Can planets form around hot stars? Some people think they can. The planets around stars should be rare or non-existent according to the evidence. Astronomers have found one.
A team of researchers found a star that is six times the mass of the Sun. It has a planet that is ten times the size of Jupiter.
The star is about as far away from Earth as possible. The stars and planet are about 10 times and 11 times larger than our Sun and Jupiter, respectively.
The paper describes a giant planet in the high-mass b Centauri system. The lead author is an astronomer at the University of Sweden.
Astronomers have found a wide variety of solar system architectures. The detail of how planets form around different stars is an important area of research. To understand the planet formation process more thoroughly, you need to look at the process through the entire range of stellar and planetary mass. The extremes are important.
Astronomers have studied planets that are very close to high-mass stars and found that the number of giant planets increases with the mass of the stars hosting them. Only until a point. The frequencies of giant planets plummet when they are at 1.9 stellar mass. It is implied that giant planets should be rare around high-mass stars.
Astronomers have found one.
Janson said that finding a planet around b Centauri changed the picture about massive stars hosting planets.
The Sun is at least six times the size of Centauri. It is the most massive stellar system to host planets. Astronomers haven't found any planets around a star of three solar mass before.
b Centauri b is 100 times more distant from its star than Jupiter is from the Sun, and detection methods that are sensitive to close-in stars aren't as sensitive to exoplanets on a wider path. It is more than the Sun-Earth distance. The separation of the detected planet is 100 times larger than that of Jupiter, according to the authors.
The planet-to-star mass ratio is shown in this figure. All of the circles are known for their exoplanet to star mass ratios. It shows the planets in our solar system. b Centauri b has a low mass ratio to the central system relative to other planets in the more comprehensive, directly imaged population. Janson et al 2021.
b Centauri is three times hotter than the Sun. Powerful radiation is emitted in UV and X-rays. The gas surrounding the star is forced to evaporate by the energy. B-type stars are considered to be quite destructive and dangerous. It was thought that it would be difficult to form large planets around them.
The team first spotted b Centauri b as a faint point source. It was one of the three that they found. A faint point source like this one is either a distant star or a planet. Other observations will show a proper motion with the star if it is a planet. Astronomers can conclude that it is bound to its star. The follow-up observation of b Cen was scheduled in 2021. They found other archival observations of the system showing the planet as a point source, which were ignored in the original studies.
The data gathered by the team confirmed that the target shared a common proper motion with b Centauri. There is clear evidence for the motion of the central stellar mass.
This is an image of the massive hot star b Centauri and the exoplanet b Centauri b. Background stars are marked as point sources. The b Centauri has auddy appearance due to residual noise. Janson et al 2021.
We are still in the early days of discovery. Inevitably, the Solar System was held as a kind of norm since there was nothing to compare it to. There are a lot of different solar system architectures out there. The discovery adds to the diversity.
The MPIA scientist and co-author says that they have always had a solar system-centered view of what planetary systems are supposed to look like. Over the last ten years, the discovery of many planetary systems in surprising and novel configurations has made us widen our historically narrow view. This discovery adds another chapter to the story.
The secret to its existence might be its distance from its star. It is the widest planetary circle ever observed. The b Centauri could not drive the gas away.
For now, we don't know how this planet formed. Did it form out there? Did it move?
It will be an interesting task to figure out how it formed, which is a mystery at the moment.
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The lead author is Markus Janson.
The core-accretion process is unlikely to have formed b Centauri b. The star likely prevented that. It may have started elsewhere and moved to its current location. The authors say that a giant planet could form from the circumstellar gas disc.
The team briefly discussed the formation of b Centauri b. They say that it might be an important mechanism in the context of massive stars. The planet would have formed more quickly in that process than in core accretion, which would take about 10 6 years. The instability mechanism is less sensitive to the rapid dispersion of discs around massive stars.
The team points to a scenario in which the star's outward pressure prevented it from moving closer. Theoretical models predict that discs around more massive stars are more likely to fragment as a result of more vigorous mass accretion.
They support the idea that b Cen is a disc instability planet.
