The field of planet detection undergoes a step change this week, as the Kepler team detail 312 exoplanet candidates, almost doubling the total number of known exoplanets. Up until now, planet-hunters have had to rely on a drip-feed of new results. While we’ve been doing an awful lot better since the first detections in the nineties (see graph), we’re still only detecting tens of planets a year.
Before Kepler’s announcement yesterday, there were only around 460 known extrasolar planets. If you want to try and understand how all planets form, this sample is simply too small to do any reliable statistics. And it gets worse: we know for a fact that we have been biased in the past towards seeing more massive planets, especially if they are closer in to their parent star. New instruments like Kepler are beating this bias, and are probing down to lower and lower planet masses. Current theory says that we should see more planets of low mass than high mass, so the more sensitive our probe, the more planets we will see.
Hence Kepler’s astonishing bounty. 306 stars with 312 planets (5 of the stars have multiple planets). Kepler detects planets using the transit method (watching for the planet eclipsing the star), so we know the planets’ radius from the shadow it casts on the star (as well as the period of the orbit by watching for multiple eclipses). What’s interesting is that most of the planets in the Kepler list have radii similar to Neptune’s, about ten times smaller than the typical planetary radii in the 460 previously known planets. They still tend to see planets on short period orbits (2 to 5 days typically), but they have a few noticeable long ones.
There’s one in particular you could be hearing a lot about in the next few weeks, the sexily named KIC11465813. It is about 1.3 times the radius of Jupiter (pretty large for a giant planet!), orbiting with a period of about 30 years around a solar-type star. It’s so big that it might not be a planet, but a brown dwarf (an object big enough to perform nuclear fusion at its core, but not the same flavour of fusion as regular stars do).
If it is a planet, then it would be a pretty cool exoplanet (about -93 degrees C on the surface), maybe a little too cool for a planet so big. If that’s the case, then it might need a set of Saturn-like rings to cast such a big shadow. Or even an Earth-like satellite! Detecting such satellites (which we call exomoons) has been the subject of theory for a while, and I know quite a few people who would get pretty excited at the first confirmed detection.
In any case, 772 planets is substantially better than 460. It could get even better – data on 400 more candidates haven’t been released yet. They’re being held back for further study, because some are ripe for a second observation using Doppler Wobble techniques. Some are particularly exciting because they have radii close to that of Earth’s. Potentially 712 planets detected by Kepler, including potential Earthlike planets, all with only 2 months of observation time!
The bottom line: exoplanet researchers are about to receive an embarrassment of riches. Watch out for some seismic shifts in planet formation theory!