Probing an exoplanet atmosphere

The quest to find planets in orbit around distant stars that resemble the Earth and could be habitable has been going on for decades. But most of the early candidates were what we call “hot Jupiters” - these are massive gas giant planets orbiting close to the parent star and are in no way capable of supporting life. Techniques and technology are improving rapidly though and are now sufficiently good that we can spot much tinier worlds and even analyse their atmospheres. Chris Smith spoke to John Southworth from Keele University, who has recently done this for a star about 40 light years away...

John - The star is GJ1132 and the planet is GJ1132b, and we’ve discovered an atmosphere around this planet which makes it the first detection of an atmosphere around what we think is a rocky planet outside our own solar system.

Chris - Why is that a big milestone?

John - It’s big because there’s an awful lot of these things. The Milky Way contains about 150 billion stars; the very, very low mass stars would be the dominant component of that.

Chris - If there are lots of them and there are planets around them, that means there are, potentially, lots of rocky worlds a bit like the Earth out there?

John - Yes, that could be the most common thing in the universe.

Chris - You’ve been studying the planet which is going round this star. Tell us about that planet then.

John - Well, the planet's small. We think it’s probably rocky. It’s going round its host star every 1.6 days so it’s really close in to the host star. But, because the star is cold and small, then the planet itself is not particularly hot.

Chris - And how hot is not particularly hot?

John - It’s 370 degrees Celsius on the surface.

Chris - And you can actually physically see this planet, or can you see its effect on the host star?

John - What we see is it pass in front of its host star once per orbit. So, every 1.6 days, the star gets a little bit fainter because the planet’s blacking out some of the surface. Now, importantly, this dip in brightness actually tells you how big the planet is relative to the star, so you get a direct measurement of the size of this planet, which is not possible to do in any other way.

Chris - Can you weigh the planet: by looking at how fast it’s going round, and how big it is, you can work out basically what it must be made of and its mass?

John - Yes. We can weigh the planet by measuring how the speed of the star changes. Because, whilst the planet is orbiting the star, the star itself is actually going round in an orbit pulled by the planet and so it actually changes its speed continually, and if we measure how much that speed changes, then we can measure the mass of the planet as well.

Chris - How much does the planet weigh?

John - Around about 1.6 times the mass of Earth. Our results are consistent with it being rocky, but also it could be entirely water.

Chris - Oh, so you have a waterworld?

John - Yes. Both these options can fit the data.

Chris - What about the atmosphere that you’re saying you can also see; how are you looking at that?

John - I mentioned that when the planet goes in front of the star it gets a little dip in brightness, which allows you to measure the size of the planet. Now what we did was to measure this change in brightness at seven different wavelengths - four in the optical and three in the near infrared. And we found that at one wavelength, which is just a little bit too red for the human eye to see, we found that the planet appears to be bigger. So our interpretation of this is that the planet has quite a large atmosphere, which is letting through light at most wavelengths, but at this near infrared wavelength it’s actually opaque and it’s sufficiently big that it makes the planet itself look bigger. Now if you try to make some theoretical models of the planet’s atmosphere, then you can get this effect by using atmospheres with either steam or methane, or a combination of the two.

Chris - Right. And you can’t tell at the moment which of the two it is?

John - No. At present our data could be entirely satisfied by either of those two things, or a combination. So the obvious next step then is to get some better data and try and work out whether it’s methane or water. Three weeks ago now I sat down to write an application to get some data from a big telescope - the very large telescope (VLT) - in Chile. I was half an hour into the application process writing the science case, and I thought I’d best check that no-one else has done this first. I checked the archive and it turns out that my colleagues in Exeter have actually observed this star and this planet with a very large telescope so, hopefully, within a few months we may get some more information about this planet.

Chris - That must be like an Egyptologist thinking they’ve walked in on the tomb of Tutankhamun and then discovering their mates had been there first?

John - Yes, but the planet was publically announced back in 2015, so we both had the chance to have a go at it. In fact, I think we may have been observing the same transits from different telescopes in the same country without even knowing it!

Chris - They would get, by using a much bigger telescope, better resolution of data then, will they?

John - Yes, better wavelength resolution which will, hopefully, enable them to work out whether it’s methane or water, and possibly find out other interesting things about the atmosphere.

Chris - What are the implications of what you’ve found?

John - Well, if you look at very low mass stars, they tend to form with a lot of magnetic fields which, in turn, causes an awful lot of X-ray emission and ultraviolet light. We think that this blows off quite a lot of the atmospheres of low mass planets near them. What we have shown here is that an Earth-like planet can survive for billions of years with this kind of radiation coming in and still retain an atmosphere. Now we know that there’s a lot of these planets in the galaxy and, therefore, that means that there can be an awful lot of planets with an atmosphere. Many of these will actually have the right temperature on their surface to support life, so the implication here is that life could be a lot more common than we thought...