K2-18b and the Search for Alien Life
Did we really find signs of life on a distant exoplanet?
You may have heard the recent claims of detecting unique molecules produced by life on Earth in the atmosphere of an exoplanet.
The headlines were mostly sensational, like these:
“Scientists find ‘strongest evidence yet’ of life on distant planet” —Pallab Ghosh for the BBC
“Strongest hints yet of biological activity outside the solar system” —Sarah Collins writing for a University of Cambridge press release
There were some headlines and some reporting that were at least a bit more reserved, though.
Some of the best reporting even included some quotes from colleagues of mine who point out that there is a lot we don’t know about the exoplanet in question and that the claims need to be verified by other research groups. Two of those more rational articles are:
“Scientists detect signature of life on a distant planet, study suggests” —Ashley Strickland for CNN
“Astronomers Detect a Possible Signature of Life on a Distant Planet” —Carl Zimmer with the New York Times
Many of us in the realm of astrobiology are certainly intrigued by these possible findings. Why wouldn’t we be?
Many of us have spent our lives not only wondering if aliens might be out there, but actively using the tools of science to take part in the search.
But, once again, it feels very much like the journalists and the media have “jumped the gun” on much of the reporting, and even the lead researcher involved in the claim seems to be causing some issues when it comes to helping the general public to understand the science and what we can reasonably say about their findings.
This isn’t the first time that we’ve had a claim related to alien life from some scientific findings.
It also isn’t the first time that some journalists have sensationalized the findings and made it seem like we were certain of the conclusion of alien life.
What are these possible signs of life?
Let’s explore a little bit about what was found (or not) on the exoplanet K2-18b and how that relates to other intriguing findings in the search for alien life.
Pouring Some Cold Water on the Alien Life Media Fire
Exoplanet K2-18b *could* be an alien biosphere, but we certainly don’t have sufficient evidence of that yet.
As Phil Plait, of Bad Astronomy, first wrote about this recent claim: No, astronomers almost certainly didn’t find biosignatures of life on another planet.
That’s a bit harsh. What has been potentially found could be a biosignature, but it also might not be and also might not even be there based on what we know right now.
There’s a bit of a history to these claims around K2-18b, which is the designation of the exoplanet in question (and which orbits around a star called K2-18 that’s about 124 lightyears away from us). For brevity, I’m just going to write about the more recent claims regarding the molecules dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) that *might* be in the atmosphere of this world.
In 2023, a group of researchers led by Nikku Madhusudhan published a paper detailing some of their findings in relation to K2-18b. This included data from the James Webb Space Telescope (JWST) that supported finding methane and carbon dioxide in the planet’s atmosphere. But they also hinted at the possible finding of DMS.
As many researchers pointed out then, though, the *hint* of DMS in the atmosphere was extremely weak and could easily be interpreted in other ways (like there being no DMS at all).
So the researchers continued their work and got more data (yay, science!). This led to a recently published paper and the subsequent sensational headlines.
In their new paper, Madhusudhan and his colleagues share their interpretations of the data they collected and suggest that not only is DMS present in the atmosphere of K2-18b but that DMDS is as well. They even report a statistically high probability that their findings of DMS and DMDS are real and that these molecules really do exist in the atmosphere of this distant world.
That’d be pretty incredible if true. On Earth, DMS and DMDS are both produced through living processes. They’re commonly produced by phytoplankton in the ocean, and contribute to that sulfury smell you sometimes pick up near the sea. DMS, in particular, plays a key role in Earth’s sulfur cycle and even affects cloud formation and climate regulation. Its presence in an exoplanet’s atmosphere could hint at the activity of microbial life in an ocean or other biosphere, making it a molecule of interest in astrobiology.
However, while DMS is biologically sourced here on Earth, it has also been detected in non-biological contexts. DMS has been observed in the coma of Comet 67P/Churyumov–Gerasimenko by the Rosetta mission, and its spectral signatures have been identified in the interstellar medium (so just floating along out there in space). Given that we know of non-biological sources for DMS and DMDS they are not really strong biosignatures. But they are super intriguing, especially if we find them in the atmosphere of an exoplanet.
