(ORDO NEWS) — Astronomers at the University of Michigan and the University of Chicago have predicted the existence of helium planets resulting from the evolution of sub-Neptunes, giant exoplanets smaller than Neptune.
It is known that planets with a radius smaller than that of Neptune, orbiting their parent stars in less than a hundred years, occur 10 times more often than planets that are larger than Neptune.
However, scientists almost never meet planets with a radius of 1.5 to 2 Earth radii, but at the same time they observe many objects with radii of about 1.3 and 2.4 Earth radii.
Such a bimodal size distribution of sub-Neptunes has long been a mystery to astronomers, since the nature of the valley between the two peak radii of this class of exoplanets was not understood.
Almost all of the few exoplanets located in the so-called valley of radii have a low average density, which is explained by the presence of bulk gaseous shells with high mass fractions of hydrogen and helium.
According to one scenario, these shells are exposed to ionizing radiation from the parent stars and thermal energy released by the cores.
At the same time, the gravitational pull of the planet is too small to prevent the loss of mass, so they end up with bare rocky cores.
An alternative hypothesis suggests that large water worlds that originally formed on the snowline (regions where water exists in the form of ice) migrate closer to the star.
This creates a second peak, while the first peak is formed by rocky planets that formed near the parent star.
The scientists ran simulations of 70,000 sub-Neptunes, varying their sizes, types of parent stars, and atmospheric temperatures to assess how internal heat and radiation affects the gaseous envelopes.
It turned out that the scenario of evaporation of gas shells is being implemented, while the planets lose more hydrogen than helium.
Thus, sub-Neptunes with a radius of 1.6-2.5 Earth radii should have atmospheres that are 40 percent helium.
If this theory is correct, planets with a helium-rich atmosphere should be especially common among the second peak planets, which have sizes close to the upper limit of the valley of radii.
Helium builds up as the sub-Neptune loses its atmosphere, shrinking in size. This can be confirmed by future observations, which will be carried out using the James Webb telescope.
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