So, more exciting news from the Kepler team: Kepler 20, a planetary system of five planets orbiting an Earth-like star 946 light years away in the constellation Lyra was announced at a NASA press conference yesterday. This time, none of the planets, from Kepler 20b through -f, were estimated to be orbiting within the habitable zone of Kepler 20 but instead had very short orbital periods of between 3 and 77 days, putting them all within the orbit of Mercury if superimposed onto our Solar System. The importance of the Kepler 20 system is that two of the planets (-e and -f) are Earth-sized, or possibly even smaller, making them the smallest extrasolar planets yet discovered. The remaining three planets -b,-c and -d are sub-Neptune sized. I’ve done my best to illustrate their orbital configurations here (click for a better view):
In terms of habitability, there is not much we can garner from the Kepler 20 planets. They are all in the ‘hot zone’ in very close proximity to the star. Their equilibrium temperatures are likely to range from 1014 °K (Kepler 20b) to 369 °K (Kepler 20d) – much too hot to host terrestrial biology within our current experience and understanding. It is possible, within ‘theoretical considerations‘ (i.e. planet formation model output), that Kepler 20e and -f are rocky, and that Kepler 20d may have a thick water-vapour atmosphere.
The larger Kepler 20 b, -c and -d were detected using a combination of photometry and astrometry, but due to their relatively small photometric and/or gravitational effect, the smaller Kepler 20 planets were uncovered using a statistical method that excluded the possibility that the detected signal was caused by a binary star eclipsing Kepler 20. The Kepler team have now provided proof that they are able to detect Earth-sized planets, even in very short period orbits. Coupled with the discovery of Kepler 22b in the habitable zone a few weeks ago, their brief of discovering Earth-like planets with the habitable zone of distant stars is partially complete – all we need now is to find an Earth-sized planet, like Kepler 20e, in the habitable zone, like Kepler 22b. It really is only a matter of time now.