Exoplanets: An explanation for the absence of sub-Neptune planets

Exoplanets: An explanation for the absence of sub-Neptune planets

To date, more than 5,000 exoplanets have been discovered. Some are conspicuous by their absence: exoplanets with radii twice Earth's are much less common than others. On either side of this value, super-Earths, which range from 1 to 1.6 times larger in size than our planet, and mini-Neptunes, whose diameter ranges from 2.4 to 3.4 times the diameter of Earth, abound. How can we explain this “hole”, also called the “Sub-Neptunian Rift”? Thanks to new numerical simulations, Remo Bern, 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 form by accretion into the disk of gas and dust surrounding a young star. The closer they get to this protostar, the more rocky and drier they become. The defect is the star's radiation, which causes water to evaporate from planetary bodies. Radiation also has an effect on the atmosphere: over time, the gas breaks off from the planet's gravity and spreads out into space. Thus, as they lose atmosphere, these planets “shrink”, to the point that their radius is barely twice that of Earth. These are giant planets, one of the most common classes of exoplanets.

Another type of exoplanet often encountered in observations is the minor planet Neptune. Since these worlds do not exist in the solar system, specialists know little about their structure and evolution, but they believe that these planets are covered by an atmosphere of hydrogen and helium, more than twice as thick as Earth.

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And between the super-Earths and minor Neptune? nothing. Or very little, as space telescope observations have shown Kepler, Which hunted exoplanets for about ten years. “This telescope focused on certain parts of the sky that it had been studying for years. Therefore, we cannot imagine that there could have been any bias in the observation.” The existence of the rift under Neptune certainly has a physical explanation. But which? It has been the preferred solution since Long is the star's radiation that “flattens” the planetary atmosphere.But if this clearly explains the formation of super-Earths that form near their stars, it does not explain how the same could be true of distant planets.

Observed distribution (in blue) And simulation (in red) Planets according to their radius. The decrease would be about 2 Earth radii due to two effects. Icy planets that migrate toward their star see their water turn into vapor and increase in size. It accumulates about 2.4 Earth radii. Conversely, planets born close to their star gradually lose their atmosphere and decrease in size. It accumulates about 1.4 radii of Earth.

© R. Burn, C. Mordasini / MPIA

For Remo Burn, the solution lies in two parameters that have long been omitted in numerical simulations: water and planetary migration. First, if planetary accretion occurs outside the ice line, far enough from the star for water to remain, it is strongly wet. “Water on Earth represents only 0.1% of its mass. Here we are talking about planets that could have half their mass composed of water ice.” Hence, a lot of evidence tends to show that planets do not stay where they were formed. So, if these frozen planets migrate Toward their star, the ice melts to create a thick atmosphere of water vapor. They then become larger than super-Earths, reaching sizes similar to the large mini-Neptunes. “This therefore predicts that these planets are not covered only by hydrogen and helium as we thought,” adds Remo Byrne. “But also a large portion of water.”

The rift under Neptune would therefore be the result of two different processes in planetary evolution: that of the star's breath reducing super-Earths on the one hand, and that of the evaporation of ice making the planets grow migratory.

Exoplanets: An explanation for the absence of sub-Neptune planets

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