Life as we know it needs water, but it would probably also like to live on a rocky world with a stable climate. That’s why huge, Earth-like exoplanets with both continents and oceans are better for harboring extraterrestrial life than waterworlds.
Super-Earths are typically thought to be covered entirely with water because their strong surface gravity would likely create a flattened topography and deep oceans. But in a study published in Astrophysical Journal earlier this year, Nicolas Cowan Northwestern University and Dorian Abbot from the University of Chicago argue that super-Earths have exposed continents and shallow oceans that allowed the planets to have a much more stable climate, making them far more habitable.
"A planet could be ten times wetter than Earth and still have exposed continents," Cowan tells Astrobiology Magazine. They used Earth as a starting point for modeling how these giants might store their water in surface reservoirs (oceans) and in underground, interior reservoirs within the mantle. They looked at how a planet’s water distribution could end up being balanced in a steady state between the surface oceans and the mantle.
"Earth is the only known planet with plate tectonics, a deep water cycle, etc., so it’s a good place to start," Cowan explains. "On the other hand, if it turns out that Earth’s deep water cycle is nowhere near a steady-state, then our conclusions are way off the mark. "
They found that super-Earths with active tectonics can have exposed continents if their water is less than 0.2 percent of the total planetary mass. "By making super-Earths 80 times more likely to have exposed continents," Abbot tells Gizmodo, "we've dramatically improved their odds of having Earth-like climate."
Water that’s deep in the mantle can re-enter the surface oceans when volcanic activity at mid-oceanic ridges splits the crust; this drop in pressure causes the mantle rock to lose its volatiles, like water. But because super-Earths have stronger gravity, the greater seafloor pressure suppresses the mantle’s loss of water. That means more of the planet’s overall water is stored in the mantle, and not overflowing their basins.
"If some of our input parameters are wildly off, then the actual water-world boundary might differ by an order of magnitude," Cowan says. "No matter how you cut it, though, the water-world boundary is unlikely to be as damning as previously thought."
You might remember how earlier this month astronomers announced a new mega-Earth: Called Kepler-10c, it’s about 17 times heavier than our planet. Researchers initially thought it was a Jupiter-like gas giant, before they realized how hefty and solid it is. No word yet on its continents.
[Via Astrobiology Magazine]