You may have already seen the headlines: Signs of life have reportedly been discovered on an alien world.
A team of astronomers led by Nikku Madhusudhan of the University of Cambridge used the James Webb Space Telescope to search for interesting molecules in the atmosphere of a planet outside our solar system called K2 18b. The team now says they’ve found molecules that, on Earth, are associated with life, in an abundance that is hard to explain otherwise.
Combining that with the planet’s mass, size and distance from its star, the team posits that this is an ocean world “teeming with life,” Madhusudhan said in a live-streamed talk about the discovery on April 17. “It is the strongest sign to date of any possibility of biological activity outside the solar system,” although more data is needed to confirm it, he said.
Other astronomers warn that crying “aliens” is premature at best. That includes Laura Kreidberg, who studies exoplanet atmospheres with JWST but was not involved in this study.
“It’s such a grandiose claim given the level of evidence that we’re currently seeing,” says Kreidberg, of the Max Planck Institute for Astronomy in Heidelberg, Germany.
Let’s break down that claim.
What did we already know about this planet?
K2 18b was discovered by scientists using the Kepler space telescope in 2015. The world orbits a dim red star about 125 light-years from Earth. It’s in the star’s habitable zone, or the right distance for liquid water to be stable on its surface — though previous researchers have noted that there’s more to being habitable than just being in the zone.
K2 18b is larger than Earth, about 2.6 times Earth’s diameter and 8.6 times its mass. That puts it in a class of planets called sub-Neptunes or mini-Neptunes that are unlike any of the worlds in our solar system.
Since we don’t have a nearby analog to study, astronomers rely on theoretical models to figure out the makeup of these planets — whether they’re rocky worlds with thick atmospheres or gaseous worlds with no rocky surfaces.
Do we know what this planet is made of?
Maybe. In earlier studies, Madhusudhan’s team suggested a novel structure for K2 18b: a planet-wide liquid water ocean underneath a thick hydrogen-rich atmosphere. They call it a Hycean world, using a portmanteau of hydrogen and ocean.
The pressure and temperature at the bottom of that atmosphere would melt rock, so there’s no solid surface like we have on Earth. But there could be a liquid water layer above a molten surface, “sort of like a floating ocean,” Kreidberg says.
“It sounds so cool. I would love for this to be real,” she says.
But she’s skeptical that such planets are common in the universe, if they exist at all. “It’s not impossible, but you have to be pretty finely tuned,” she says. “You’ve got to make the ingredients of the planet just right — an extreme Goldilocks situation.”
It’s hard to know for sure, but if that is what’s going on on K2 18b, it could be the first habitable zone planet whose atmosphere has been searched for hints of life.
How do we know what’s in its atmosphere?
K2 18b made itself known because it passes directly in front of, or transits, its host star from the point of view of Earth. Astronomers can deduce the planet’s size based on how much the star dims when the planet is blocking it.
Astronomers can also see how starlight changes as it travels through a planet’s atmosphere. Molecules in the atmosphere absorb light in specific wavelengths, leaving a chemical fingerprint on the light that reaches our telescopes. It’s like how you can’t see through your hand with your eyes, but X-rays let you see all the way to the bone.
Many of the molecules that could be signs of life absorb infrared wavelengths of light. That’s where the James Webb Space Telescope, which launched in December 2021, excels.
“JWST started revolutionizing this field almost immediately,” Madhusudhan said in the live-streamed talk. He and his colleagues observed K2 18b with two of JWST’s instruments in the telescope’s first year of operations.
In 2023, the team reported tentative signs of a molecule called dimethyl sulfide, a possible biosignature, in K2 18b’s atmosphere. In April 2024, they looked again with a third JWST instrument. Those observations strengthened the case for dimethyl sulfide and detected another potential biosignature, dimethyl disulfide, to boot. The team reports their results April 17 in the Astrophysical Journal Letters.
“We are seeing [dimethyl sulfide] or [dimethyl disulfide], or both, in this habitable zone planet, for which other data has been suggesting the most likely plausible explanation right now is a Hycean world with an ocean and a hydrogen-rich atmosphere,” Madhusudhan said. “This takes that evidence just a little bit further. Trying to explain [dimethyl sulfide] is even harder without life at this stage.”
