The Kepler telescope found a massive number of planets from May to September in 2009. What's the breakdown?
William Borucki: We released data on 155,000 stars, including 1235 candidate planets. Of those, 68 are Earth-size; 288 super-Earth-size, which means they're almost twice the size of Earth; 293 are Neptune-size, about four times the size of Earth; and 165 candidates are the size of Jupiter, 11 times the size of Earth. We now have over 1200 candidatesmore than twice the number of planets yet discovered by humansfound in our first four months of collecting data. We'll be putting out a paper in the next day or two to let people see what we found.
Were there results you did not expect? What surprised you?
One of the biggest surprises is that we're finding 170 stars that have multiple planetary candidate systems. That's an enormous number that blows everything else away. You're seeing systems of planets, like the solar system, and we're learning so much. It's a big, important discovery.
The thing that's special is that we've found 68 Earth-size candidate planets, and 54 candidates in the habitable zone of their stars. Of those in the habitable zone, one of these is a little smaller than Earth; four are a little bit bigger, up to twice the size of Earth; and the other 49 are quite a bit bigger. Some of them are the size of Jupiter, which means their moons could be as large as the Earth. All of the moons orbiting Jupiter are in the habitable zone, because Jupiter is in the habitable zone. You can imagine, among these moons, several have atmospheres.
When you do find an Earth-size planet in the habitable zone, how do you determine the chances that it has life?
We have no way of knowing, and I don't want to guess. We don't know if these planets have atmospheres or not, but I can tell you that we've worked very hard to prove that these candidates aren't planets and we've failed. We need a bit more evidence before we can announce them, but that's what we'll be doing in the coming years. We'll have never-before-seen candidates in or near the habitable zone, but we'll be also be spending a lot of our effort to confirm whether the candidates we know about today are planets.
Finding meaning in so much data, over such a vast amount of space and over a long period of time, must have its difficulties. How did you review the data?
One of the things that we spent a lot of time on was looking at the data and seeing if it was a false positive. For example, if you see two eclipses, then it's a small star. There shouldn't be a big eclipse and a small eclipse if it's a planet. There should be just the transit.
If the planet is hot enough, the life of the planet is cut off when it goes behind the star, so we look for that to see if the shape of the transit is right. The shape of a star passing over another star looks different from a planet crossing over a star. Another thing that bothers us quite a bit is that when you look into the galaxy as we do, there are a lot of little starsbillions of little starsand some of them are close to your targets. So a little star that's going through eclipses, an eclipsing binary, makes you think that it's your star, but instead, it's this background star.
When Kepler operates, it has a blurry image, so it doesn't saturate these stars and can't tell the difference. We have to go through a great deal of effort to make sure what we're finding isn't one of these background stars. We take the Kepler data and, using programs we built that search for what happens during a transit, we can tell if a background star is actually doing the transiting, because its brightness is changing.
We also go to the really big telescopes, and we get very sharp images to see if there's anything around that star that could explain it. When we're through with all those tests, we go to the Keck or the HET or another large telescope and we measure the spectra. If the spectra show that the star is wobbling, we get the mass of a planet. In particular, we can go to the Keck, get the period of the planet and get the epoch (where the plane