"Super-Earths" could have atmospheres more similar to the Blue Planet than researchers previously believed.
These planets are common in the Milky Way, and new research suggests they may contain vast continents surrounded by liquid oceans, a Northwestern University news release reported.
Past theories have suggested super-Earths' surfaces were completely submerged in liquid; new research challenged that idea, claiming tectonically-active super-Earths would store liquid in their mantle leaving continents exposed. This type of composition could allow the planets to have an Earth-like "stable" environment.
"Are the surfaces of super-Earths totally dry or covered in water?" Nicolas B. Cowan, a postdoctoral fellow at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics said. "We tackled this question by applying known geophysics to astronomy. 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 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."
The researchers believe plate tectonics cause liquid to cycle between the super-Earth's surface and its mantle. Their model focuses on factors such as seafloor pressure and high gravity. They believe that larger super-Earths have higher pressure in both fields.
"We can put 80 times more water on a super-Earth and still have its surface look like Earth," Cowan said. "These massive planets have enormous seafloor pressure, and this force pushes water into the mantle."
"If Earth was 1 percent water by mass, we'd all drown, regardless of the deep water cycle," Cowan said. "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."
Exposed continents play a huge role in climate; the surface temperatures work to mediate the carbon cycle, producing a stabilizing 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," Dorian Abbot an assistant professor in geophysical sciences at UChicago, 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."
Two "uncertainties" in the researcher's model are that that super-Earth's actually do possess plate tectonics and exactly how much water would be stored in the mantle.
"These are the two things we would like to know better to improve our model," Cowan said. "Our model is a shot from the hip, but it's an important step in advancing how we think about super-Earths."
