NASA’s Kepler mission is proving to be a huge success, better than I thought was possible. Prior to Kepler, most planets were found using the Doppler spectroscopy method, in which telescopes look for the tell-tale to-and-fro movement of a star’s spectral lines as the star itself wobbles due to the gravitational tug of a planet. Kepler uses a different technique. It looks for the tiny dip in a star’s brightness when a planet transits – it’s the equivalent of looking for the dimming of a car’s headlight when an insect flies across it.
For any given star it’s unlikely that we’ll see a transit. But Kepler observes the brightness of so many stars at once – there are about 145,000 main sequence stars in its field of view – that the numbers stack up in its favour. Thanks to Kepler, the transit photometry method of exoplanetary discovery is proving to be much more productive than the Doppler method: at the time of writing it has found 2326 candidate planets. (Progress in this field never ceases to amaze me. When I was a student it seemed it impossible to detect exoplanets on this scale.)
The Kepler discovery announced by NASA on 5 December 2011 is perhaps the most interesting yet. The planet Kepler- 22b is in the habitable zone: in other words it’s possible that liquid water flows on this planet. We shouldn’t get carried away after this announcement: this planet is much bigger than Earth (about 2.4 times Earth size) and we don’t know whether it has an atmosphere (and the presence of an atmosphere will strongly influence the planet’s surface temperature). Nevertheless 22b is the most Earth-like planet we’ve ever seen. It orbits a star that’s like our Sun, it orbits in the Goldilocks zone, and it even has a year (290 days) that’s similar to Earth’s year.
The big question is: does 22b, like Earth, possess liquid water? And that’s important, of course, because ‘life as we know it’ requires water.
Even if it turns out that 22b isn’t an Earth-twin – and my guess is that in the end it will turn out to be not much like Earth at all – Kepler will eventually find a planet that’s truly Earth-like. And even if Kepler doesn’t, then some other mission will do so. (I’m not sure what that mission will be: the European Darwin mission has been cancelled, as have the American Terrestrial Planet Finder and SIM Lite missions. But some experiment, some time, will find an Earth twin.) I’m certain that there are myriads of truly Earth-like planets out there, all of them potentially homes for life. To my mind, then, the really interesting thing about planets like Kepler-22b is how it makes Fermi’s question even starker: