Artist’s impression of the exoplanet K2-18b
A. Smith/N. Mandhusudhan
The search for life beyond our solar system heated up this year when scientists reported a tantalising signal from an exoplanet of a molecule that is known to be produced exclusively by life on Earth. Those hopes soon faded when other teams failed to confirm the detection, but the resulting vigorous debate was a good learning process for would-be alien spotters, say exoplanet researchers.
In April, Nikku Madhusudhan at the University of Cambridge and his colleagues announced in a press conference that they had seen the “first hints… of an alien world that is possibly inhabited”. Those hints came from K2-18b, a planet around eight times as massive as Earth, 124 light years away and in the habitable zone of its star, which they had observed with the James Webb Space Telescope (JWST).
The infrared light from K2-18b suggested that its atmosphere might contain a molecule called dimethyl sulphide (DMS), which, on Earth, is only produced by living organisms, primarily marine phytoplankton.
The news predictably caused a stir among the world’s media and scientific communities. But alongside the excitement, many researchers also urged caution. The DMS signal was extremely weak, and would require many follow-up observations and further analysis to confirm, they said.
Now, after months of additional observations and careful analysis, most astronomers agree that we can’t say that DMS, or anything resembling a biomolecule, exists in K2-18b’s atmosphere – and if it does, we can’t currently detect it. “The only two things that we know for sure are that there is methane and carbon dioxide in the atmosphere of this planet,” says Luis Welbanks at Arizona State University.
The claim that we might have seen alien life was premature, says Welbanks. “It has been repeatedly proven to not be accurate or correct. New observations show that the presence of those gases is not there,” says Welbanks.
However, the spike in the data that was originally attributed to DMS still requires explanation, says Jake Taylor at the University of Oxford. “There is this bump there. It’s physical. We see it. We just don’t know what the explanation is right now.”
Working out what molecule is causing the spike will require more observations of the planet, which are being planned with JWST next year, says Taylor. Scientists can only measure what is in the planet’s atmosphere using the starlight that passes through it when the planet moves in front of its host star, which happens four times in each Earth year.
For all the strife over the disputed discovery, it has led to some positives, says Taylor. “It has been a really good learning process for the exoplanet community as a whole. We’ve now gone back to the drawing board in terms of what definitions we should be using for different statistical methods. It’s been really, really useful for us,” he says.
“It helps us learn how to realign our expectations,” says Welbanks. “This is a lesson that if you have to play around with numbers to claim the presence of something, that is really challenging. Someone smarter than me said that there’s lies, damned lies and statistics. This whole thing about DMS falls into that category.”
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