Large rocky planets excel at ocean building

SEATTLE — The best oceanfront real estate in the galaxy may be on a planet two to four times as massive as Earth. Such super-Earths steadily grow oceans for many billions of years whereas less massive planets deplete their seas relatively quickly, researchers reported January 5 at a meeting of the American Astronomical Society.

Astronomer Laura Schaefer of the Harvard-Smithsonian Center for Astrophysics used computer simulations to study how the mass of a planet affects its underground water cycle. She found that small planets build oceans quickly but can’t sustain their water cycle, in which plate tectonics draws water into the mantle and volcanoes return it to the atmosphere. The planet’s water eventually becomes trapped in the mantle. Inside super-Earths, on the other hand, high pressure can keep the water cycle running for at least 10 billion years, creating oceans that grow deeper over time.

Planet mass is just one ingredient for creating and sustaining oceans. 

Venus and Earth, for example, weigh nearly the same, yet Venus is so hot that its water evaporated long ago. Schaefer plans to look at how planet mass and proximity to the host star work together to determine an ocean’s fate.