Planets around other stars

It's 20 years since astronomers found the first direct evidence that many of the stars of the night sky have planets around them. But until recently, we've known very little about what these worlds might be like. Using a technique called transmission spectroscopy though, astronomers are now starting to gleam some clues about what the atmospheres of these planets are made of. Dominic Ford caught up with Catherine Huitson from the University of Exeter and as she explained the technique relies on waiting for planets to pass in front of their host stars and then studying the light that's filtered through their atmospheres.

Catherine - We're using a technique called transmission spectroscopy which has been in use for about 10 years or so. So, what it does is when you see the planet pass in front of the star like the recent transit of Venus, you can actually see the starlight be filtered through the atmosphere and characteristic absorption lines of specific elements and molecules are imprinted on the filtered spectrum. So, we use that to look for different chemical species in the atmosphere of these planets.

Dominic - So essentially, by seeing what colours of light are passing through and being absorbed by the atmosphere of the planet, you can start to work out what that atmosphere is made of.

Catherine - Yes. That's why essentially, we know what we expect to see and then we look for absorption in these particular wavelengths that tell us whether those compounds are present in the atmosphere.

Dominic - I guess, given that planets are much smaller than stars in general, the diminishment of light that you see when a planet transits a star is quite small, it must be really incredibly difficult to get a spectrum of that light that's being absorbed.

Catherine - Yes, so we're looking at Jupiter-size planets. Well, we're looking at hot Jupiter, so they tend to be a little bit bigger than Jupiter, a bit puffy, and they're very close to the star, so that's why we see quite a large signal. So, that's why we're not able to look at earth-like planets yet. So, we're still developing that technique to be able to do that. So, what we're looking for is, we try to pick targets or planets which are very large which are orbiting stars which are quite small, so that the light diming when the planet goes in front of the star is large.

Dominic - Because these planets are puffy as you put it, I guess there's a lot more atmosphere there to absorb light whereas the Earth's atmosphere is quite a thin layer on its surface.

Catherine - Yeah, so because they're very close to the star, they get heated so that they increase their size. So, for that reason, there's features that we see are quite large. Essentially, the atmosphere is inflated.

Dominic - What instruments are you using to make these measurements?

Catherine - We're using the Hubble Space Telescope and we're using spectrometers from the optical to the infrared.

Dominic - I guess the molecule that everyone is interested in is water because that's obviously the molecule that's needed for life. Have you found any evidence of that?

Catherine - Well, we have found evidence of water as steam because these planets are so hot, so it's not going to be liquid water, but we have seen a water feature. So, that's interesting again because we have seen the feature that we expect whereas in previous planets, observations have shown that the feature is muted. It's like the planet is covered in clouds and you only see part of the feature. So, it's interesting to see that in this planet, we actually see the feature we expect, so there's a surprising diversity in the planets that we have studied so far, which is the reason we're doing this survey to try and understand whether planets are different and why.

Dominic - We've also got Hannah Wakeford here from the University of Exeter.  Hannah, what else do we know about the environments of these planets?

Hannah W. - So, these are really strange worlds. They're actually tidally locked to their star which means that one face of the planet is continuously facing the heat, the radiation from that star. That means that the day side of that planet is really very, very hot, up to about 1,500 degrees Kelvin. What we're actually looking at through transmission spectroscopy is the limb, the kind of edge of atmosphere around the side, kind of the bridge between the day side and the night side of that planet. So, we're really also looking at the different temperature ranges that we're getting between those two. These water features that we're seeing are actually slightly cooler than what has been measured for the day side of these planets, which is another really, really interesting result, seeing how the winds, how the different environments on those planets are transporting that heat from the day side to the cold night side of them.

Dominic - So, it's fascinating how much we're learning about these exoplanets beyond just their masses, their sizes. We're really starting to learn what these might be like as worlds, but these aren't very earth-like worlds. You hinted that you want to move this technique to something that's a bit more similar to our own planet. How much more difficult is that going to be?

Hannah W. - Yeah, you're right. These are really not hospitable worlds. You wouldn't want to go there for a holiday. So, to move that technique forward a bit, we really need to develop the technology a lot more. But what we're doing is really kind of laying the foundation for what we understand about looking at these molecules, looking at these fingerprints. If we can understand them in something as big as these hot Jupiters where the signatures should be really very clear because the atmosphere is so extended. We can really refine the techniques right down to the point when we can confidently say when we've found these other planets and we've got better instruments to look at them, we know what that feature is. We have evidence from previous studies that we know what we're looking for and we've got it right here.

Dominic - I guess the molecules that everyone is interested in are molecules like oxygen which are bio tracers of life. How far away are we from detecting those, do you think?

Hannah W. - Oxygen is particularly a different one and that's difficult because of the wavelength ranges it lies in the intensity of the feature. Due to the intensity of the feature, it's quite small compared to other molecules. It's going to be quite difficult to find oxygen itself. But what we can look for is an imbalance in these atmospheres. So, where is there being CO2 or other molecules like water, where is there more methane and is there an imbalance from what we would expect from these worlds. That would kind of be indicative of something that is producing these different molecules rather than it occurring naturally in nature.

Dominic - Now Catherine, you've been using the Hubble Space Telescope for these observations. I guess to move this further, are you going to have to use purpose-built observatories?

Catherine - Yeah, so hopefully, James Webb will be flying soon and that will be specifically looking more in the infrared and the near infrared which is where we'd expect to see features such as water and methane. That should hopefully really take over.

Dominic - Hannah, there has been talk of purpose-built observatories for looking at the spectra of planets. I'm thinking of Darwin in particular. Does it look like any of those missions are actually going to fly anytime soon?

Hannah W. - So unfortunately, Darwin is dead. It's not around. We're not flying that mission, but there's always talks, there's proposals every few years where you can put forward these different telescope ideas. It's really important that we going to space for these, so that means that there's that higher cost and that higher risk with it. So yes, there are things in the works, but of course, space mission proposals are years and years in the future. It took from '84 to 2009 to get Keppler up, so watch this space. We'll definitely have something.