January 9, 2014
Super Earths May Be More Like Our Planet Than Previously Theorized
[ Watch the Video: More Like Earth? Exoplanet Scientists Weight In ]
Brett Smith for redOrbit.com - Your Universe Online
It seems like the discovery of Earth-like planets is always announced with one caveat – the planet discovered probably isn’t much like Earth. Many of these are "super-Earths," several times larger than our own planet and thought to be completely covered in water.
However, a new study to be published in an upcoming edition of the Astrophysical Journal, has suggested that these massive Earth-like planets do have both oceans and exposed continents.
"Are the surfaces of super-Earths totally dry or covered in water?" rhetorically questioned study author Nicolas B. Cowan, a postdoctoral fellow at Northwestern University. "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."
In the study, the research team treated exoplanets like Earth, which has a significant amount of water in its mantle. Rock within the mantle contains tiny amounts of water, but because the mantle is so large – those small amounts of water add up to a large quantity. A water cycle deep within the Earth moves water between oceans and the mantle. The division of water between the oceans and mantle is determined by seafloor pressure, which is relative to gravity.
When the study team considered the effects of seafloor pressure and the high gravity of exoplanets in their model, they found that as the size of 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," Cowan said. "These massive planets have enormous seafloor pressure, and this force pushes water into the mantle."
The team noted that the ability of super-Earths to preserve exposed continents is necessary for planetary climate. Exposed continents mean that the carbon cycle on a planet is mediated by surface temperatures, which produces a thermostat-like stabilizing feedback.
"Such a feedback probably can't exist in a waterworld, which means they should have a much smaller habitable zone," said study author Dorian Abbot, an assistant professor in 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."
The study team conceded two major uncertainties in their model: knowledge of super-Earths’ tectonics and the amount of water Earth has in its 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."
He added that it doesn't take that much water to push a planet into being covered in water.
"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."