There comes a time in the life of every intelligent species when it confronts a fundamental question: Are we alone in the universe? For humanity, that moment may have arrived. Not with alien spacecraft in our skies, not with mysterious radio signals from the depths of space, but with hard scientific data from an exoplanet 124 light-years away. K2-18b it's not a particularly evocative or poetic name, but could soon become synonymous with one of the greatest discoveries in human history. Through the artificial eyes of the space telescope James Webb, astronomers have glimpsed something that shouldn't have been there: molecules that, as far as we know, are produced exclusively by living organisms.
K2-18b and the signs of life
“These are the first traces we are seeing of an alien world that is possibly inhabited,” he said. Nikku Madhusudhan ofUniversity of Cambridge during a press conference on March 15. This is no small statement. It's one of those statements that make you stop for a moment, like when you hear a strange noise in the middle of the night.
K2-18b has been discovered in 2015 and immediately proved to be a promising candidate for the search for life. About eight times more massive than Earth, it orbits in the so-called "habitable zone" of its star, 124 light years away; that band of space where water can exist in a liquid state. Observations Part 2019 had already found traces of water vapor, suggesting the possibility of oceans beneath a hydrogen-rich atmosphere (although not all astronomers agreed).
In 2023, Madhusudhan and colleagues used the James Webb to analyze K2-18b's atmosphere in infrared light, confirming the presence of water vapor, carbon dioxide, and methane. But there was also a tempting clue: the dimethyl sulfide (DMS), a molecule that on Earth is produced exclusively by living organisms, mainly marine phytoplankton. The signal, however, was extremely weak.
The scientific bomb
Now the same researchers have used a different instrument on JWST, the mid-infrared camera, to observe K2-18b again. And what they found is, frankly, shocking; a much stronger signal for DMS, along with another related molecule, dimethyl disulfide (DMDS), also produced on Earth only by living beings.
What we are finding is an independent line of evidence in a different wavelength range, with a different instrument, of possible biological activity on the planet.
The team supports, I'll link the paper here, that the detection of DMS and DMDS is at a level of statistical significance “three sigma“: means that there is only a 3 in 1000 chance that a data pattern like this will end up being a fluke. Of course, it must be said that in physics the gold standard for accepting something as a real discovery it's "five sigma", which equates to a 1 in 3,5 million chance that the data is a random occurrence. But the difficulty of observing a planet's atmosphere is immense. As he explains Thomas beatty ofUniversity of Wisconsin-Madison:
“The relative size of the atmosphere to the size of the planet is very similar to the thickness of an apple peel on an apple. That’s what we’re trying to measure.”
Caution required
Nicholas Wogan of the NASA Ames Research Center in California admits that the evidence is more compelling than the 2023 results, but still requires verification by other groups. Once the data is made public, other researchers can begin to confirm the results, but that could take weeks or months because of the difficulty of interpreting the JWST data.
There are also those who are more skeptical. Ryan MacDonald ofUniversity of Michigan warns: “We have a ‘wolf, wolf’ situation for K2-18b, where previous three-sigma detections have completely vanished when subjected to closer examination.” Madhusudhan and his team estimate that 16 to 24 hours of additional observations with JWST could help them reach the five-sigma level, but they cannot guarantee it because of the difficulty of observing the planet's atmosphere.
K2-18b, a surprising abundance
If the concentrations of DMS and DMDS that appear to be present on K2-18b (over 10 parts per million) are confirmed, would be thousands of times higher than the concentrations in the Earth's atmosphere. This could indicate a much greater amount of biological activity than on Earth, if the signal turns out to be correct.
Sarah Seager of the MIT suggests that K2-18b could remain in the “viable candidate” category for a long time, perhaps decades, as the question may never be fully resolved with the limited data that exoplanets offer. In this whole story, a profoundly human aspect strikes me. Regardless of whether these molecules are the product of life or not, we are still looking at something extraordinary: an intelligence that arose from single-celled life billions of years ago and is now able to peer into the atmosphere of a planet 124 light-years away.
Whether or not there is life on K2-18b, the real wonder, in the end, is we who look.
