So, last week you could’ve hardly missed the news, since it was everywhere: astronomers detect signs of life on an exoplanet! Or, astronomers find best evidence yet of life in space!

This is at best an exaggeration.

Here’s the story: The exoplanet K2-18b was discovered in data from the Kepler spacecraft, designed to look for mini-eclipses (called transits) as planets passed directly in front of their host stars as seen from Earth. This is the most successful method we have of finding such alien worlds, and thousands have been spotted this way.

K2-18, the star, is a red dwarf 124 light-years from us, a dim bulb half the diameter and mass of the Sun, and just 2% of its brightness. In 2015, a planet was detected orbiting it. K2-18b (note the lower-case “b”, denoting it’s a planet) is about 2.6 times the Earth’s diameter and 8.6 times its mass. Some people call it a super-Earth, but in my opinion that’s misleading; it’s more likely a mini-Neptune at that size and mass (so a small gas giant instead of gigantic rocky world). Very interestingly, though, it orbits its host star at a distance of about 22 million kilometers, putting it in the star’s habitable zone, where liquid water could exist on the surface.

Mind you, the planet may not even have a surface per se. If it’s like Neptune it’s basically atmosphere all the way down (well, a few hundred kilometers down the gas phases into a liquid, but still, no real surface). Or it might be more like a lava planet. For a while it was thought it might represent a kind of planet called a hycean world, called this because these would have water oceans and an atmosphere of hydrogen (so, hy-cean). We have no solid evidence these kinds of worlds even exist, however. They might, but we just don’t know yet.

There’s been a lot of speculation that K2-18b might be a hycean world. To help nail down what kind of planet it is, a few years ago a team of astronomers used Hubble to try to detect constituents in the planet’s atmosphere. As I’ve written before:

As a planet transits the star, some of the starlight passes through the planetary atmosphere on its way to us. Atmospheric constituents like carbon dioxide, water, and others absorb light at very specific wavelengths, and that absorption can be in principle be detected (it’s been done many times for gas giants orbiting other stars).

Mind you, this is a very tricky observation, and analyzing it even harder. If you see absorption at a given wavelength, how do you know what it is? It could be from any number of molecules or elements. Using models of how gases behave, we have standardized tables of how various things absorb light, which helps, but isn’t a slam dunk. It takes careful analysis, and even then it’s possible to misidentify the constituents.

With the Hubble observations, that’s just what happened. The astronomers thought they had found water in K2-18b’s atmosphere [link to journal paper]. However, a different team led by an astronomer named Nikku Madhusudhan at the University of Cambridge Institute of Astronomy used JWST to look at K2-18b, and showed what the first team thought was water was actually methane and carbon dioxide. So right away, to be very clear, we don’t know what kind of world K2-18b is. Astronomer Ethan Siegel wrote extensively on this aspect of the story at that time.

This is where things ramp up. In that same JWST paper, Madhusudhan and his collaborators made an interesting claim: they had a marginal detection of a molecule called dimethyl sulfide, or DMS [link to journal paper]. As they mention, this is a potential biomarker, a molecule made by life here on Earth (usually by sea life, which is why the ocean can smell sulfury sometimes). Their detection wasn’t strong, just a hint, so they got longer observations from JWST. They just released their paper about those observations, and that’s what caused all this stink (haha, ha, sulfur, get it?). 

They claim that they find a much stronger signal for DMS (and a similar molecule called dimethyl disulfide, or DMDS). In the paper they show the spectrum, and also a model of what DMS and DMDS would look like if they’re what indeed are in the atmosphere. Certainly by eye the comparison looks favorable. They also claim the detection is statistically significant, basically better than a 99% chance it’s real. I’ll note that in the paper, they acknowledge that this is not good enough to claim a discovery (the detection “…is at the lower end of the robustness typically required for scientific evidence”).

Here’s where things start to go sideways. First, off, it’s not at all clear that what they see in the spectrum are DMS and DMDS. According to planetary scientist Ryan McDonald on Bluesky, their statistical analysis is suspect, and the significance of their detection is far lower than they claim. Another planetary scientist, Raymond T. Pierrehumbert, also laid out his argument that the claims are overconfident.

In other words, there’s a good chance the JWST observations don’t even really show DMS and/or DMDS at all.

So there’s good reason to doubt even the existence of those molecules in K2-18b’s atmosphere, let alone make any claims about them being biogenic (created by life).

But it gets worse, and that’s how this played out in science communication.

At first, things seemed OK. In a video released by Cambridge, Madhusudhan talks about all this and is decently circumspect.

Despite the video title, had this been the only release to the public, I’d be somewhat mollified.

But it wasn’t.

The New York Times posted an article about this, and the overall tone is reasonably calm and more or less skeptical (many scientists expressing doubt are quoted, though near the bottom). In it, Madhusudhan is quoted as saying, “It is in no one’s interest to claim prematurely that we have detected life.” OK, cool, but then was also quoted as saying, “This is a revolutionary moment… It’s the first time humanity has seen potential biosignatures on a habitable planet.”

So which is it? Is this premature, or a revolutionary moment? I find his choice of words a little uncomfortable. 

In case you think I’m splitting hairs here, Madhusudhan was also quoted (in the awful UK tabloid The Sun, which I won’t link to here because they’re awful) as saying, “There is no mechanism in the literature that can explain what we are seeing without life.”

But that’s not correct. DMS has been seen on a comet as well as in a gas cloud in deep space. Clearly there are abiotic (meaning non-life or non-biological) ways of making DMS. To be fair that doesn’t mean we know of ways it could be made specifically in the atmosphere of a mini-Neptune orbiting a red dwarf like K2-18b, but not knowing how does not mean impossible. His statement, or at least the way he is quoted here, is misleading.

You could argue that his statements are a little ambiguous and I’m looking at them negatively. But then, about finding life or evidence of it in space, Madhusudhan also said, “It’s a question humanity has been asking for thousands of years. It’s a shock to the system. It takes time to recover from the enormity of it.”

That line is what really hit me. He is strongly implying they have found evidence of life. Mind you, this was not a statement in a journal paper, but to the press, which guarantees it will get repeated and interpreted as his team having found life or evidence thereof. And that’s exactly how it’s getting interpreted.

A lot of other venues picked up on the “best evidence yet of life on another planet” claim; I saw many videos from major news organizations on TikTok saying this with very little skepticism. Reading the comments on those videos makes it clear people really think astronomers have found evidence of alien life.

All this, from what might not even be an actual detection of those molecules in the first place. And even if they are real that’s a far, far cry from them being biosignatures. On Bluesky my friend and colleague Sarah Hörst, who also studies planetary atmospheres, has plenty to say about that. So does another friend and colleague Chris Lintott (he also posted something about this on The Last Word on Nothing). Other skeptical takes can be easily found.

So no, I am not convinced at all about any of this. I’d like to see the data analyzed by other astronomers before any claims are made. 

Still, the JWST observations are certainly interesting. If more can be obtained, then perhaps the robustness of the detection can be amplified. Or negated. Or put into context. Any of these would help. We also need more and better modeling of planetary atmospheres and interiors in general (and a lot of astronomers are working on that!). 

As always, we very much need to keep our skeptic hats on when we see claims of aliens in the press. That’s a claim of last resort, when everything else has been adequately eliminated, and we are nowhere near that here.

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