Free Astronomy Magazine November-December 2019

44 NOVEMBER-DECEMBER 2019 of lakes, seas and oceans, on average less deep than ours. It goes without saying that, in order to have a larger surface, a planet must be bigger and therefore more massive − but there is obviously a limit to the size. Planets more massive than the Earth develop much higher pressures at the mantle level, which increases viscosity, hampering (or mak- ing impossible) plate tec- tonics, a geological process essential to the redistri- bution of continents and oceans, and to the regen-