“I think the observations are really exciting,” says astronomer Caroline Morley of the University of Texas at Austin. “This object is of great interest to the community, [so] we’ve been highly anticipating these observations. Everybody knew these were coming.”
Why is this molecule a sign of life?
The hallmark of a biosignature is something that is known to be produced by life and can’t be explained by anything but life. In exoplanet atmospheres, this usually means a gas that is out of chemical equilibrium — there’s too much of it to explain without something on the planet actively producing it.
That’s the case for dimethyl sulfide on K2 18b, Madhusudhan said. “These molecules need to be present in large quantities for them to be observable.”
Dimethyl sulfide has been proposed as a good biosignature before. On Earth, it’s produced by phytoplankton, and there’s no known way to create it naturally without biological processes involved.
It has been produced in at least one lab, however. It’s also been detected in environments where there is no life. It’s even been found on a comet that is definitely not inhabited.
“Even if there is dimethyl sulfide, connecting that to biosignatures is an enormous leap that we’re not ready to make yet,” Kreidberg says.
Is the biosignature really there?
The evidence isn’t strong enough to say for sure. The detection is at a 3 sigma level of statistical significance, meaning there’s a 0.3 percent probability of being due to chance. The gold standard for science is 5 sigma.
“The significance of the detection of [dimethyl sulfide] is right on the border of what we would consider significant,” Kreidberg says. “I think the discovery team did a careful job of exploring a lot of different possibilities, but they didn’t look at everything.”
There are several ways the signal could turn out not to be real, Krediberg says. First, there’s the data itself.
“This is just a really frickin’ hard measurement,” she says. The changes in the appearance of the planet that JWST is detecting are less than one one-hundredth of a percent. That’s right at the limit of what JWST can do. If one pixel on its detectors is more sensitive than the others, for instance, that could produce a signal that looks like dimethyl sulfide, but is actually nothing.
“JWST can do this science,” Kreidberg says. “But no telescope is perfect. As wonderful as JWST is, it has its warts and all.”
Morley agrees. “I’m deeply skeptical of the robustness of the detection of dimethyl sulfide and dimethyl disulfide,” she says. “These observations are tricky to do, tricky to do the data analysis, tricky to interpret the data once you get it.”
Even if the signal is real, connecting it to any specific molecule is a hard problem. Sometimes one molecule swamps the signal from another, or two molecules combine to masquerade as a third.
That’s already happened for K2 18b: In 2019, astronomers using the Hubble Space Telescope thought they saw water in K2 18b’s atmosphere. It turned out to be indistinguishable from methane.
“We could be seeing a very similar phenomenon here,” Kreidberg says. “I’ve seen this movie before.”
So will we ever know for sure?
Madhusudhan called for more observations and more studies of ways to produce dimethyl sulfide and dimethyl disulfide without biology. “We have to remain extremely cautious,” he said. “Maybe there are ways of producing these molecules that we haven’t thought about as a field.”
JWST could easily observe K2 18b again, Kreidberg says. “This seems like a no-brainer for follow-up,” she says. It wouldn’t take much more telescope time to get to a 5 sigma detection. “With 20 or 30 hours, we could learn a lot more.”
“To actually claim a detection of life,” Morley says, “I think we would need to have a better understanding of what this atmosphere looks like at other wavelengths and with other methods.”
She suggests observing the planet as it moves behind its star to learn more about its temperature and geologic context. “All of this data is taken during the planet’s transit,” she says. “We can get different information during a planet’s eclipse.”
But there may not ever be a moment when scientists definitively declare they’ve found alien life. Detections will probably trickle out like this one has: first a tentative hint, then a little bit more statistical significance, then calls for more data, then a little bit more significance. There will probably always be room for doubt.
“The path to solid confirmation of exoplanet biosignatures may be long,” says anthropologist Kathryn Denning of York University in Toronto. “And that future is quite uncertain right now.” With proposed funding cuts to NASA and U.S. research, planned telescopes that could give more definitive evidence, like the Habitable Worlds Observatory, may not end up launching.
All the fuss about this one detection “stresses me out,” Kreidberg adds. “In general, in this climate, the credibility of scientists is on the line. We have a big responsibility to do a good job here.
“For exoplanet astronomy, one of the biggest things we want to do is find evidence for life,” she says. “I don’t want us to be in the situation where we’re the boy who cried wolf.”
