Habitable "super-Earths" with up to five times the mass of our own planet could possess vast, long-lasting oceans, new research suggests. Oceans cover more than two thirds of the Earth's surface and are continually replenished by volcanic activity dragging up water buried deep underground.
But a new study indicates that "habitable zone" planets with two to four times the Earth's mass may be even better at establishing and maintaining oceans. A planet occupying a star's habitable zone is in an orbit where temperatures are mild enough to allow the existence of liquid surface water, making lakes, rivers and oceans possible.
Most scientists agree that liquid water is a prerequisite for the development of life as we know it.
Dr Laura Schaefer, from the Harvard-Smithsonian Center for Astrophysics in the US, who led the new research, said: "When people consider whether a planet is in the habitable zone, they think about its distance from the star and its temperature. However, they should also think about oceans, and look at super-Earths to find a good sailing or surfing destination."
Studies have shown that the Earth's mantle holds several oceans' worth of water that has been pulled underground by the movements of tectonic plates and sinking ocean floors. Without water being brought back to the surface by volcanism, the oceans would soon disappear.
Dr Shaefer's team used computer simulations to verify that the same recycling process that keeps the Earth's ocean basins filled with water also takes place on super-Earths. In fact oceans were even more likely to be found on planets having two to four times the mass of the Earth.
Super-Earth oceans also persisted for a very long time - at least 10 billion years, assuming they were not boiled away by a swelling "red giant" star near the end of its life.
The largest planet studied, with five times the Earth's mass, did not develop oceans for about a billion years due to its thicker crust.
"This suggests that if you want to look for life, you should look at older super-Earths," said Dr Shaefer. Complex life was most likely to be found on super-Earth planets about 5.5 billion years old, around a billion years older than the Earth, said the scientists.
The findings were presented at the American Astronomical Society annual meeting in Washington DC.