These two planets are made of water, NASA says


Two distant planets are likely made of water, according to studies conducted with NASA’s Hubble and Spitzer telescopes.

The so-called “water worlds” orbit a red dwarf star, the smallest and coolest type of star, according to a press release from NASA’s Jet Propulsion Laboratory.

The water planets are 218 light-years away in the constellation of Lyra, NASA says. And they are “unlike any planets found in our solar system,” the agency said.

The finding that the planets are likely composed mostly of water comes from a study published Thursday in the journal Nature Astronomy. A research team led by Caroline Piaulet, a graduate student at the University of Montreal’s Institute for Exoplanet Research, used NASA’s Hubble and Spitzer Space Telescopes to observe the distant planetary system.

The planetary system they studied is called Kepler-138 because it is in the field of view of NASA’s Kepler spacecraft. Researchers were aware of the existence of three exoplanets — the term for planets outside our solar system — within the Kepler-138 system, but have only just discovered that two of them are likely made of water.

They also discovered evidence of a fourth planet that had not previously been described.

But the determination is not as simple as it might seem. The scientists could not directly detect water on the exoplanets Kepler-138c and Kepler-138d. Instead, they compared the size and mass of the planets to reference models.

When they compared the planets to the models, they found “that a significant portion of their volume – up to half – should be made up of materials lighter than rock but heavier than hydrogen or helium.”

Water is the likeliest candidate for a material that’s lighter than rock but heavier than hydrogen or helium, NASA says.

This image shows cross-sections of Earth and the exoplanet Kepler-138d.  Kepler-138d density measurements suggest it may have a water layer that accounts for more than 50% of its volume down to a depth of about 1,243 miles (2,000 kilometers).

“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, co-author of the study and Professor of Astrophysics at the University of Montreal, in the 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.”

“It’s the best evidence yet for water worlds, a type of planet that astronomers have long suspected existed,” Benneke continued.

The high temperatures on these planets mean they could be surrounded by a steamy atmosphere, according to NASA.

“The temperature in Kepler-138d’s atmosphere is likely above the boiling point of water, and we expect a thick, dense atmosphere of steam on this planet,” Piaulet said in the press release. “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.”

The “water worlds” are outside of their star’s habitable zone — the range where temperatures allow liquid water on the surface of a rocky planet and potentially support life. But the new planet the researchers described, Kepler 138-e, falls in just that zone, according to the publication.

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