Stern evaporates nearby planets

For a long time astronomers were amazed: why are there no medium-sized planets near stars? Researchers at the University of Geneva have found evidence for an answer: such a planet quickly loses ground through its central star.

If sport fishermen only catch big and small, but no medium-sized fish, they would be surprised. Astronomers face a similar riddle, as the University of Geneva wrote in a statement on Thursday.

Planets that are close to their star and are therefore called "hot" seem to be barely average among the previously known exoplanets, so about the size of the planet Neptune. Larger "hot Jupiter" and smaller "hot super herds" are plentiful.

The researchers around Vincent Bourrier from the University of Geneva are now on the trail of a possible explanation for the lack of "hot Neptune". In the magazine "Astronomy & Astrophyics" they report on a "warm Neptune", which is a bit further away from its central star. Because of the radiation of his star, this planet quickly loses hydrogen from its atmosphere. The researchers came to this conclusion with observations from the Hubble Space Telescope.

This planet, called GJ 3470b, is 97 light years away from the earth and not the first of its kind: a few years ago the same research team discovered an evaporating "warm Neptune". So a planet in Neptungrösse on the edge of the zone, in the "Neptune" does not seem to exist, and is also lost by hydrogen.

Both planets are about 3.7 million kilometers away from their stars – less than a tenth of the distance from the sun closest to the planet Mercury to the sun. GJ 3470b, however, is a much younger and more energetic star than the other "warm Neptune". As a result, GJ3470b loses about 100 times faster hydrogen in its previously described counterparts.

"This is the first time a planet is observed that quickly loses its atmosphere that it can affect evolution," said Bourrier, according to a statement from the University of Geneva. He and his colleagues suspect that the GJ 3470b has lost more than a third of its mass.

The discovery of the evaporating "warm neptune" supports the theory that the "hot neptune" – which is closer to the stars – shrank or even completely eroded until only their planetary nuclei remained out of rocks.

"That could explain why a large number of super-earths was discovered," said study author David Ehrenreich of the University of Geneva. Researchers refer to planets that are slightly larger than the earth, but whose diameter does not exceed 1.5 times the diameter of the earth.

However, to further test this theory of evaporated Neptune, it is necessary to study other planets of this size as close as possible to their central star, wrote the University of Geneva. The observation of hydrogen, however, only works at a distance of 150 light-years from the earth, because interstellar gases, moreover, interfere with the analysis.

The observation of helium in the atmosphere of exoplanets can be remedied with the successor of Hubble, the James Webb Space Telescope: infrared radiation from helium permeates the interstellar medium. To expand the search field, Bourrier said.

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