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'Super-Earth' exoplanets may be more Earth-like than previously thought

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With the number of known exoplanets being discovered now numbering in the thousands (and estimated to be in the billions in our galaxy alone), astronomers have already found an amazingly diverse plethora of worlds. Some of the most common are the "super-Earths," rocky planets which are larger than Earth but smaller than Neptune or Uranus. It has been thought by scientists that such worlds might often be true water worlds, with their surfaces completely covered by water with no land visible at all, causing less stable climates than on our home planet.

But now new studies suggest that these planets might tend to be more Earth-like after all. Two scientists, Nicolas B. Cowan from Northwestern University and Dorian Abbot from the University of Chicago have announced a new model for super-Earths which shows that ones which are tectonically active would probably store most of their water in their mantles, producing both oceans and continents and subsequently a more Earth-like climate. This would likely be true regardless of the mass of the planet(s).

As Cowan explains:

“Are the surfaces of super-Earths totally dry or covered in water? 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. 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.”

Plate tectonics allow a water cycle to exist between the oceans above and the mantle below, which helps to stabilize the climate. Even for such rocky planets larger than Earth, this could still create both oceans and continents, due to increased gravity and seafloor pressure.

“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.”

On Earth, the carbon cycle, essential for life as we know it on our planet, is also regulated by surface temperatures, producing a stabilizing feedback, a sort of thermostat on geological timescales.

As Abbot notes, “Such a feedback probably can’t exist in a waterworld, which means they should have a much smaller habitable zone. 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 findings suggest that super-Earths are much more likely to have an Earth-like surface than previously thought. The new model depends on these planets have plate tectonics and a similar amount of water, or more, stored in their mantles, which are still two unknowns at this point. But the odds are probably good that at least some of them will, increasing the chances of life of some sort on these worlds.

The findings were presented on Jan. 7 at the 223rd meeting of the American Astronomical Society (AAS) annual meeting in Washington, D.C.

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