Two distant planets could be filled with water

An artist's rendering of the Kepler-138 system with the terrestrial water world Kepler-138d in the foreground.

An artist’s rendering of the Kepler-138 system with the terrestrial water world Kepler-138d in the foreground.

Astronomers using the Hubble Space Telescope have observed two exoplanets that could be examples of a long hypothetical planetary classification: a water world.

Known as Kepler-138c and Kepler-138d, the planets are located in a star system 218 light-years distant, according to a press release from the European Space Agency. The team, led by Caroline Piaulet of the Institute for Exoplanet Research at the University of Montreal, used data from Hubble and NASA’s Spitzer Space Telescope to determine that the worlds are mostly made of water, making them even wetter than our notoriously wet home planet . Their results were published in Nature Astronomy.

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“We used to think that planets slightly larger than Earth were big spheres of metal and rock, like enlarged versions of Earth, and that’s why we called them super-Earths,” said Björn Benneke, one of the study’s authors . in the ESA publication. “However, we have now shown that these two planets, Kepler-138c and d, are quite different in nature and that a large proportion of their total volume is likely to be water.”

Kepler-138c and Kepler-138d are estimated to be about three times the size of Earth and twice the mass of our own planet, but at much lower densities.

The research team hasn’t observed water on the planets directly, but by comparing the observational data to existing models, they found that they should be made of materials with densities between the gases hydrogen and helium and rock – and the likely candidate is water. With that in mind, researchers believe these planets may have impossibly hot atmospheres, meaning their surfaces may not be teeming with vast liquid oceans.

“The temperature in Kepler-138d’s atmosphere is probably above the boiling point of water, and we expect this planet to have a thick, dense atmosphere of steam,” Piaulet said. “Only under this vapor atmosphere could there possibly be liquid water at high pressure, or even water in another phase that occurs at high pressure, a so-called supercritical fluid.”

Some of the first direct evidence for the existence of aquatic worlds was found in 2012, but subsequent observations have been hard to come by. A 2017 study suggested that these aquatic worlds could make up a large percentage of potentially habitable exoplanets. Other research in 2019 found that exoplanets larger than Earth – like the Kepler 138 planets – could likely be water worlds with incredibly deep oceans.

While the latest results are exciting, the research team looks forward to follow-up work with the Webb Space Telescope to further study the atmospheres of Kepler-138c and d.

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