The hunt for Hoth: Do Star Wars-like planets exist?

Exoplanets are planets that orbit a star other than our Sun.  So far, more than four thousand of them have now been discovered. But where are they, and what are they like? Could any of the Star Wars planets be a reality? Chris Smith spoke to space scientist Paul Rimmer from Cambridge University, asking firstly how scientists look for exoplanets...

Paul - One of the ways in which they find these planets is just by looking for them. They can see these planets either from the light that’s reflected from a star of the planet or from the light of the planet itself. It’s very important, if you’re going to do that, to block out the light of the star because the star is so much brighter than the planet itself.

Another way is very much like if you’ve looked at the transit of Venus, it’s the same sort of idea. You have a planet passing in front of our Sun, you can also have planets passing in front of other stars. Now these stars are so far away that you’re not really going to be able to see the planet passing in front of the star but what you can see is you can see a little dip in the light of the star.

Chris - You see the effect of the planet on the light rather than see the planet itself?

Paul - Exactly. Depending on how prominent that effect is, you get an idea about the size of the planet.

Chris - How? Because it makes a big hole in the light coming to you so the light dips or something?

Paul - Yeah. Some of that light is blocked and so the light goes down a little bit, and the amount that the light goes down depends on that cross-sectional area of that planet.

Chris - Presumably, how often it does that tells you how fast it’s going round the star and that tells you, therefore, how far away from the star it is?

Paul - Absolutely, absolutely.

Chris - What else can you do because, presumably, the planet is exerting a gravitational effect on the star, so can you exploit that as well?

Paul - Absolutely, yeah. You can actually look at the star and the star will have a certain colour. As the planet goes around, the planet pulls on the star and causes the star to wobble away from us and towards us a little bit, and that makes the light a little bit bluer and a little bit redder. And from that, how fast that happens, you also get an idea of how far away the planet is from the star, and by how much that happens you get an idea of the mass of the planet.

Chris - So you can physically weigh a planet you can’t see, around a star that’s light years away?

Paul - Yeah.

Chris - If you know the mass of star, does that then tell you roughly what it’s made of because you can get some idea as to where it's orbiting, and how fast, and how much it’s making the star wobble, so can you infer the mass of the planet from that?

Paul -Yeah. With all of these methods though, it’s very important to understand the star very, very well. With the transit method also, you only know the size of the planet as a ratio of the size of the star, so you really need to know the size of that star.

Chris - Can we work out what these planets are made of though?

Paul - Yeah. As we were just talking about from this transit method, you get an idea about the size of the planet. And from this radial velocity method, this is looking at the wobble of the star, you get an idea about the mass of the planet. If the planet has a density of around 5 grams per centimetre cubed, then you know that is probably rocky like the Earth. If it’s something more like 2 grams per centimetre cubed then it probably has a great deal of water, and if it’s density is much less than that then it probably has a very gaseous envelope of hydrogen and helium.

Chris - What about the conditions one might experience if you were transported to this world?

Paul - One of the things that you could look into is whether the planet actually has an atmosphere and you can tell that through the transit method. You can look at the planet passing in front of its star at different wavelengths of light and maybe the planet looks smaller in red light and larger in blue light. That tells you that the red light passes through something more easily than the blue light passes through, and that’s generally an atmosphere.

Chris - How far away are these planets that you and your colleagues are looking at an exploring? Are they in our cosmic neighbourhood or are they a considerable distance away?

Paul - Quite a few of them are, in fact, in our cosmic neighbourhood. It is still vast distances by the way that I tend to travel. Some of them range from just a few light years all the way to thousands of light years away.

Chris - If you can tell all this about the atmospheric composition, and the likely temperature, and what the planets made of itself, does that mean you can also ask hard questions about the possible existence of life processes because we know there are some molecular signatures that go along with life on Earth?  If you were looking at the Earth from space and you asked those questions of the light coming from the Earth, you could tell there’s probably life here, so can we do that for these exoplanets?

Paul - Potentially so. It’s a challenging problem and, again, it’s a problem where you really need to be able to understand the star itself. So, if it was an Earth-like planet around a Sun-like star you could, in fact, look for these particular signatures in the transit. You could maybe see oxygen, and methane, and nitrous oxide, and if you saw all three of those that would be a very good indication that there was life.

One of the problems though is that a lot of these planets are found among much smaller, cooler stars. And these smaller, cooler stars are much more active in the ultraviolet, and that can end up producing some of these things like oxygen in great abundances without life being there at all, so you really need to be careful.

Chris - Anything that we’ve spotted so far that bear any remote resemblance to those sorts of worlds that they go visiting in Star Wars?

Paul - One of the planets, which comes out of my favourite exoplanetary systems right now is Trappist 1F which is, as far as we understand it, a pretty cool planet. We don’t know if any of these planets have an atmosphere. But, if it does have an atmosphere and the atmosphere was very much like what the Earth was in its infancy, then it would be very, very cold there. In fact, it would be almost entirely covered with ice except for this one side which is always facing its star, which would be like a giant blue ocean, and it might be likened to the ice planet Hoth, at least on one of its sides.

Chris - Would it be a nice place to go or not?

Paul - If you were a bacterium it may not be so bad in the ocean. Otherwise I probably wouldn’t recommend it as your first travel destination.