How common are water-containing exoplanets?

Published - September 10, 2022 08:40 pm IST

A new study suggests that many more planets may have large amounts of water than previously thought — as much as half water and half rock. The catch? All that water is probably embedded in the rock, rather than flowing as oceans or rivers on the surface.

"It was a surprise to see evidence for so many water worlds orbiting the most common type of star in the galaxy," Rafael Luque from the University of Chicago said in a release.

Thanks to better telescope instruments, scientists are finding signs of more and more planets in distant solar systems. Scientists from the Institute of Astrophysics of the Canary Islands and the University of La Laguna decided to take a population-level look at a group of planets that are seen around a type of star called an M-dwarf. These stars are the most common stars we see around us in the galaxy, and scientists have catalogued dozens of planets around them so far.

As the scientists looked at the numbers — 43 planets in all — they saw a surprising picture emerging. The densities of a large percentage of the planets suggested that they were too light for their size to be made up of pure rock. Instead, these planets are probably something like half rock and half water, or another lighter molecule.

These planets are so close to their suns that any water on the surface would exist in a supercritical gaseous phase, which would enlarge their radius, which was not seen during the study. That suggests the water is not in the form of surface ocean.

Instead, the water could exist mixed into the rock or in pockets below the surface. Those conditions would be similar to Jupiter's.

The finding matches a theory of exoplanet formation that had fallen out of favor in the past few years, which suggested that many planets form farther out in their solar systems and migrate inward over time.

Though the evidence is compelling, other scientists would still like to see more evidence that one of these planets is a water world.

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