Can planets form around massive, hot stars? Some astronomers think they can’t. According to the evidence, planets around stars exceeding three solar masses should be rare, or maybe even non-existent. But now astronomers have found one.

A team of researchers found a binary star that’s six times the mass of the Sun. And it hosts a planet that’s about ten times more massive than Jupiter.

The binary star is named b Centauri, and it’s about 324 light-years from Earth. The pair of stars are six to 10 times more massive than our Sun, and the planet, named b Centauri b, is about 11 times more massive than Jupiter.

The paper detailing the discovery is titled “A wide-orbit giant planet in the high-mass b Centauri binary system.” It’s published in the journal Nature, and the lead author is Markus Janson, an astronomer at Stockholm University, Sweden.

In this age of exoplanet discovery, astronomers have found a wide variety of solar system architectures. The detail of how planets with different masses form around different stars is an important research area. The only way to understand the planet formation process more thoroughly is to examine the process through the whole range of stellar and planetary masses. The extremes are particularly important.

Astronomers have studied planets that orbit very closely to high-mass stars and found a curiosity: the frequency of giant planets increases with the mass of the stars hosting them. But only up until a point. At about 1.9 stellar masses, the frequency of giant planets drops precipitously. This drop-off implies that giant planets should be rare, or even non-existent, around high-mass stars.

But now astronomers have found one.

“Finding a planet around b Centauri was very exciting since it completely changes the picture about massive stars hosting planets,” explained Janson.

b Centauri is only about 15 million years old and is at least six times more massive than the Sun. It’s the most massive stellar system to host any planets that astronomers have found. Before this discovery, astronomers haven’t found any planets orbiting a star of three solar masses.

Detection methods sensitive to close-in stars aren’t as sensitive to exoplanets on wider orbits, and b Centauri b is a whopping 100 times more distant from its star than Jupiter is from the Sun. Put another way, it’s 560 times greater than the Sun-Earth distance. “The planet-to-star mass ratio of 0.10—0.17% is similar to the Jupiter-Sun ratio, but the separation of the detected planet is ~100 times wider than that of Jupiter,” the authors write in their paper.

This figure from the study shows the planet-to-star mass ratio for the b Centauri system. All of the small circles are known exoplanet to star mass ratios. Planets in our Solar System are shown for comparison. Notice that b Centauri b, shown with a blue diamond, which has an unusually low mass ratio to the central system relative to other detected planets in the wide, directly imaged population. Image Credit: Janson et al 2021.
This figure from the study shows the planet-to-star mass ratio for the b Centauri system. All of the small circles are known exoplanet to star mass ratios. It shows planets in our Solar System for comparison. Notice that b Centauri b, shown with a blue diamond, has an unusually low mass ratio to the central system relative to other detected planets in the more comprehensive, directly imaged population. Image Credit: Janson et al 2021.

Massive young stars like b Centauri are extremely hot. b Centauri is a B-type star and is three times hotter than the Sun. It emits powerful radiation in UV and X-rays. All that energy forces the gas surrounding the star to dissipate, which impedes large planet formation. “B-type stars are generally considered as quite destructive and dangerous environments. It was believed that it should be exceedingly difficult to form large planets around them,” Janson explains.

When the team first spotted b Centauri b, it was just a faint point source. It was one of three that they found. In general terms, a faint point source like this one is either a distant star in chance alignment or a planet. If it’s a planet, other observations will show a common proper motion with its star. Then astronomers can conclude that it’s physically bound to its star. “We therefore scheduled a follow-up observation of b Cen,
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