  Light and cold extrasolar planets such as OGLE 2005-BLG-390Lb, a 5.5
Earth-mass planet detected via microlensing, could be frequent in the Galaxy
according to some preliminary results from microlensing experiments. These
planets can be frozen rocky- or ocean-planets, situated beyond the snow line
and, therefore, beyond the habitable zone of their system. They can nonetheless
host a layer of liquid water, heated by radiogenic energy, underneath an ice
shell surface for billions of years, before freezing completely. These results
suggest that oceans under ice, like those suspected to be present on icy moons
in the Solar system, could be a common feature of cold low-mass extrasolar
planets.
