Exoplanète eau évaporation

More than 5,000 exoplanets have been discovered so far. And some are conspicuous by their absence: exoplanets with a radius about twice that of Earth are much less common than others. On either side of this value there are super-Earths, between 1 and 1.6 times larger than our planet, and mini-Neptunes, between 2.4 and 3.4 Earth radii. How can we explain this ‘hole’, also called ‘sub-Neptunian rift’? Thanks to new numerical simulations, Remo Burn, an astrophysicist at the Max Planck Institute in Heidelberg, Germany, and his colleagues have shown that this can be explained by the migration of planets covered by an icy ocean.

Planets are formed by accretion in the disk of gas and dust that surrounds a young star. The closer they are born to this protostar, the rockier and drier they are. The fault is the star’s radiation, which causes water to evaporate from the planets’ bodies. Radiation also affects their atmosphere: over time, the gas is torn away from the planet’s gravitational pull and dispersed into space. As they lose atmosphere, these planets therefore ‘shrink’ to the point that their radius is barely twice that of Earth. These are super-Earths, one of the most common types of exoplanets.

Another type of exoplanet often found in observations is that of mini-Neptunes. Because these worlds are not found in the solar system, specialists know very little about their structure and evolution, but they believe that these planets are covered with an atmosphere of hydrogen and helium that is more than twice as thick as Earth’s.

And between super-Earths and mini-Neptunes? Nothing. Or very little, as observations with space telescopes have shown Kepler, who hunted for exoplanets for almost a decade. “This telescope focused on certain parts of the sky that he had been studying for years. We therefore cannot imagine that there would have been any observational bias,” reports Remo Burn. The existence of the sub-Neptunian rift therefore certainly has a physical explanation. But which ? The preferred solution has long been that of radiation from the star ‘planing’ the atmospheres of the planets. But if it clearly explains the composition of the super-Earths that form near their star, it doesn’t explain how the same can be true for more distant planets.

Observed distribution (in blue) and simulated (in red) planets according to their radius. The drop to 2 Earth radii would be due to two effects. Icy planets that migrate toward their star see their water turn to vapor and grow larger. They accumulate about 2.4 Earth radii. Conversely, planets born close to their star gradually lose their atmosphere and become smaller. They accumulate about 1.4 Earth radii.

© R. Burn, C. Mordasini / MPIA

For Remo Burn, the solution lies in two parameters long left out of digital simulations: water and the migration of planets. First, if the accretion of the planets occurs beyond the ice line, that is, far enough from the star to leave water behind, then they are highly hydrated. “Water on Earth represents only 0.1% of its mass. We are talking about planets where half of the mass could consist of ice-cold water,” the researcher compares. Many clues then point to the fact that the planets do not stay where they are formed. So when these frozen planets migrate to their star, the ice melts, creating a thick atmosphere of water vapor. They then grow larger than super-Earths and reach sizes comparable to large mini-Neptunes. “So this predicts that these planets are not only covered with hydrogen and helium, as we thought, but also with a large portion of water,” adds Remo Burn.

The sub-Neptunian rift would therefore be the result of two different processes in the evolution of the planets: on the one hand, the process by which the breath of the star shrinks the super-Earths, and on the other hand, the process by which the evaporation of the ice makes the planets grow. migrants.

Exoplanets: an explanation for the absence of sub-Neptunes

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