“This is a great discovery in exoastronomy,” explains University of Montreal astronomer Bion Benneke, who is one of the authors of the study published in the journal Thursday. Natural Astronomy. “This is the first time we have two exoplanets, and a good explanation for their low density is that a good fraction of their mass is in water or less dense matter. »

Kepler-138c and Kepler-138d, located 218 light-years from Earth in the constellation Lyra, are slightly larger than Earth. Thanks to the pictures from the space telescope Hubble etc Spitzer, the Montreal researchers found, their density means these exoplanets must be composed of material lighter than rock but heavier than the hydrogen or helium that make up most of gas giants like Jupiter. “We’re talking about nitrogen, methane and water,” says Benneke. Water should dominate. A mostly watery ocean should cover a quarter to a half of the exoplanet’s mass.

In August, the same team discovered another “ocean” exoplanet, TOI-1452 b. “But it was just a more likely candidate than the other ocean planets discovered so far,” Bennecke said. With Kepler-138 c and d we are almost certain. »

Great pressures

A 2,000-kilometer-deep ocean implies enormous pressure that can turn a mixture of water, methane, and nitrogen into a “supercritical” fluid, and has the properties of mixing liquids and solids.

Two exoplanets have been described Natural Astronomy Three times the diameter of Earth, but twice the mass. “Most Earth-like planets are rocky. Kepler-138c and D are close to the icy moons of Saturn, Jupiter Enceladus and Europa. »