Free Astronomy Magazine May-June 2016

KEPLER-36’S NON-IDENTICAL TWINS Another recent, puzzling case concerned the two known planets of Kepler-36, a sun- like star located 1,530 light-years away in the constellation Cygnus. The two planets have the particular character- istic of orbiting extre- mely close to one an- other, such that (at the minimum distance) they are separated by only 2 million kilometres, or 5 times the average Earth- Moon distance. But even more peculiar is the fact that despite having formed at the same time, approximately at the same distance from their star (0.115 AU and 0.128 AU), the two objects have very different average densities which are difficult to interpret if we consider that the size of the two planets are not significantly different: the inner one, Kepler-36b, is a super-Earth, while the outer one, Kepler-36c, is a mini-Neptune (two proximate planetary classes). One is thus rocky, with a negligible atmosphere in terms of mass, while the other has a solid core surrounded by an extended gaseous envelope of hydrogen and helium. In order to make sense of the glaring differences between the two planets, James Owen (In- stitute for Advanced Study, Princeton) and Timothy Mortom (Princeton University) re- processed the archival data of the Kepler space telescope, with the aim of establish- ing the initial physical conditions of the two planets and the weight of the stellar radia- tion in the evaporation process of the atmo- sphere during the 6 billion years since the birth of this system. It was found that when the two planets were formed, they were A bove, an ex- ample of how planet BD +20594b might appear to a hy- pothetical visitor. With a mass 16 times that of the Earth and a dia- meter more than twice that of the Earth it is the larg- est rocky planet discovered so far. Its revolution pe- riod only lasts 42 days. [NASA/JPL- Caltech] C omparison between the positions and sizes of the Kepler-36 planets and those of Mercury (the stars are not to scale). Below, an imaginative representation of Kepler-36b and Kepler-36c which, due to their very close orbits, are as close as two planets can get. [Rodrigo Luger/NASA images]