The larger the star, the normally larger the average radius of its planets. Until recently, the reasons for this were not entirely clear. Now astronomers have shown that the situation cannot be explained only by the fact that large stars have more massive planets: as it turned out, a completely different factor plays an important role.
Of the 4434 candidates for exoplanets, the main part was found in red dwarfs - stars of small mass and size. Planets around more massive stars - orange dwarfs like Alpha Centauri B or yellow dwarfs like our Sun - are larger on average. In a new paper, forthcoming in Astronomy & Astrophysics, an international team of researchers has shown the reasons for this pattern. It turned out to be not only the mass, but also the other chemical composition of the planets in more massive stars. The text of the corresponding article can be found on the arXiv.org preprint server.
The real reasons why the radius of the planets in more massive stars is larger than that of less massive stars may be different. So far, the scientific literature has offered three possible explanations. First, larger stars have a higher luminosity - heating from their rays could "inflate" the gas envelopes of their planets due to thermal expansion. Second, the planets of more massive stars were formed from a larger protoplanetary disk and could have a higher average mass. Thirdly, exoplanets in massive stars could have a higher content of light gases - and, therefore, a lower average density and larger sizes, even with an ordinary, average mass.
The authors of the work decided to check all three versions. To do this, they made three predictions, the right or wrong of which corresponded to each of the above options. If the first explanation - about the expansion of atmospheres from heating - is correct, then the calculated temperature of the planets in more massive stars should be the greater, the larger the radius of such a planet (since it "swells" from heating). If the second explanation is correct - that is, around massive stars and planets are formed more massive - then the higher the mass of the exoplanet, the larger its radius should be. Moreover, such a relationship should be relatively linear and predictable. Checking the correctness of the third explanation is the most difficult of all, because it is difficult to accurately determine the composition of an exoplanet. However, if it has a very large radius at a moderate mass and calculated temperature, then it is clear that it contains more light elements than planets in less massive stars.
With the help of calculations, the researchers were able to show that the planets around less massive stars have a lower proportion of light elements - especially helium and hydrogen. Only this can explain the radii observed in them at a fixed mass. The mass of exoplanets can be determined by the radial velocity method. In their analysis, astronomers this time focused on the planets of orange and yellow dwarfs, since the planets of red dwarfs know much more and there the analysis would be more laborious. Regardless, they note that the findings clearly extend to exoplanets in red dwarf systems. That is why, according to scientists, there is a much higher proportion of terrestrial planets and a noticeably smaller number of giant planets with a high content of gases - such as Jupiter or Saturn in the solar system. From this it follows indirectly that the chances of the emergence of terrestrial life in less massive stars may be higher than in more massive ones like our Sun.
As the researchers noted, the pattern they discovered does not apply to stars more massive than yellow dwarfs - for example, to white-yellow luminaries of spectral class F. hydrogen and helium) in a relatively short time.
It is not yet completely clear why the planets of more massive stars contain more hydrogen and helium - the details of the process of planet formation are still not clear to astronomers. But it is known that in the solar system, giant planets with hydrogen and helium in the atmosphere were the first to form, and only then did the terrestrial planets - Mercury, Venus, Earth and Mars. At the same time, the influence of massive Jupiter, close to Mars, deprived the Red Planet of the opportunity to have a significant mass and, as a result, a dense atmosphere and high vitality.