Other habitable planets?

Dan - Well, the story I've got this week is about exoplanets - planets around other stars. There's an announcement this week that the Kepler satellite has discovered the best candidate so far for a habitable planet around a distant sun-like star. So, it could have oceans, it could have land, it could be habitable by some sort of creature, but we don't know that much about it. Only that it's the right size, the right distance from its sun, and you know, it looks a little bit like Earth.

Ben - So, what are we looking for when we're looking for habitable planets rather than just any old lump of rock or gas? What are the indicators?

Dan - Well, all they can tell with this particular method that Kepler uses, because planets are so small and so dim compared to the star they're orbiting around, you can't see them directly. It's very, very hard to see them directly, so you have to detect them by other methods.

Kepler uses the method of transits. So, it essentially looks at a star, measures how bright it is and keeps watching it. And if ocasionally, it gets a very, very tiny bit dimmer, that means that a planet has passed in front of it. So, the only information you get are about its orbit - so, how long it takes to orbit the star and the size of the planet so you can tell its radius. So, the stellar system it has discovered in this case has 5 planets that they've detected so far, ranging from half the size of Earth to twice the size of Earth. But the most distant one is the one that looks the best candidate and they judge that by its distance from the star and essentially how much energy it's getting from the star.

If it's too far away, any water and other gases on it would freeze out. If it's too close, it would be too hot and they would boil off. So, you're looking for one that's in the middle of - between those two extremes, so youre going to get the most temperate atmosphere. But sadly, we can't know if it has an atmosphere. We don't know what it's made of. We don't even know for sure that it's got a rocky surface. It could be made of gas. But it's 40% bigger than earth, so the likelihood is that it could be a solid planet.

Ben -  And Kepler has been out there for awhile now and it's found hundreds of them. It seems to turn up a few hundred more every month in fact. How do we now start asking the questions you were talking about? Does it have an atmosphere? Is it rocky? Surely, Kepler can' do that because of the way it's looking for planets.

Dan - That's true. It's not there to characterise the planets. It's there to see how many there are and how they're distributed. So, to find out more about individual planets, we have to have another method, and the ideal method is direct observation. But that's extremely hard because it's like looking into the beam of a search-light in trying to identify a firefly flying around nearby. And it can be done.

The Hubble Space Telescope and some ground base scopes have identified a few planets. But they tend to be special cases like very bright planets around very dim stars and in very wide orbits. So, they're a long way away. But you need special telescopes or special satellites to get any further. Later this year, there are a couple of new instruments that are going to be attached to some of the world's biggest ground base telescopes and they'll be able to directly image some bigger planets, but not ones that are earth-sized. To get earth-sized planets, you need a specially purpose-built telescope in space and at the moment, they're just too expensive.