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Oceans on super-Earths can last for billions of years

Jenny Winder, News Writer
Jan 6, 2015, 19:17 UTC

Sen—New research suggests that some exoplanets are even better at establishing and maintaining oceans than our Earth. Oceans on super-Earths, exoplanets up to five times the mass, or 1.5 times the diameter, of our own planet, can last for billions of years, once they are established. 

A super-Earth is an extrasolar planet that is more massive than Earth, but less so than a gas giant like Uranus or Neptune, which hold up to 10 times the mass of our world. The first super-Earths to be found in a star's habitable zone, where liquid water could exist on a planet's surface, were discovered in 2007.

For other planets to develop life as we know it, those planets would need liquid water, or oceans. Geologic evidence suggests that Earth's oceans have existed for nearly the entire history of our world.

"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 a study presented at the annual meeting of the American Astronomical Society in Seattle.


Relative sizes of Kepler habitable zone planets discovered as of April 18, 2013 (except for Earth, these are artists' renditions). Image credit: NASA/Ames/JPL-Caltech

Earth is mostly rock and iron. Though water covers 70 per cent of Earth's surface, it makes up only about a tenth of a per cent of the planet's overall bulk.

"Earth's oceans are a very thin film, like fog on a bathroom mirror," explains study co-author Dimitar Sasselov (CfA).

However, studies have shown that Earth's water is not just on the surface. Our planet's mantle holds several oceans' worth of water. Earth maintains its oceans through planet-wide recycling as oceans are dragged underground by plate tectonics and subduction of the ocean seafloor, to return to the surface via volcanism (mainly at mid-ocean ridges). 

Schaefer's team used computer simulations to see if this recycling process would take place on super-Earths. They also studied how long it would take oceans to form after the planet cooled enough for its crust to solidify.

The team found that the oceans of super-Earths, two to four times the mass of Earth, would persist for at least 10 billion years (unless boiled away by an evolving red giant star).

The oceans on the largest planet (five times the mass of Earth) that the team studied didn't develop for about a billion years, due to a thicker crust and lithosphere that delayed the start of volcanic outgassing.

"This suggests that if you want to look for life, you should look at older super-Earths," Schaefer says.

"It takes time to develop the chemical processes for life on a global scale, and time for life to change a planet's atmosphere. So, it takes time for life to become detectable," added Sasselov.

According to the research, and assuming evolution takes place at a similar rate to Earth's, the search for complex life should begin on planets that are about five and a half billion years old, a billion years older than Earth.