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.
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