The effort to determine the circumstances in which large rocky planets have an ocean offers good news, but also suggests some planets may take a while to get properly wet.
Few questions are as debated, or as important, to the question of life on other planets as the circumstances under which oceans exist and persist. Microbes might survive on a largely desert world, but all our visions for advanced life rely on some sort of large body of water, be it Earth's marine expanses or the oceans thought to exist beneath the ice of Europa and Ganymede.
"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," says Laura Schaefer of the Harvard-Smithsonian Center for Astrophysics (CfA) and lead author of the paper presented at the American Astronomical Society's conference this week. “The abundance of water on the Earth's surface is not controlled by the atmosphere, but rather the deep water/silicate cycle,” the paper argues.
From our surface view, oceans seem to be the dominant phase of matter on the planet, covering 70% of the Earth. However, co-author Dimitar Sasselov, also of the Center for Astrophysics, points out, “Earth's oceans are a very thin film, like fog on a bathroom mirror.” If oceans have disappeared from Mars and Venus (albeit of carbon dioxide in the latter case), might their survival be an interstellar anomaly?
So far the search for planets like our own has mostly turned up “super-Earths,” those with between one and ten times the mass of our planet, probably because they are easier to find than smaller objects. However, with nothing rocky in this range to study up-close, we know little about them.
Schaefer and Sasselov modeled the behavior of water on rocky planets that were both more and less massive than Earth to see how quickly oceans would form, as well as how water would be processed through the crust. Our direct experience is that where one tectonic plate is subducted under another, water is dragged down into the Earth's mantle, to be returned through volcanoes at mid-ocean ridges.
The modeling suggests that thicker crusts delay the release of volcanic water. Consequently, larger planets may remain dry on the surface for a long time – a billion years in the case of a five Earth masses object. Their oceans also never get as deep as on Earth. However, once started they last a long time, at least a billion years unless orbiting a large, and therefore short-lived, star. On the other hand, while lighter planets gain oceans quickly, they lose much of the water fast enough to endanger evolution.
"This suggests that if you want to look for life, you should look at older super-Earths," Schaefer says.
The work accompanies a study by the same institution suggesting a similar process of planetary evolution for the super-Earths so far identified and the rocky worlds of our own solar system.
