The formation of wet planets like Earth seems easy.© GettyImages

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.

They simulated a scattering of planetary embryos inside the orbit of a Jupiter-like giant. These embryos accrete smaller bodies of rock and ice, called planetesimals, under the strong pull of the giant. The researchers explored a range of possibilities by, for example, altering the position, mass and orbit of the Jupiter-like planet.

In all cases, one to four Earth-like planets appeared, with masses ranging between 20% and 400% of Earth's. In our own Solar System there are three such planets: Venus, Earth and Mars.

These simulated planets acquired a water masses ranging from almost zero to 300 times the amount on the Earth's surface. The formation of wet planets like ours "seems easy", say the researchers. More surprising still is the fact that more than half the worlds were much more water-rich. Land dwellers like us would be impossible on planets entirely covered by ocean.

The existence of planets like this, for which there is no analogue in our Solar System, must be seriously considered, the team concludes. 

• References

  1. Raymond, S. N., Quinn, T. R. & Lunine, J. I. Making other Earths: dynamical simulations of terrestrial planet formation. Preprint, http://xxx.arvix.org/abs/astro-ph/0308159, (2003).