Twin planets orbiting a distant star may be half water

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


15 December 2022

Kepler-138 d (front), Kepler-138 c (left), and Kepler 138 b passing in front of its parent star

NASA, ESA, Leah Hustak (STScI)

Two strange planets 218 light-years away may be completely covered by oceans 500 times deeper than Earth’s. While these twin water worlds, designated Kepler-138 c and d, probably do not have life, there may be many more scattered throughout the universe.

Both exoplanets orbit a star called Kepler-138 and were found in 2014. Observations at the time implied that these were quite different worlds, but largely made of rock. Now, Caroline Piaulet of the University of Montreal and her colleagues have received a new set of observations that show otherwise, using the Hubble and Spitzer space telescopes, as well as the WM Keck Observatory in Hawaii.

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

By tying these new numbers to their model, the researchers discovered that up to half of each planet’s volume must be made of something lighter than rock but heavier than the ubiquitous hydrogen and helium on gaseous worlds—the most likely explanation is water. “There may be other molecules with similar density to water — methane or ammonia would be good alternatives — but the reason we think it’s most likely water is because it’s the most abundant of these alternatives in the universe,” Piaulet says. .

However, despite its importance to life, water does not necessarily make a planet habitable. Kepler-138 c and d are relatively close to their stars, so they likely have dense vapor atmospheres rather than the icy crusts that characterize most of the water-laden worlds in our own solar system. Below the atmosphere, temperatures are expected to rise above 200°C and pressures will be at least 100 times, perhaps thousands, times higher than the surface pressure on Earth.

“These are probably not the best planets for life,” Piaulet says. “But the fact that they exist means that there may be planets with compositions like this, but only slightly further away from their host stars, which opens the door to an entirely new habitable world.”

Journal reference: Nature AstronomyDOI: 10.1038/s41550-022-01835-4

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