So far, astronomers have not 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, is orbited by a larger planet. The mass of the giant gas is ten times the mass of Jupiter, making it one of the largest known exoplanets. Also, it is the first planet to have discovered such massive and extremely hot central stars. Until now it was not clear whether planets could also form around such stars.
Astronomers have already discovered several thousand extrasolar planets around many different stars. Typically, low-mass red dwarfs have smaller planets, while larger, more massive gas giants tend to cluster around larger, more massive stars. Planetary researchers attribute this to the fact that the size and density of the protoplanetary disk and thus the building material for future planets increases with the mass of the central star. Small stars do not provide enough raw material for the formation of very large planets. But the nearly linear mass relationship between stars and their planets only lasts up to a certain point: the curve is bent by a star mass of 1.9 solar masses. For stars greater than three solar masses, observations so far have shown that there are no more planets.
Gas giant around bright double star
The discovery by Markus Jansson of Stockholm University and his colleagues now refutes this notion. In their study, the astronomers investigated an abnormality in the double star B Centauri in the constellation Centaurus, about 525 light-years away. Both stars in this pair are only about 15 million years old and are significantly brighter, hotter, and more massive than the Sun. Their weight together is six to ten times the mass of the Sun. In 2019, telescopic images of this system showed three faint points of light in the vicinity of two stars whose origins could not be clearly determined. That’s why Jensen and his team have now once again targeted Centauri with the high-resolution SPEAR instrument on the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile.
Observations showed that two of the three points of light are background stars. However, one point turned out to be an exoplanet. It orbits one of the two central stars at a distance of 560 astronomical units – thus it is 560 times farther from the Sun than Earth and 100 times more distant from the Sun than Jupiter. This planet has one of the longest orbits ever observed. From the images, the researchers concluded that B Centauri (AB) b must have a mass of about 10.9 that of the baptized planet Jupiter. It is one of the few gas giants of this size and mass known to date.
riddle of origin
However, the biggest feature of the newly discovered planet is that it is also present in this system. “The discovery of a planet around B Centauri was very exciting because it completely changed the way we think about massive stars as planetary homes,” Jansson explains. Because no planet had ever been detected around such a massive and hot star before – and it was questionable whether planets could arise around such stars. Reason: Massive stars of these masses emit so much radiation immediately after their formation that they expand their protoplanetary disks faster than smaller, Sun-like stars. This leaves little time for the planets to grow through the gradual accumulation of dust and gas. “B-stars are generally considered to have quite destructive and dangerous environments. It was previously believed that it would be extremely difficult to form large planets around them,” explains Jensen.
But the discovery of B Centauri (AB) b has now proved that planets can arise 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 – there was little time for this, even in the dense disk region close to the star. Furthermore, there is no evidence that the planet was ejected from there into its extremely wide orbit. Instead, the gas giant could have been created directly from the gravitational collapse of the local region of its parent cloud. “Since this happens much faster than the approximately one million years required for accretion, this mechanism is less sensitive to the rapid disintegration of disks of matter around such massive stars,” the researchers explain. The double star’s strong stellar wind could then have prevented the young planet from moving closer to its parent stars. “This would explain why it spins so far,” the team said.
However, this is still speculation. “It would be a fascinating task to figure out how this could have become. At the moment it is still a mystery,” says Jensen. However, it seems clear that reality has once again overtaken common ideas about how and Which stars can have planets?
Source: Marcus Jansson (Stockholm University) et al., Nature, doi:10.1038/s41586-021-04124-8
