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, is orbited by a large planet. The gas giant has a mass ten times that of Jupiter, making it one of the largest known exoplanets. At the same time, it is the first planet that such massive and extremely hot central stars were discovered. Until now it was unclear whether planets could even form around such stars.
Astronomers have already discovered several thousand extrasolar planets around a multitude of different stars. Typically, low-mass red dwarfs tend to have small planets, while large, 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 simply do not offer enough raw material for the formation of very large planets. But the almost linear mass relationship between the stars and their planets only lasts up to a certain point: the curve tilts from a star mass of 1.9 solar masses. For stars with more than three solar masses, observations to date have shown that there are no more planets at all.
Gas giant around bright double star
The discovery by Markus Janson of Stockholm University and his colleagues now refutes this assumption. 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 around 15 million years old and significantly brighter, hotter and more massive than the sun. Together they weigh six to ten times the mass of the sun. In 2019, telescope images of this system showed three faint points of light in the vicinity of the two stars, the origin of which could not be clearly determined. That is why Janson and his team have now once again targeted Centauri with the high-resolution SPHERE instrument on the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile.
The observations showed that two of the three points of light are background stars. One point, however, turned out to be an exoplanet. It orbits one of the two central stars at a distance of 560 astronomical units – it is thus 560 times further away from them than the earth from the sun and 100 times further than the Jupiter from the sun. This planet has one of the longest orbits that has ever been observed. From the images, the researchers conclude that the b Centauri (AB) b baptized planet must be around 10.9 masses of Jupiter. It is one of only a handful of gas giants of this size and mass known to date.
Riddle of the origin
The greatest peculiarity of the newly discovered planet, however, is the fact that it even exists 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 the home of planets,” explains Janson. Because never before has a planet been detected around such a massive and hot star system – and it was questionable whether planets could even arise around such stars. The reason: Giant stars of these masses emit so much radiation immediately after their formation that they disperse their protoplanetary disk faster than smaller, sun-like stars. This leaves far less time for planets to grow through the gradual accumulation of dust and gas. “B-stars are generally considered to be quite destructive and dangerous environments. It was previously believed that large planets would have to be extremely difficult to form around them, ”explains Janson.
But the discovery of b Centauri (AB) b now proves that planets can also arise around such giant stars – but the question arises as to how. According to the astronomers, it is rather unlikely that the exoplanet was formed by classical accretion – there would have been too little time for this even in the denser disk region close to the star. In addition, 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 by the gravitational collapse of a local area of its primordial cloud. “Since this takes place much faster than the approximately one million years required for accretion, this mechanism is less sensitive to the rapid dissolution of the 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 drifting closer to its parent stars. “That would explain why it circles so far out,” said the team.
However, this is still speculation. “It will be a fascinating task to explore how it could have formed. That’s still a mystery at the moment, ”says Janson. It seems clear, however, that reality has once again overtaken the common ideas about how and around which stars planets can arise.
Source: Markus Janson (Stockholm University) et al., Nature, doi: 10.1038 / s41586-021-04124-8