Surprise! Exoplanets can also form around massive stars

Regret André Brahic used to say that exploring the Solar system carried out with probes like those of the Voyager missions in the first place had shown its extraordinary diversity and had brought its share of surprises. It is clearly the same with the exoplanets and just remember the reaction of astrophysicists during the serial discoveries of Hot jupiters at the start of our exploration of these worlds. It was then necessary to bring in the processes of planetary migrations, the importance of which we did not yet imagine in the cosmogony.

Today, a new surprise comes from the observations of an international team ofastronomers having used the Sphere instrument (Spectro-Polarimetric High-contrast Exoplanet REsearch) installed on the Very Large Telescope of the European Southern Observatory (the VLT of the’Eso) in Chile.

A binary system with a massive B-type star

It allowed them to zoom in on the star b Centauri located at approximately 325 light years of Sun in the Milky Way and that we can see at theeye naked in the constellation of the Centaur. It is therefore not a question of stars of the triple system Alpha Centauri which is only about 4.3 light years away. Nevertheless, b Centauri, also known as HIP 71865 as an Eso release explains, is itself a double star.

It’s clear that b Centauri is a member of the catalog Hipparcos (HIP) which is a catalog ofstars resulting from Esa’s Hipparcos astrometric space mission preceding the mission Gaia between November 1989 and March 1993 and which made it possible to obtain parallax of 118,218 nearby stars with an accuracy of one arc millisecond.

With regard to the denomination of b Centauri, it follows the rules introduced by the astronomer Johann Bayer (1572-1625) in his Uranometria, the first atlas covering the entire celestial sphere. They essentially consist of taking a Greek letter followed by the genitive of the name of the constellation where the star is located.

But why b Centauri is it in the spotlight today to the point of being the subject of a publication in the newspaper Nature ?

A beginning of explanation is given by Markus Janson, astronomer at the University of Stockholm (Sweden) and first author of the article: “ The discovery of a planet around b Centauri is very exciting because it completely changes the image of massive stars as hosts of planets. “.

Indeed, the catalog of exoplanets has just been enriched with b Centauri b and its existence is unexpected because b Centauri contains about 6 solar masses with in particular a star of type B, more massive than the Sun therefore; however, it was expected that the radiation of this kind of binary system massive inhibits the formation of exoplanets.

” Type B stars are generally considered to be quite destructive and dangerous environments, and it was believed that it would be extremely difficult to form large planets around them. “, Explains Markus Janson in a press release from Eso accompanying the publication in Nature.

In fact, until now, no planet has been spotted around a star more than three times as massive as the Sun. Corn b Centauri b is indeed real although at a distance equivalent to 100 times that of Jupiter from the Sun in orbit around b Centauri.

An exoplanet with 10 times the mass of Jupiter

Markus Janson’s colleague and co-author, Gayathri Viswanath, doctoral student at Stockholm University, also explains in the Eso press release that “ the planet of b Centauri is an alien world in a completely different environment from what we experience here on Earth and in our Solar System. It’s a harsh environment, dominated by extreme radiation, where everything is on a gigantic scale: the stars are bigger, the planet is bigger, the distances are greater. “.

The conjunction of the theory of stellar structure, which describes the properties of stars and the origin of these properties, combined with that of the formation of planetary systems, leads to believe that stars of at least several solar masses must blow much faster than did the young Sun or young stars less massive than him – the gas and the dust of protoplanetary disc surrounding them initially – so that the formation of exoplanets should be very difficult, if not impossible.

We know that radiation can exert a pressure on matter, we can see it in part with the comets but above all it is he who opposes the gravitational contraction of the stars on the main sequence.

Now, as has been said, the main star of b Centauri is a so-called type B star, more than three times hotter than the Sun. We can easily calculate with this temperature that it must emit large amounts of radiation ultraviolet and of X-rays which should have caused a rapid evaporation of a protoplanetary disc associated with this double star.

Cosmogonic models are for the moment silent on the existence of b Centauri b who is one of exoplanets most massive ever discovered with more than 10 times the mass of Jupiter. At the most, we can notice that being also one of the exoplanets in one of the largest orbits ever discovered, this distance from the main star could have contributed to its formation.

We may know more in a few years when the commissioning of theELT (Extremely Large Telescope) will allow you to zoom in again on b Centauri.