Researchers claim to have found the “strongest evidence” of biological activity outside the solar system. The findings are tantalizing, but we wouldn’t draw any conclusion just yet.
Life, possibly
When the James Webb Space Telescope (JWST) first opened its gold-coated eye to the cosmos, we were all thinking it. Finding alien life wasn’t its main goal, but we were all hoping astronomers might glimpse signatures of life beyond Earth. Now, a team led by the University of Cambridge believes it just might have.
Their latest study, published in The Astrophysical Journal Letters, reports the detection of either dimethyl sulfide (DMS) or dimethyl disulfide (DMDS) in the atmosphere of K2-18b, a planet 124 light-years away. Here on Earth, both of these molecules are produced only by living organisms.
“These are the first hints we are seeing of an alien world that is possibly inhabited,” said Nikku Madhusudhan, the team’s lead researcher.
We know a bit about K2-18b. Its mass is roughly 8.6 times heavier than the Earth’s and its radius is 2.6 times bigger. It belongs to a class of planets dubbed “sub-Neptunes” — bigger than rocky worlds, smaller than gas giants. Such planets do not exist in our own solar system, but they dominate the Milky Way
In 2021, Madhusudhan and colleagues proposed that K2-18b might be a “Hycean” world: covered in a global ocean and wrapped in a hydrogen-rich atmosphere. That year, they detected carbon-based molecules — methane and carbon dioxide — in its skies. Then came a faint spectral signature that hinted at DMS, a sulfur-based compound known to be produced by phytoplankton and other marine life.
The initial detection, using JWST’s near-infrared instruments (NIRISS and NIRSpec), was not statistically conclusive. “We didn’t know for sure whether the signal we saw last time was due to DMS, but just the hint of it was exciting enough for us to have another look with JWST using a different instrument,” said Madhusudhan, from Cambridge’s Institute of Astronomy.
But when the team observed the planet again using JWST’s Mid-Infrared Instrument (MIRI), which operates in an entirely different part of the spectrum, the results were clearer. “The signal came through strong and clear,” said Madhusudhan. This second line of evidence pointed once more to DMS or DMDS — this time at a strength thousands of times higher than Earth’s atmospheric levels.
Why this is not 100% clear
“Extraordinary claims require extraordinary evidence,” cautions Laura Kreidberg of the Max Planck Institute for Astronomy, for NPR. She notes that even detecting the composition of a distant planet’s atmosphere is “an insanely difficult measurement.”
At present, the statistical confidence in the detection stands at three sigma — roughly a 99.7% probability that the signal is real. For most fields, that is a solid result. But not here. The scientific community generally demands five sigma — 99.99994% — before declaring a true discovery.
But there’s another issue.
On Earth, both DMS and DMDS are biosignatures. No known non-biological process produces them in large amounts. But K2-18b is not Earth. Its thick hydrogen atmosphere, high temperatures, and deep oceans could host unfamiliar chemistry. Scientists will need to run lab tests to see how these molecules behave in conditions like K2-18b.
“The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them” said co-author Subhajit Sarkar of Cardiff University.
“Our work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings,” adds co-author Savvas Constantinou, also from Cambridge’s Institute of Astronomy.
Very exciting, but hard to confirm
Researchers have mostly reacted with cautious excitement to this announcement, because in truth, there’s much we don’t know about this planet.
K2-18b, was first discovered in 2015 by NASA’s Kepler mission. Its existence was later confirmed with the Spitzer Space Telescope. K2-18b orbits a cool dwarf star that lies about 124 light years away, in the constellation of Leo. It lies in the so-called “Goldilocks zone” around the star, where temperatures are not too hot and not too cold to have liquid water and, presumably, possible life.
But that doesn’t necessarily mean there is life on it.
Some scientists think it’s not habitable at all A rival model suggests it could be a searing, rocky world with a magma ocean beneath its atmosphere — no place for life as we know it. But this is all tantalizing.
Madhusudhan says they’re not “currently claiming that this is due to life”. He admits the enormity of the claim. Yet he stands by the results.
For now, the consensus seems to be: promising, but premature.
There will undoubtedly be more studies on this planet. The prospect of alien life has never been closer, but we don’t have a smoking gun yet.
Journal Reference: The Astrophysical Journal Letters DOI: 10.3847/2041-8213/adc1c8
