The record for the smallest planet orbiting a sun-like star has once again been broken. The newest planet to hold the record in its tiny hand is COROT-Exo-7b, which has only twice the diameter of the Earth!
COROT is a European satellite that stares at stars and looks for tiny drops in their brightness. There are many causes of such events, but one is from a planet passing in front of the star, making a mini-eclipse. This has a distinctive signature in the way the light of the star dims, so a planetary transit can be distinguished from other sources.
We know so much about the way stars behave now that by looking at how much the star dims, we can get a decent handle on the size of the planet (a big planet blocks more light, and a smaller one makes a smaller dip). In this case, the planet has a diameter of about 25,000 kilometers or so. That's far smaller than the usual "hot Jupiters" detected, which are more than 100,000 km across. Like them, however, it's very close to its parent star: it only takes about 20 hours to make an orbit! Even Mercury takes 88 days to round the Sun, so this new dinky ball must be practically touching the surface of its sun. Even though the star is a K0 type (slightly cooler than the Sun), the surface temperature of the planet must be well over 1000 C. Just so's you know, this system is over 450 light years away from us.
The big questions is, what's the mass of this planet? These observations cannot determine that; they only get its size. To find its mass, astronomers will have to take spectra of the star and look for a shift in the wavelengths from the Doppler shift as the planet tugs the star. I have not read the journal paper for this discovery -- it's not published yet -- but it seems to me that kind of observation may still be beyond our current technology. The planet may not have enough mass to pull on the star hard enough for us to detect it.
Until we know that, we can't say much about this little guy. Is it solid metal, or rock? What characteristics does it have? We simply don't know. I'll add that a lot of news stories -- even the press release itself -- are saying this is a planet that has a surface we could walk on. That's baloney, no matter what. First of all, walking on a surface glowing red hot may be your idea of a fun perambulation, but it ain't mine! Second, if the planet is rocky, the surface may very well be molten. Third, planets have surprised us before. While I doubt this is a gaseous body, we simply don't know enough about it to say much about it past its size and temperature. In other words, saying anything more is pure speculation, and should be regarded as such.
Still, a planet this size strongly implies it's rocky, like Earth, as opposed to a gas giant like Jupiter. We are very, very close to detecting an Earth-sized planet orbiting another star, maybe even one in an Earth-like orbit. I wouldn't be surprised if we find one in the next year or two... but even then, we won't know much about it besides its size and temperature. But that's certainly enough to make for some exciting news!
Image credit: CNES