Even Really Massive Stars Seem to Have Planets

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. 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, which was executed in 2021,” the authors write. They also found other archival observations of the system showing the planet as a point source, which those original studies disregarded at the time.

According to the team, all of the data they gathered confirmed that the target shared a common proper motion with b Centauri. They also found that “…there is clear evidence for orbital motion consistent with the expected orbital speed around the central stellar mass.”

This is an image of the massive hot star b Centauri and the exoplanet b Centauri b, marked ‘b’ in this image. The point sources marked as ‘bg’ are background stars. The ‘muddy’ appearance of the b Centauri is due to residual noise. Image Credit: Janson et al 2021.

We’re still in the early days of exoplanet discovery. Inevitably, astronomers held our Solar System as a kind of norm since there was nothing to compare it to. But now we’re finding out that there are a wide variety of solar system architectures out there. This discovery adds to the diversity.

“We have always had a very solar system-centric view of what planetary systems are ‘supposed’ to look like,” MPIA scientist and co-author Matthias Samland points out. “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 exciting chapter to this story, this time for massive stars.”

b Centauri b’s vast distance from its star might be the secret to its existence. It’s the widest planetary orbit ever observed. Even the powerfully luminous b Centauri couldn’t drive the gas away at such a great distance.

But how exactly this intriguing planet formed is unknown, for now. Did it form way out there? Did it migrate?

“It will be an intriguing task to try to figure out how it might have formed, which is a mystery at the moment.”

Markus Janson, lead author, Stockholm University.

It’s doubtful that b Centauri b formed in-situ by the core-accretion process. The ultra-energetic star likely prevented that. It may have originated elsewhere and then moved to its present orbit. “Alternatively, a giant planet could form directly from the circumstellar gas disc through gravitational instability,” the authors write.

In their paper, the team briefly discussed the formation of b Centauri b. They say that gravitational instability “…might be a particularly important mechanism in the context of massive stars.” They point out that the planet would’ve formed more quickly in that process rather than in core accretion: about 10 4 years rather than about 10 6 years. That’s because “…the instability mechanism is less sensitive to the rapid dispersion timescales of discs around massive stars,” they write.

The team points to one possible scenario where b Centauri b formed rapidly through gravitational instability, but the star’s powerful outward pressure prevented it from migrating closer. “Theoretical models predict that discs around more massive stars are more likely to fragment as a result of more vigorous mass accretion, which further
supports the interpretation of b Cen (AB)b as a disc instability planet,” they explain.

Another possibility is that the planet formed in isolation, and the star system captured it. But captured planets tend to have highly eccentric orbits, which b Centauri b doesn’t have.

But for now, the team’s data suggests that the planet formed in its current orbital position. “Our measurements of the orbital properties of b Cen (AB)b disfavour a dynamically violent past, favouring instead a formation close to its present location with little subsequent orbital evolution. Since core accretion is challenging at such large separations, disc instability might represent a more probable formation scenario.”

Thanks to this work, we know that massive stars can host planets. Exactly how the planet formed will be a subject of future work.

“It will be an intriguing task to try to figure out how it might have formed, which is a mystery at the moment,” said Janson.

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Evan Gough

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