Massive super-Earths might not be waterworlds

The artist's concept depicts Kepler-62e, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the sun, located about 1,200 light-years from Earth in the constellation Lyra. Scientists do not know if Kepler-62e is a waterworld or if it has a solid surface. (Credit: NASA Ames/JPL-Caltech)

The climate on massive terrestrial planets called “super-Earths” may be far more Earth-like than previously believed.

A new model challenges conventional wisdom that the surface of super-Earths are completely covered in water. Instead, the model suggests that most tectonically active super-Earths—regardless of mass—store most of their water in the mantle and have both oceans and exposed continents, allowing for a stable climate like the one on Earth.


“Are the surfaces of super-Earths totally dry or covered in water? We tackled this question by applying known geophysics to astronomy,” says Nicolas Cowan, postdoctoral fellow at the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern University.

“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,” he says. “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.”

In the model, researchers 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.

An exoplanet, or extrasolar planet, is a planet outside our solar system.

80 times more water

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.

Accounting for the effects of seafloor pressure and high gravity are two novel factors in the 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 says.

It doesn’t take that much water to tip a planet into being a waterworld.

“If Earth was 1 percent water by mass, we’d all drown, regardless of the deep water cycle,” Cowan says. “The surface would be covered in water. Whether or not you have a deep water cycle really matters for planets that are one one-thousandth or one ten-thousandth water.”

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 stabilizing feedback (a thermostat on geological timescales).

A shot from the hip

“Such a feedback probably can’t exist in a waterworld, which means they should have a much smaller habitable zone,” says Dorian Abbot, assistant professor of geophysical sciences at the University of Chicago. “By making super-Earths 80 times more likely to have exposed continents, we’ve dramatically improved their odds of having an Earth-like climate.”

Cowan and Abbot accede that there are two major uncertainties in their model: that super-Earths have plate tectonics and the amount of water Earth stores in its mantle.

“These are the two things we would like to know better to improve our model,” Cowan says. “Our model is a shot from the hip, but it’s an important step in advancing how we think about super-Earths.”

The findings will be published in the Astrophysical Journal . The Alfred P. Sloan Research Foundation funded the project.

Source: Northwestern University