Twin planets orbiting a distant star may be half water

A pair of twin planets called Kepler-138 c and d appear to be water worlds, with steamy atmospheres and oceans that take up half their total volume

By Leah Crane

Two strange planets 218 light years away may be completely covered in oceans 500 times deeper than Earth’s. While there probably isn’t life on these twin water worlds, called Kepler-138 c and d, there may be a lot more of them scattered throughout the universe.

Both exoplanets orbit a star called Kepler-138 and were found in 2014. The observations at the time hinted that they were fairly different worlds from one another but that they were made largely of rock. Now, Caroline Piaulet at the University of Montreal and her colleagues have taken a new set of observations using the Hubble and Spitzer space telescopes, as well as the W. M. Keck Observatory in Hawaii, that indicate otherwise.

While the star was previously thought to have only three planets, these observations showed evidence of a fourth world. Including that extra planet in simulations of the system revealed that Kepler-138 c and d are far more similar than researchers initially thought. Each is a little more than twice as massive as Earth with about 1.5 times Earth’s radius.

By plugging those new numbers into their models, the researchers found that up to half of the volume of each planet must be made of something lighter than rock but heavier than the hydrogen and helium ubiquitous in gaseous worlds – the most likely explanation is water. “It can be other molecules that would have a similar density to water – methane or ammonia would be good alternatives – but the reason why we think it’s most likely water is that water is the most abundant of these alternatives in the universe,” says Piaulet.

However, despite its importance for life, water does not necessarily make a planet habitable. Kepler-138 c and d are both relatively close to their star, so instead of the icy shells that mark most of the water-laden worlds in our own solar system, they probably have dense atmospheres of steam. Beneath the atmosphere, temperatures are expected to reach beyond 200°C and pressures would be at least 100 times the surface pressure on Earth – perhaps as much as thousands of times higher.

“These probably aren’t the best planets for life,” says Piaulet. “But the fact that these exist means that there could be planets with compositions like this, but just a tiny bit further from their host stars, and that opens the door to a completely new type of habitable world.”

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-022-01835-4

Sign up to our free Launchpad newsletter for a voyage across the galaxy and beyond, every Friday