Discovery of a huge exoplanet around the most massive star system known to date

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The Very Large Telescope at the European Southern Observatory has captured images of a gigantic planet orbiting the binary system b Centauri. However, the latter is a particularly hot and massive system (6 to 10 solar masses according to experts). Scientists previously thought that it was unlikely that a giant planet could form around such massive stars. This discovery thus questions some of the theories about the formation of planets.

Previous studies of planets in close orbit around high-mass stars have found that the frequency of giant planets tends to increase with the mass of the star up to a certain point (up to 1.9 solar masses), beyond which their frequency decreases. quickly. This suggested that planet formation around more massive stars was a priori more difficult, if not impossible, and that giant planets around stars of more than 3 solar masses were therefore rare or non-existent.

This belief is now in doubt: Astronomers provide evidence that planets can orbit much more massive star systems than theoretical models predicted. The planet in question here, called b Centauri b, is located at a distance from its star equivalent to 560 times the Earth-Sun distance (or 560 AU). It is 10 times more massive than Jupiter. The planet / star mass ratio, estimated at 0.10-0.17%, is similar to the Jupiter / Sun ratio, but the distance between b Centauri b and its star is approximately 100 times greater than that between Jupiter and the Sun.

The most massive star system ever discovered

Planet b Centauri b is 325 light years from Earth, in the constellation Centauri. This is the first time that the observation of a planet around such a massive star system has been confirmed. “The discovery of a planet around b Centauri is very exciting, because it completely changes the picture of massive stars as planet hosts,” says Markus Janson, an astronomer at Stockholm University and co-author of the study linking the discovery.

Image of planet b Centauri b, near the star system b Centauri, captured by the SPHERE instrument of the Very Large Telescope. © ESO / Janson et al.

“We’ve always had a very solar system-centric view of what planetary systems are ‘supposed’ to be like. During the last ten years, the discovery of many planetary systems in surprising and unprecedented configurations has made us broaden our historically narrow view, “said Matthias Samland, a researcher at the Max Planck Institute for Astronomy and a co-author of the study. The image was taken by the SPHERE instrument (Spectro-Polarimetric High-contrast Exoplanet REsearch) installed in the Very Large Telescope, using a coronagraph, which allowed to block the intense light emitted by the binary system to better distinguish the planet. SPHERE has already successfully imaged several planets orbiting stars other than the Sun, ESO said in a statement.

The B Centauri binary system is the most massive star system ever discovered to date. Most massive stars are also very hot, and this system is no exception: its main star is the so-called B-type star, which is more than three times hotter than the Sun. Due to its high temperature (of the order 10,000 to 30,000 K), these stars, called blue giants, emit intense ultraviolet rays and X-rays. However, these high-energy radiations cause more rapid evaporation of light. the process of formation of nearby planets. “Type B stars are generally considered destructive environments,” confirms Janson.

A training process that remains a mystery

However, the discovery of b Centauri b shows that planets can form successfully even under such difficult conditions. The extremely large orbit followed by this planet (560 AU), one of the largest detected so far, could explain how it was able to survive radiation from its star.

Another distinguishing property of this new exoplanet is the relative youth of its star system, each of the two stars of which is 15 million years old (compared to our Sun’s 4.6 billion years). According to the researchers, it is unlikely that b Centauri b was formed in situ by the conventional heart accretion mechanism, a model of planet formation in which solid particles from the star’s protoplanetary disk aggregate to form a solid body, big enough to have a gravitational pull.

In contrast, they suggest that it may have formed elsewhere and then ended up in its current location through dynamic interactions, or it may have formed through “gravitational instability,” a model that assumes that ‘part of the protoplanetary disk collapses in on itself. himself under the effect of his own gravity. “When this happens, it creates a small secondary body and begins to orbit the star,” says Kaitlin Kratter of the Steward Observatory at the University of Arizona.

Thanks to ESO’s Extremely Large Telescope, due to open in 2025, and new technical advances ahead, astronomers will certainly be able to learn more about the formation and characteristics of this planet. “It will be an intriguing task trying to understand how it could have been formed, which is a mystery at the moment,” concludes Janson.

M. Janson et al.

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