K2-18b Redux!

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There was definitely one space story that stood out from all the rest this week, the tale of the Webb Telescope’s hunt into the atmosphere of the exoplanet K2-18b for gases that could serve as possible biosignatures. It was picked up by every news-y outlet from my usual space news sites to The Late Show with Stephen Colbert.

And it’s a cool study! And it led to some slightly breathless headlines. So let’s talk about what exactly it was that Webb found, what it means (and why some of those headlines might be a little exaggerated), and why it’s awesome anyway. 

Ocean World?

In a nutshell, Webb found what may have been traces of the gas dimethyl sulfide (DMS) in the atmosphere of K2-18b. And that is potentially exciting. This is a gas we see in Earth’s atmosphere as well, and the only way we see it getting produced on Earth is by biological processes, aka life! On Earth the source is generally oceanic algae, and it is partially responsible for that smell you think of when you think of the ocean.

At first glance K2-18b does not look like the sort of world that would be a great place to look for ocean-based lifeforms. It’s quite large, over twice the size of Earth and over eight times as massive orbiting in the habitable of a tiny red dwarf star. But astronomers have found certain strong gas signatures in its atmosphere that suggest it could be a Hycean world, a planet with a hydrogen-rich atmosphere that hides a global ocean (hydrogen + ocean = Hycean…look, sometimes astronomers are very clever at naming things, but not all the time).

So a possible ocean world with possible gas signatures in its atmosphere that we know can be made by ocean-dwelling organisms. It sounds like a slam dunk! But it’s not, at least not according to the traditional requirements that physics (and astronomy by extension) uses when pronouncing discoveries. 

The Gold Standard

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Astronomers and physicists don’t like to say a discovery is certain until they’ve passed the “five-sigma” mark. This is a reference to the fact that scientists use the Greek letter sigma (σ) to represent something called “standard deviation” when they look at the results of their analyses. You can think of how certain you can be that your data indicates something is really happening and isn’t just showing random stuff happening in the background. 

Let’s say you’re someone using Webb and you want to look for DMS in the atmosphere of K2-18b. You point Webb at it and take a spectrum of the atmosphere and you see what might be a spike where you’d expect DMS to be on that spectrum. But it’s a small spike (you are, after all, hunting for a specific gas in an atmosphere 124 light years away)—so it really DMS, or is just a fluctuation in the background that happens to be in the same spot you’d look for DMS to be?

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The sigma standard is essentially a way of stating of how likely it is that the phenomenon you’re observing is real as opposed to something random. A one-sigma result means there is a 68% chance that you’ve really got something. A two-sigma result means there’s a 95% chance it’s real, three-sigma means there’s a 99.7% chance. Four-sigma means 99.99% By the time you reach a five-sigma result you can be 99.99994% sure you’ve actually seen something. And that’s when a physicist or astronomer will say you’ve actually made a discovery. 

You might be shaking your head at the idea that being 95% or 99.7% sure isn’t good enough. The truth is that for a lot of disciplines it is! But physics has been burned by seeing even three or four-sigma results turn out to be statistical noise. So it’s five-sigma or get yourself side-eyed at the next academic conference.

DMS at K2-18b…Again!

You might be wondering where you’ve heard about K2-18b before. That’s because this isn’t the first time it’s been in the news for potential signs of DMS in its atmosphere. Back in 2023 Webb’s near-infrared instrument made a roughly one-sigma detection of this very same thing. Enough to be a major tease, but certainly not enough to say anything for sure.

(That didn’t stop me from getting super excited though. Even if I’m required to put out all sorts of disclaimers when talking about it, I’m still allowed to get myself worked up about possibilities. I’m a professional space nerd after all!)
This time Webb looked with its mid-infrared instrument and made a three-sigma detection of DMS. That’s a 99.7% chance that it’s really there! But it’s not officially a discovery. That said, the fact that Webb saw the signature twice with two different instruments is a pretty good sign.

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What will it take to get to a five-sigma result? Time. The more you can observe something, the more clearly actual phenomena will stand out against random background noise. It’s why Hubble and Webb’s Deep Field images are so full of galaxies we can’t see otherwise—they’re created by having the telescopes stare at a single patch of space for hours (Webb) or weeks (Hubble). When all those observations are stacked on top of each other the background noise melts away and faint galaxies normally lost in that noise become visible.

Webb can do the same thing at K2-18b—if researchers can get the time. Observation time with Webb is precious currency, and the lead researcher behind this latest result says it will take 16-24 additional hours of Webb time to make it to five-sigma. That may not sound like much, but it is when you’re talking about something as in demand as Webb time.

The good news is that it doesn’t have to be gotten all in one go. It can be spread out over multiple observations at different times. But it will likely take years to accumulate that much time with Webb.

And Then?

Let’s say we do get that time. Let’s say in a couple of years we have a perfect, undeniable, crystal clear five-sigma detection of dimethyl sulfide in the atmosphere of K2-18b. Will that be it? The holy grail of astronomical science, the confirmation of life beyond our planet?

No, probably not. Carl Sagan, God of Science Communication, once said “extraordinary claims require extraordinary evidence.” To say that the presence of DMS in this planet’s atmosphere is definitely, for sure, no question caused by life, we’d need to be definitely, for sure, no question certain that there was no other way for the DMS to get there except via life processes.

And that’s not the case. In 2024 astronomers found dimethyl sulfide on the comet 67P/Churyumov-Gerasimenko. Being an airless conglomeration of dirt and ice it’s certainly not life generating DMS on this comet. In fact we have no idea how that DMS was generated. But it’s proof that you don’t need life to make it.

So What’s the Point?

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Look, if we find hardcore evidence that there’s a strong DMS presence in K2-18b’s atmosphere, it won’t be absolute proof of anything. Not by itself anyway. And it’s 124 light years away, so nobody’s flying out there to take a look up-close. But we’re definitely not done with this planet, not by a long shot!

As Webb continues to obsess over this planetary atmosphere, it may very well find other things that could be made by life, even if they could also be made by other means (technically it already has, methane and carbon dioxide). Each individual detection might not be a smoking gun—but it’s possible that each will turn out to be a puzzle piece, and when you put it all together there’s one picture that’s far more likely than any other.

And that may be the day we can say that the evidence heavily suggests that we’re not alone in the universe. I really want to see that day. 

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