Are Water Worlds Habitable?

By Shannon Hall

Roughly 39 light-years away toward the constellation Aquarius is a planet that hosts a global ocean so deep it drowns the land. Set sail anywhere on that water world and you will never spot mountains, hills or even beaches on the horizon, just deep blue tides. And this planet is not alone. A new analysis of the exoplanets circling TRAPPIST-1—which a 2017 study estimated were all roughly the size, mass and composition of Earth—suggests that four of the seven worlds are actually soaked in water. Two of them are more than 50 percent water, by mass, and the other two are less than 15 percent (that is still far wetter than Earth, which is less than 0.1 percent water). What is more: Multiple lines of evidence suggest that water worlds might be abundant throughout the cosmos.

That might sound like good news. After all, wherever we find water on Earth—be it the acidic pools of Yellowstone or the cracks within frozen glaciers—we find life. The correlation is so strong that NASA has adopted the mantra “follow the water” when searching for life beyond our pale blue dot. But these wet worlds have sparked a lively debate about how much water is too much.

Take the fifth planet within the TRAPPIST-1 system as an example. Cayman Unterborn, an exogeologist at Arizona State University in Tempe, and his colleagues think that the liquid water here extends down about 200 kilometers—roughly 20 times deeper than Earth’s Mariana Trench. That much water would create a large ice layer at the bottom of the ocean which would seal the ocean from the land and effectively shut down a geochemical cycle that plays a crucial role in Earth’s habitability.

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