Closeby water world replete with methane and CO2

How the latest exoplanet discovery is getting astronomy very excited...
15 September 2023

Interview with 

Matt Bothwell, University of Cambridge


Artist rendering of three exoplanets


Are we alone in the universe? Events this week may have taken us a step closer to answering that question. Observations made with the James Webb Space Telescope on the star behind the stir - K2-18 - found evidence of methane and carbon dioxide in the atmosphere of a watery world in its orbit. These chemicals are thought to be essential for life. The planet in question - K2-18b - also sits in the so-called ‘habitable zone’ temperature wise, and is a mere 120 light years away: in astronomical terms, that’s practically our own backyard! Matt Bothwell is the University of Cambridge’s Public Astronomer, based at the Institute of Astronomy, where the findings were made. I started by asking him how they did it…

Matt - So they used a technique called transmission spectroscopy, which is a very fancy way of saying essentially you look at the starlight passing through the atmosphere of the planet and then the light passing through the atmosphere, all the chemicals in the atmosphere will leave their fingerprints in the light. So when you take a spectrum of the light, the fingerprints of all those chemicals, like methane, like carbon dioxide, will be left in the light for us to discover.

Chris - And when one looks at that spectrum, what is the recipe and the atmosphere they're seeing?

Matt - So this type of planet is something called a hycean world. Hycean being a portmanteau of hydrogen and ocean. But the stuff that's got everyone really excited is the carbon dioxide and methane in the atmosphere. It's been a problem for a few years that whenever we look at gassy planets or planets with thick atmospheres, we don't find methane. And of course here on earth methane is one of the chemicals associated most with life. But now loud and clear, we see plenty of methane in this planet's atmosphere, which is very exciting.

Chris - They're also talking about the chemical dimethyl sulphide, which we used to think when we went to the seaside and smelled this, we were smelling ozone. Now we realise it's this chemical which comes, we are told, exclusively from life. And that's there too.

Matt - You are right. So that was reported in the paper as a tentative finding. I think this one is proving to be a little bit controversial in the field. Like you said, dimethyl sulphide is a pretty slam dunk bio indicator, at least here on Earth. It's only produced by life. There is what the authors report as a tentative detection. So they're not claiming a full detection. They're not saying yes, we've definitely found it. They're saying hopefully if we go back and get more data, we might be able to prove this definitively. Other people in the field are a bit more sceptical. I think it's an ongoing controversy. I think we've all got our fingers crossed and we hope that it turns out to be real.

Chris - It's a fairly big planet. I think the paper suggests it's about nine times bigger than Earth. Is that right?

Matt - Yeah. Just about, yeah. Eight or nine times bigger than Earth, which it really has to be to hold onto hydrogen in its atmosphere. Hydrogen's a very light gas, so the atoms and molecules travel very fast. Unless the planet's very massive, it can't hang onto a hydrogen atmosphere.

Chris - You've been saying ocean. Is that a liquid ocean? And if so, how do we know that?

Matt - So we are pretty sure it's a liquid ocean that's based on the temperature of the planet that we infer. We can see how far the planet is away from the star, and we can, based on the star's temperature, we can work out how much radiation is falling on the planet and how hot it's going to be. So this planet has an equilibrium temperature of somewhere ish, roughly zero, it's very possible that this planet is 10 or 20 degrees Celsius, like a nice warm room temperature. So I think it is very possible that this is liquid.

Chris - And if it is as big as we think it is, does that mean it's sort of all ocean or will there be any rocky stuff there? Do we have any idea of its overall composition?

Matt - Yeah. So based on the density, this is a rocky world, but I think based on the atmospheric composition, I think it's most likely to be almost entirely ocean. Because we can't really see the planet, all of this is based on looking at starlight filtering through the atmosphere. So it's all based on our best guesses. But this is consistent with a world that is completely covered in ocean, but has a rocky core.

Chris - Waterworld. Are these common?

Matt - That's the million dollar question. It's very difficult to talk about how common different types of planets are because the different observational techniques that we used to find planets like the transits I mentioned before, they have these biases built in. It's very easy to find very big planets close to your star. It's hard to find smaller rocky planets far away from your star. We are finding more and more of them as time goes on. And so we think at the very least there are almost certainly hundreds and hundreds of millions of these in the galaxy.

Chris - And will the aim now be to pursue these signals to a check that they're right, but go after this dimethyl sulphide question, this possible hallmark as you dub it, the 'slam dunk' of life. Because if that's there, that's pretty important, isn't it?

Matt -
Absolutely. I think that's the thing that has got everyone very, very excited. If we can confirm the existence of this biosignature, it's gonna be enormous news. So the real aim is to get more time on the James Webb space telescope. Look at this planet for more transit and confirm that line. So far we've only seen this planet going past its star twice. So if we go back, we get more of those transits, get better data and see if this thing is real.

Chris - It does that quite often though, doesn't it? Because you're saying it's quite close in, so it must have quite a fast orbit.

Matt - It does. I think about five or six months. We saw it going past in January and then in June of the same year. So yes. So we shouldn't need much time to confirm this.

Chris - You should have said, watch this space, Matt

Matt - <laugh> watch this space.


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