So far, astronomers have never discovered an exoplanet around a star with more than three solar masses – that has now changed. According to recent observations, the binary star b Centauri, 325 light-years away, orbits a large planet. The gas giant is ten times more massive than Jupiter, one of the largest known outer planets. At the same time, it was the first planet to discover such massive and very hot central stars. It is not yet clear whether planets will form even around such stars.
Astronomers have already discovered several thousand extrasolar planets orbiting various stars. In general, low-mass red dwarfs have smaller planets, while larger, massive gas giants cluster around larger, more massive stars. Planetary researchers say this is due to the fact that the size and density of the protoplanetary disk and the mass of the building material for future planets increase with the mass of the central star. Small stars do not provide enough raw material to form very large planets. But the almost linear mass relationship between stars and their planets lasts only to a certain point: the curve 1.9 tilts from the star with the mass of the Sun. For stars with more than three solar masses, observations to date show that there are not many planets.
The gas giant around the bright double star
The discovery of Marcus Johnson and his colleagues at the University of Stockholm now refutes this assumption. In their study, astronomers explored abnormalities in the binary star B Centauri, about 525 light-years away in the constellation Centaurus. The two stars in the pair are about 15 million years old and significantly brighter, hotter, and heavier than the Sun. All of these are six to ten times as heavy as the sun. In 2019, telescopic images of the system showed three dim light points in the vicinity of two stars, whose appearance could not be clearly detected. That’s why Johnson and his team are now re-targeting Centauri with the high-resolution SPHERE instrument at the European Southern Observatory (ESO)’s Largest Telescope (VLT).
Observations show that two of the three points of light are background stars. However, one point became an extraterrestrial. It orbits one of the two core stars at a distance of 560 astronomical units – 560 times farther from the Sun than Earth and 100 times farther from the Sun than Jupiter. This planet is one of the longest orbits ever observed. From the images, b Centauri (AB) b baptized planet Jupiter’s mass should be 10.9, the researchers conclude. It is one of the few gas giants of this size and mass known to date.
Puzzle about origin
However, the biggest uniqueness of the newly discovered planet is that it is even in this system. “The discovery of a planet around P Centauri was very exciting because it completely changed the way we think of massive stars as home to planets,” Johnson explains. This is because no planet has ever been found orbiting such a large and hot star system – and it was questionable whether even planets could form around such stars. The reason: Giant stars in these masses emit more radiation as soon as they form, scattering their protoplanetary disk faster than smaller, sun-like stars. This leaves very little time for the planets to grow by the gradual accumulation of dust and gas. “B stars are generally considered to be the most destructive and dangerous environment. It was previously believed that large planets would be very difficult to build around them,” Johnson explains.
But the discovery of b Centauri (AB) b now proves that planets can also form around such giant stars – but the question arises as to how. According to astronomers, it is unlikely that the Exoplanet was created by classical accretion – even in the smallest dense disk region closest to the star. In addition, there is no evidence that the planet was ejected from there into its vast orbit. Instead, the gas giant may have been created directly by the gravitational pull of the local area of its Adi cloud. “Because it takes place much faster than it takes to accumulate nearly a million years, this mechanism is less sensitive to the rapid disk disk of matter around such massive stars,” the researchers explain. The binary star’s strong stellar winds may have prevented the young planet from moving closer to its parent stars. “It explains why it hovers so far,” the panel said.
However, this is still speculative. “It’s a fascinating task to explore how it formed. At this point it is still a mystery,” Johnson said.
Source: Marcus Johnson (University of Stockholm) et al., Nature, doi: 10.1038 / s41586-021-04124-8
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