Exoplanets: Few sub-Neptunes around red dwarfs, apart from super-Earths
Most stars in the Milky Way are red dwarfs. It was difficult to find out which exoplanets exist there. Now there is an unexpected discovery.
Artistic representation of a sub-Neptune in front of a sun-like star
(Image: NASA, ESA, CSA, Dani Player (STScI))
The most common type of planet around sun-like stars is almost entirely absent around the by far most common class of stars in the Milky Way. This is the result of an analysis of the exoplanets discovered so far, which has now been presented. While sun-like stars are predominantly orbited by celestial bodies similar to Neptune but smaller, as well as rocky planets larger than Earth, the former are almost entirely absent around red dwarf stars. There, so-called super-Earths are the dominant form of exoplanets. This suggests that planet formation around these two types of stars differs fundamentally.
Super-Earths favored around red dwarfs?
As the research group responsible for the analysis explains, so-called sub-Neptunes and super-Earths have dominated the lists of discovered exoplanets for almost a decade. However, this was also because the search for companions of red dwarf stars was made more difficult by their low brightness. It was only the TESS (Transiting Exoplanet Survey Satellite) space telescope that changed this, providing extensive insights into the population of exoplanets around this by far most common class of stars. The analysis of the data now available has revealed that there are almost no sub-Neptunes there. This is a discovery “that calls into question existing theories of planet formation”.
It is currently assumed that the difference between sub-Neptunes and super-Earths is due to a process in which intense radiation from a star literally sweeps away a planet's atmosphere, explains the team. Red dwarf stars are active enough for this, but sub-Neptunes should not disappear completely there. Since this is the case, it is plausible that planets with a lot of water are favored over those with a gas-rich atmosphere during planet formation. This now needs to be further investigated.
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“It was already astonishing to learn that the most common planets in our galaxy do not exist within our own solar system,” recalls lead researcher Ryan Cloutier, referring to sub-Neptunes and super-Earths. For a long time, the solar system was the only example for researching planets, but now there is a much more extensive database, and this is now forcing us to rewrite assumptions: “If we want to understand the origins of planets and the origins of life, we need a complete picture of how planets form and what they're made of. This research brings us closer to that goal.” The analysis was published in The Astronomical Journal.
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