You-6894b: the planet far too big for its star which intrigues astronomers

The discovery of the planet Tou-6894b is so strange that it could call into question the planetary training models. In our system, for example, the sun acts as a colossal ogre and the planets, even giants like Jupiter or Saturn, are Lilliputians. Indeed, our star alone concentrates 99.86 % of the mass, while all of the planets counts only for 0.14 % of the whole system. And in terms of size, Jupiter is 11 times smaller than the sun.

To-6894b, a giant exoplanet that intrigues astronomers

Everywhere in the observable universe, these proportions are more or less respected. Astronomers have of course noted a few deviations, but never as much as around the Red Dwarf to 6894. This is only a fifth of the mass of the sun, but it is orbited in 3.36 days by a planet comparable to Saturn, or even a little larger, while being half less massive.

TOI-6894 is 238 light years from us and its type (red dwarf, therefore) constitutes between 60 and 75 % of the stellar population of the Milky Way. Nevertheless, only 1.5 % of these red dwarfs have a giant orbit planet, even less have seen the birth of a planet so disproportionately large than you-6894b. This kind of family is even so rare that the authors of this discovery have sought, in vain, other similar scenarios among 91,000 cases. “We don’t really understand how a star with so little mass can form such a massive planet”said Vincent Van Eylen from Mullard Space Science Laboratory of London, member of the team behind the ad.

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The problem is that the consensual models within the community of astrophysicists do not allow that a sparkling giant can be born with so little mass disparity with its parent star. And for good reason, a star appears by gravitational collapse of the majority of the gas and dust nebula, only a few remains forming the planets. Today there are two major scenarios for gaseous planets and none really sticks with our giant:

• The so -called accretion model by the nucleus: in the gas and dust disc around the young star, dust of heavy elements, like rocks and a little iron, agglomerate to form a nucleus. When the latter makes around 10 terrestrial masses, he begins to strongly attract the gas present around him.

In statistical studies on 70 protoplanetary discs around red dwarfs, only five contained in total more than 12 terrestrial masses in heavy elements. However, according to the calculations, the nucleus of Tou-6894b alone “weighs” 12 terrestrial masses! Without being impossible, it is therefore very improbable.

• There is another model called “by disk instability”. It is indeed possible that gravitational instability causes a collapse of matter at a point – this is also formed by the stars. This theory is interesting, because our giant planet has a very very large nucleus. Problem, there is a debate on the possibility that such an instability of disk occurs around a red dwarf, where the material is in small quantities.

A request for observation by the James-Webb space telescope was addressed to NASA, which will allow to analyze its atmosphere, therefore its composition, in order to lift the mystery hovering on this giant of the cosmos.