Simulation suggests formation of wet worlds is easy. If there are planets like Earth around other stars, they'll probably be water worlds, with awesomely deep, pole-to-pole oceans and no land in sight. So say Sean Raymond, of the University of Washington in Seattle, and colleagues. They have computer-modelled the late stages of planet formation, when Earth probably acquired its oceans1.The 110 planets detected so far around other stars are all much bigger than Earth, because they are the only ones that can easily be seen. But astronomers think that where there are big planets there may well be small ones - habitable ones, even. Many researchers think that liquid water is a prerequisite for habitability, as it is on Earth. But it's hard to explain why our world is wet. Models of our Solar System's formation suggest that the rocky objects that coalesced to form Earth about 4.6 billion years ago were too hot to carry much water. Many scientists think that the oceans arrived after the planet was formed, on icy comets and other cosmic debris that collided with the Earth.If so, whether a planet is habitable - wet, in other words - depends on how it formed as well as how close it is to its star. So Raymond's team looked at the statistics of planet formation in 42 different scenarios based upon our own Solar System.
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