First rocky exoplanet found with an atmosphere

Why astronomers are excited by 55 Cancri e...
10 May 2024

Interview with 

Matt Bothwell, University of Cambridge


Up into space now and a new study using the James Webb Space Telescope has found a so-called exoplanet - in other words a planet orbiting a star outside our own solar system - with what looks like a thick atmosphere around it. This is the first time anyone has discovered a rocky planet with an atmosphere like this. To find out more, I went to meet Matt Bothwell, Public Astronomer at the University of Cambridge, and asked him to take me through what the paper shows…

Matt - Astronomers, for the first time, have found an atmosphere around a rocky planet using the James Webb Space Telescope. It's orbiting a star called 55 Cancri in the slightly ridiculous naming system astronomers use. The planet is 55 Cancri e. It's about 40 light years away or so.

Chris - What do we know about the planet itself?

Matt - It's a terrible place to live. We know that for sure. It's a super Earth, about 10 times the mass of Earth. It's a roughly rocky planet, although rocky is probably the wrong word to use. It's so close to its parents star, this planet is certainly a molten blob of lava. It's about 1/25th of Mercury's distance to the sun. This thing is so close to its star it whizzes round once every 17 hours. It's a planet that's made of rock, but think of a molten blob of lava whizzing around a sunlike star.

Chris - How do we know it's got an atmosphere, then?

Matt - That's a good question. The team used this very interesting technique called a secondary eclipse. So listeners might have heard of planetary transits. That's when the planet goes in front of a star and from the missing starlight that tells you about the planet. In this case, they use the planet going round the back of the star and the way that can tell us about the planet is that if you imagine the planet just to the side of the star, our telescope will pick up the combined light of the planet and the star all at once. Then, when the planet dips behind the star, at that point we only get the star light. By comparing the before and after, the planet plus the star compared to just the star, we can subtract one from the other and get our best guess out: what the light from the planet is doing.

Chris - So how does that light tell you there's an atmosphere there and how does it tell you what's in the atmosphere, and indeed can it?

Matt - It can. I think the answer is to do some quite complicated modelling. Traditionally, in astronomy, when we look at things, we are looking for spectral lines, these characteristic fingerprints of particular atoms and molecules. That's not what's been done here. Here, the team have made some complex models of this planet's atmosphere and tried to reproduce what they saw in this planet's light using this secondary eclipse technique. The best model that fit to the data is a complex mix of carbon dioxide and carbon monoxide. We think that's what's going on.

Chris - I understand why you said it's not a home from home now. Not only is it roasting hot and hot enough to melt metal, it's also got an atmosphere that sounds pretty horrible. Why then should we be excited about this discovery?

Matt - Well, I think it's still incredibly cool that we have been able to find an atmosphere around a rocky planet. The holy grail for this whole area of science is of course discovering life living on an earth like planets. A big step in that direction is the ability to find atmospheres around rocky planets. Even though this one might not be your ideal holiday destination, I think it's a huge step in the right direction.

Chris - It's also not that far away in the sense that, 40 light years away, which is where we think it is, is in space terms just around the corner. Does that give us any clues as to the likelihood of finding planets that fit the bill that you've just outlined: how likely it will be, how frequently they are out there?

Matt - That's a very good question. It's one of the hot things that exoplanet scientists talk about a lot. I think the best guess right now is somewhere between 30 and 50% of stars have something roughly earth-like. Proxima Centauri, for example, our next door neighbour in space, has a pretty Earthlike planet. I think the good news is that these things are pretty common.


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