An ocean with a depth of 2000 km. Water at unimaginable pressures. The two exoplanets whose discovery by astronomers from the University of Montreal was announced Thursday are truly surprising.
“It’s a great discovery in exoastronomy”, explains Björn Benneke, astrophysicist at the University of Montreal, who is one of the authors of the study published Thursday in the journal Nature Astronomy. “This is the first time that we have two exoplanets where the only good explanation for their low density is a good fraction of their mass in water or low-density materials. »
Kepler-138 c and Kepler-138 d, which are located in the constellation of Lyra 218 light years from Earth, are slightly larger than Earth. Thanks to images from space telescopes Hubble and Spitzer, the Montreal researchers discovered that their density was such that these exoplanets must be composed of materials lighter than rock, but heavier than hydrogen or helium, which form the bulk of gas giants like Jupiter. “We’re talking about nitrogen, methane and water,” says Benneke. Water should be dominant. The ocean, composed mostly of water, should occupy a quarter to half the volume of the exoplanet.
In August, the same team had discovered another “oceanic” exoplanet, TOI-1452 b. “But it was just a more likely candidate than other oceanic exoplanets identified so far,” Benneke said. With Kepler-138 c and d, we are almost certain. »
Immense pressures
A 2000 km deep ocean implies immense pressures, which could transform the mixture of water, methane and nitrogen into a “supercritical” fluid, with properties mixing those of liquids and solids.
The two exoplanets described in Nature Astronomy have a diameter three times greater than the Earth, but a mass twice as great. “Most Earth-like exoplanets are rocky. Kepler-138 c and d are closer to the icy moons of Saturn and Jupiter Enceladus and Europa. »
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- 4km
- Average depth of Earth’s oceans
Source: University of Montreal