Water Worlds

With the end of winter still weeks away, people in colder climates may be dreaming of dips in a warm ocean under a steamy sky. And we know a couple of places where they wouldn’t have to wait for the change of seasons — a pair of “twin” planets orbiting a star in Lyra.

Kepler-138 is more than 200 light-years away. It consists of a star that’s much smaller, fainter, and cooler than the Sun, and perhaps four planets. Two of the planets are a little bigger and heavier than Earth. Such worlds typically are balls of rock and metal, just as Earth is. But these planets — Kepler-138 c and d — are different.

Studies with telescopes in space and on the ground showed that both planets are much less dense than Earth. But they’re too dense to be made of gas. Instead, they probably have dense centers surrounded by a layer of water more than a thousand miles deep.

The planets are so close to the star, though, that not all of the water is liquid. Each may have a thick atmosphere made of water vapor — like a giant steam bath. The atmosphere pushes down on the layer of water below, forming a deep ocean. But there’s probably not a sharp boundary between them. Instead, there may be a deep transition zone — part liquid and part gas — creating hot, wet worlds in a distant star system.

Lyra is high in the east at dawn, marked by its brightest star, Vega. But you need a telescope to see Kepler-138.
 

Script by Damond Benningfield