On the possible properties of small and cold extrasolar planets: Is OGLE 2005-BLG-390Lb entirely frozen?

Extrasolar planets as light as a few Earth masses are now being detected. Such planets are likely not gas or ice giants. Here, we present a study on the possible properties of the small and cold extrasolar planets, applied to the case of the recently discovered planet OGLE 2005-BLG-390Lb. This planet (5.5_-0.6^+1.5 M_⊕) orbits 2.6 _-0.6^+1.5 AU away from an old M-type star of the Galactic bulge. The planet should be entirely frozen given the low surface temperature (35-47 K). However, depending on the rock-to-ice mass ratio in the planet, the radiogenic heating could be sufficient to make the existence of liquid water within an icy crust possible. This possibility is estimated as a function of the planetary mass and the illumination received from the parent star, both being strongly related by the observational constraints. The results are presented for water-poor and water-rich planets. We find that no oceans can be present in any cases at 9-10 Gyr, a typical age for a star of the bulge. However, we find that in the past, when the planet was ≲5 Gyr old, liquid water was likely present below an icy surface. Nevertheless, the planet is now likely to be entirely frozen.