Planets: An unexpected gas giant is orbiting a massive pair of stars

We thought that planets couldn’t form around stars with more than three times the mass of the sun, but planet b Cen (AB)b challenges the idea


8 December 2021

most massive planet-hosting star pair to date

The twin stars of the most massive planet-hosting star system to date, b Centauri, and its giant planet b Cen (AB)b

ESO/Janson et al

A gas giant planet orbiting two stars with a combined mass of at least six times that of our sun is the largest planet-hosting star system ever detected, and may challenge our currents ideas of planetary formation.

Previous studies of planets in close orbit to high-mass stars have suggested that planets orbiting stars of more than three times the mass of the sun may be rare or even non-existent. This is because higher-mass stars emit larger amounts of radiation, which should cause the dense discs of material like gas and dust around such stars to evaporate before they can coalesce into planets.

However, Markus Janson at Stockholm University in Sweden wondered whether giant planets would still form around massive stars as long as they orbited at a great enough distance.

Now, Janson and his colleagues have found such a planet: a gas giant orbiting a young binary star system called b Centauri (b Cen) that is between six and 10 times the mass of our sun.

The researchers first directly imaged the system in March 2019 using the Very Large Telescope in Chile and then conducted follow-up observations in April 2021. They found that the planet, known as b Cen (AB)b, is 10.9 times the mass of Jupiter and orbits the two stars at 560 times the distance between Earth and the sun.

“We know for the first time that planets really can exist about really quite massive stars,” says Janson.

The researchers suggest that the planet must have formed relatively rapidly through gravitational instability, which is when massive clumps of gas and dust cool and contract into a planet. This method is much faster than the traditional core accretion model, which is when solid particles collide and slowly snowball into a planet. Even at the distance it lies from the stars, the disc of material this planet formed from would have been likely to evaporate quickly.

They also found the planet to have a reasonably circular orbit. This suggests that b Cen (AB)b was formed close to its current orbit, because planets that have been knocked off their original orbits typically follow an elliptical path around their star.

These are important insights for our limited understanding of planet formation around high-mass stars.

“This is a major breakthrough because it establishes that finding wide-separation exoplanets around a massive host star is possible,“ says Meiji Nguyen at the University of California, Berkeley. “Though I don’t think the study can conclusively prove any of the leading theories we have on how exoplanets like this form, it does provide an exciting new piece of evidence to support some of our current understanding.”

Journal reference: Nature, DOI: 10.1038/s41586-021-04124-8

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