Evolutionary dichotomy for rocky planets
A simple model shows that a rocky planet close to its star may solidify so slowly that its water is lost to space and the planet becomes desiccated, whereas a planet farther out may solidify quickly and retain its water. See Letter p.607 Accepted theories of planet formation commonly assume that planets with identical initial sizes and compositions would have the same early cooling history. But here Keiko Hamano and co-authors show that terrestrial planets fall into two distinct types based on their evolutionary history during solidification from an initially hot molten state. Type I planets, formed beyond a certain critical distance from the host star, solidify within several million years and retain most of their water, which forms the earliest oceans. On a type II planet, formed inside the critical distance, a magma ocean can be sustained for as long as 100 million years, and hydrodynamic escape desiccates these planets during this slow solidification process. Earth can be classified as a type I planet, but Venus formed close to the critical distance and its dry surface and mantle indicate that it might be a type II planet.