But did Madhusudhan and his colleagues really find these molecules on K2-18b?
Well, for starters, Ryan MacDonald has a fantastic breakdown of the statistical inferences from their paper, and he shows that it’s actually rather unlikely based on the current data that they really have found DMS and DMDS.
As Ryan points out:
“In exoplanet atmosphere research, we use something called 'Bayes factors' (B) to express how confident we are in a spectral signal.
• No evidence (B < 3)
• Weak evidence (3 < B < 12)
• Moderate evidence (12 < B < 150)
• Strong evidence (B > 150)
The Bayes factor for DMS and DMDS (both combined) in this paper is 2.62, which falls in the 'no evidence' category. We normally don't start using the word 'detection' until we have B > 150.”
On top of this issue with the statistics in the paper based on the model that Madhusudhan and his colleagues used, the model itself could be wrong.
Another exoplanet researcher, Jake Taylor, has a paper out where he shows that another model, just a simple flat line with no spectral features, is just as good of a fit to the data that Madhusudhan and colleagues have.
Taylore concludes in his paper:
“Despite the claim of a 3.4-σ deviation from a flat line in N. Madhusudhan et al. (2025), I find a flat line is an acceptable fit…
Therefore, there is no strong evidence for detected spectral features in K2-18b’s MIRI transmission spectrum.”
So, as of now, we don’t really have the data to support the conclusion of DMS and DMDS being present in the atmosphere of K2-18b. It’s probably still worth exploring and collecting more data. We may find with more observations and better data that there is more to these possible detections.
And this kind of back-and-forth isn’t unusual in science—it’s actually how science works. Claims are made, tested, contested, and refined. But the public conversation around alien life doesn’t always leave space for that nuance—especially when some journalists race to get the most sensational headlines out there (more clicks means more ad revenue, after all).
But on top of the fact that Madhusudhan and his colleagues really don’t have the data to support the claim of these findings, it turns out that Madhusudhan himself is behaving a bit questionably in the public arena when addressing the scientific issues that others have raised. In one BBC interview (here, starting at 2:39:30), Madhusudhan himself refers to this as a “transformational moment” and a “pinnacle moment” for astronomy, which is unwarranted as a claim, but he also responded to some skepticism of his claim by calling a fellow astrophysicist a commentator and not a practitioner which is a silly and unnecessary attempt at disparaging those who question the resilience of the claim. That’s certainly unfortunate, though it’s also not the first time a researcher making extraordinary claims without extraordinary evidence has taken to grandstanding about their conclusions while showing resentment or stooping to disparaging remarks about others.
As of right now, we can’t say with any sufficient reason that DMS and DMDS are present on K2-18b.
It will be awesome to get more data and better explore the claims of these molecules potentially being there, even if it is sad to see some of the behaviors of Nikku Madhusudhan in response to criticisms of his claims.
But in the search for alien life, we need to be humble and true to ourselves.
The Biosignature Challenge
This isn’t the first time we’ve seen claims about potential biosignatures spark excitement followed by skepticism. In 2020, researchers announced the possible detection of phosphine in the clouds of Venus—a molecule that, on Earth, is associated with anaerobic life (and found in penguin poop and around swamps and bogs). The paper made headlines around the world. But follow-up studies suggested the spectral lines might have been misidentified or overstated, and that phosphine might not be present at all. But if the phosphine is there, that alone doesn’t mean there is certainly an alien biosphere on Venus. We’ve also found phosphine produced without life in other places, and we need to know more about Venus itself before we could say whether or not phosphine there is certainly a sign of alien life.
The problem here is foundational: for most molecules, even those commonly produced by life on Earth, there is no certainty that their presence is proof of life elsewhere. Chemistry is complex. Non-biological processes can mimic biological signatures, and our instruments, especially when probing distant atmospheres through layers of noise and interference, have their limits.
And yet we keep searching. Many of us hope to find signs of life out there, and that has often led to sensational headlines and a lot of hype around some of our findings (often before other teams of researchers have had a chance to explore the data and make their own interpretations).
