Super-Earths may have oceans and continents

Super-Earths - common in our Milky Way galaxy - may have both oceans and exposed continents, suggesting these exoplanets have a stable climate like that of our Earth, scientists have found.
Researchers found that the odds of massive terrestrial planets, called 'super-Earths', having an Earth-like climate are much greater than previously thought.
Nicolas B Cowan, a Northwestern University astrophysicist and Dorian Abbot, a University of Chicago geophysicist developed a new model that challenges the conventional wisdom which says super-Earths actually would be very unlike Earth - each would be a waterworld, with its surface completely covered in water.
The new study concluded that most tectonically active super-Earths - regardless of mass - store most of their water in the mantle and will have both oceans and exposed continents, enabling a stable climate such as Earth's.

"Super-Earths are expected to have deep oceans that will overflow their basins and inundate the entire surface, but we show this logic to be flawed," Cowan said.
"Terrestrial planets have significant amounts of water in their interior. Super-Earths are likely to have shallow oceans to go along with their shallow ocean basins," he said.

In their model, Cowan and Abbot treated the intriguing exoplanets like Earth, which has quite a bit of water in its mantle, the rocky part that makes up most of the volume and mass of the planet.
The rock of the mantle contains tiny amounts of water, which quickly adds up because the mantle is so large. And a deep water cycle moves water between oceans and the mantle.

Water is constantly traded back and forth between the ocean and the rocky mantle because of plate tectonics.
The division of water between ocean and mantle is controlled by seafloor pressure, which is proportional to gravity, Cowan and Abbot said.
Accounting for the effects of seafloor pressure and high gravity are two novel factors in their model. As the size of the super-Earths increase, gravity and seafloor pressure also go up.
"We can put 80 times more water on a super-Earth and still have its surface look like Earth. These massive planets have enormous seafloor pressure, and this force pushes water into the mantle," Cowan said.

The ability of super-Earths to maintain exposed continents is important for planetary climate.
On planets with exposed continents, like Earth, the deep carbon cycle is mediated by surface temperatures, which produces a stabilising feedback (a thermostat on geological timescales).
"Such a feedback probably can't exist in a waterworld, which means they should have a much smaller habitable zone," Abbot said.
"By making super-Earths 80 times more likely to have exposed continents, we've dramatically improved their odds of having an Earth-like climate," Abbot said.