, you’ll come to a sort of reversed question mark – and that is part of the constellation of Leo,” Ian says.
The planet orbits within the habitable zone of its star, a cool red dwarf less than half the size of the Sun. When the Hubble space telescope appeared to detect water vapour in its atmosphere in 2019, scientists hailed it as “the most habitable known world” beyond our solar system.
What did the scientists find this time?
There is no way for people to photograph or reach planets beyond our solar system. However, scientists can investigate and assess their properties, including chemical composition using the James Webb telescope. It is so powerful that it can track the chemical signatures of K2-18 b by observing the planet’s transit as it passes in front of its host star, measuring the starlight that has been filtered through its atmosphere. “All molecules absorb light at a particular wavelength, and you can use that as a chemical signature for that molecule in the atmosphere of another planet,” Ian explains. “So in this case, scientists found that the atmosphere of K2-18 b contains a lot of dimethyl sulfide [DMS] and dimethyl disulfide [DMDS].”
These two compounds are why so many scientists are excited. On Earth they are only produced naturally by living organisms, specifically marine phytoplankton. There is no natural process on Earth that produces these molecules otherwise.
Is this a big deal?
“There’s no doubt this is cool and a really interesting detection … but there is a but,” Ian says.
Sceptical scientists have queried the findings and what they reveal, pointing out that there is still some debate about the conditions of K2-18 b. “The team in Cambridge think there’s a vast water based ocean there, but others will argue that the data suggests the oceans on this planet are made of magma or that it is a gas planet, which obviously isn’t particularly conducive to life as we know it,” Ian says.
A few more years of measurement are required to say more confidently that the astronomers are sure of these two compounds in the atmosphere. But there is also a possibility that, even if astronomers confirm robustly that DMS and DMDS are in the atmosphere of this planet, they may be created by unfamiliar chemical processes that do not involve life.
There is no way to go to the planet to see for ourselves, of course, but Prof Nikku Madhusudhan, who led the observations, told Hannah Devlin this has not been a barrier to the discovery of black holes or other cosmic phenomena. “In astronomy the question is never about going there,” he said. “We’re trying to establish if the laws of biology are universal in nature. I don’t see it as ‘We have to go and swim in the water to catch the fish’.”
What kind of life are we talking about?
If and when those debates are settled, the kind of life we are talking about here is not little green three-eyed men or giant tentacled beings. K2-18 b, and planets like it, are not expected to host intelligent alien life, but more likely tiny microbes similar to those that lived in Earth’s oceans billions of years ago.
For intelligent life to exist, there would presumably need to be a long period of stable environmental conditions for evolution to take place. “With microbes, the requirement is lower,” Ian says.
Detecting signs of intelligent life might not involve biology and biosignatures at all. “You would be looking for what they call technosignatures, which would be a form of radio or electromagnetic waves,” Ian says. “They may be so advanced that they’re way ahead of using those kinds of technologies but that’s another thing to think about.”
What now?
Conducting this kind of research requires long-term, reliable funding. The James Webb space telescope took three decades to build, with an initial price tag of $10.8bn, which includes $861m set aside to support five years of operations.
Nasa has been experiencing funding cuts set out by the Trump administration and the team overseeing the telescope could face budget cuts of up to 20%, affecting every aspect of the flagship observatory’s operations.
“If you start dismantling, undermining, sacking, redirecting research at a whim, you will completely destabilise all the work, and there is no doubt that cuts in the US will have a global impact because the US is such a dominant player in science,” Ian says. Even the feeling of uncertainty alone can have a chilling effect on research. “One scientist in the US told me that this could set US science back a couple of decades.”