A History of Martian Hope and Hype
Nowhere has this cycle of hope and hard scrutiny played out more than on Mars.
In the late 1800s, Percival Lowell claimed to see canals on Mars—evidence, he thought, of an intelligent civilization. The canals turned out to be optical illusions, a case of eager human imagination interpreting noise as signal.
In 1976, the Viking landers conducted biology experiments on the Martian surface. While most of the data revealed no signs of life, one experiment had intriguing results that seemed to show possible signs of metabolic activity in the Martian soil. Follow-up interpretations suggested chemical, not biological, reactions (though the lead researcher for that experiment lived the rest of his life believing he had indeed found signs of Mars life).
In 1996, NASA scientists announced that Martian meteorite ALH84001 contained potential fossilized microbial structures and hints of biological activity. President Clinton even addressed the nation about the finding. But again, skepticism prevailed, and all of the possible signs of life have been shown to be capable of being formed abiotically.
Then there's the infamous “face on Mars,” a rock formation photographed by Viking that, under certain lighting, looked eerily like a humanoid face. Later, higher-resolution images showed it was just a mesa—our brains playing connect-the-dots with shadows and stone.
More recent findings from the Curiosity and Perseverance rovers have uncovered organic molecules, potential seasonal methane spikes, and ancient lakebeds. These hint at a potentially habitable past, but they stop short of offering sufficient proof of life.
Honestly, for me, I’d be willing to bet that if there is extinct or even extant life on Mars, that we’re going to need to dig a lot deeper into the surface to find it. And that might require us sending humans there before we can!
Technosignatures and the Wow! Signal
Our search for alien intelligence has yielded similar stories to our explorations of Mars.
In 1977, a radio telescope at Ohio State University picked up a powerful, narrowband signal from deep space that lasted 72 seconds. The astronomer who saw the printout a few days later scrawled “Wow!” in the margin—giving the signal its name. But the Wow! signal never repeated, and its origin remains unknown (though Abel Mendez and his colleagues have a tantalizing conclusion suggesting that the signal was caused by a strong, moving stellar source exciting a cold hydrogen cloud in space).
Other possible technosignatures—like fast radio bursts, Dyson sphere candidates, unexplainable signals from distant stars, or inferences that an interstellar object could have been an alien spacecraft—have likewise often turned out to be easily explained as natural phenomena (even if the conclusion of aliens is still on the table in some cases).
Still, we keep listening and looking. Because what if?
Science, Headlines, and Public Trust
Every time a claim like those for K2-18b makes headlines, the cycle repeats: excitement, doubt, correction, and often disappointment.
For many outside the scientific community, this looks like inconsistency. For those of us within it, it’s a sign of integrity.
Science is a process of continuous refinement. We move forward by testing our assumptions and acknowledging uncertainty. But in a world hungry for definitive answers—and headlines that often bank on driving internet traffic more than getting things right—this process can seem like backpedaling or incompetence.
If we want the public to trust science, we have to communicate not just our discoveries, but our doubts. We must show that wonder and skepticism are not opposites—they’re allies.
The Long Road to “Yes”
The search for alien life is a long, winding road that’s been filled with false positives, ambiguous data, hopeful speculation, and a lot of human issues. But that doesn’t mean the journey isn’t worth it.
Each potential biosignature or technosignature, whether it proves to be real or not, helps us sharpen our tools, refine our thinking, and build a better understanding of what life might look like elsewhere.
The recent claims of DMS/DMDS on K2-18b are intriguing, but, as others have pointed out, the data just aren’t good enough yet to support that finding, nor does the finding of DMS/DMDS in an atmosphere of an alien world certainly mean that there are alien beings present anyway.
While many of us wish we could reel in the clickbait headlines and develop more journalistic integrity across the globe whenever science or other news is being reported, that’s just not something we can easily orchestrate. We need more understanding and empathy in our society. We need more skepticism and rationality to go along with our hope and wonder. We need to be better.
Because one day—maybe soon—we’ll come upon a finding that overtly and sufficiently tells us that we’re not alone. That, yes, there are aliens out there. And if and when we do, we’ll know what it took to get there.