“Everything on a Gigantic Scale” – Alien Planet Imaged Orbiting b Centauri’s Massive Two-Star System

b Centauri


“The planet in b Centauri is an alien world in an environment that is completely different from what we experience here on Earth and in our Solar System,” explains co-author Gayathri Viswanath, a researcher in exoplanet detection at Stockholm University. “It’s a harsh environment, dominated by extreme radiation, where everything is on a gigantic scale: the stars are bigger, the planet is bigger, the distances are bigger.”

Until now, no planets had been spotted around a star more than three times as massive as the Sun, reports the European Southern Observatories (ESO). The Very Large Telescope (ESO’s VLT) in Chile has captured an image of a planet orbiting b Centauri, a two-star system that can be seen with the naked eye. This is the hottest and most massive planet-hosting star system found to date. The planet was spotted orbiting it at 100 times the distance Jupiter orbits the Sun, which might guarantee its survival.

Transforms What We Know about Massive Stars as Planet Hosts

“Finding a planet around b Centauri was very exciting since it completely changes the picture about massive stars as planet hosts,” explains Markus Janson, an astronomer at Stockholm University, Sweden and first author of the new study published online today in Nature.

The massive planet was imaged by the sophisticated Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) mounted on VLT. SPHERE has successfully imaged several planets orbiting stars other than the Sun before, including taking the first ever-image of two planets orbiting a Sun-like star.

Located approximately 325 light-years away in the constellation Centaurus, the b Centauri two-star system (also known as HIP 71865) has at least six times the mass of the Sun according to dynamical measurements, making it by far the most massive system around which a planet has been confirmed. The spectral type of the primary star, an early B-type star, suggests it is even more massive, likely eight times the mass of the sun, sufficient to end its life as a core collapse supernova. 

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Orbiting a B-type Star –Emitting large amounts of ultraviolet and X-ray radiation

Most massive stars are also very hot, and this system is no exception: its main star is a so-called B-type star that is three times hotter and a thousand times more luminous than our Sun. Owing to its intense temperature, it emits large amounts of ultraviolet and X-ray radiation.  

The large mass and the heat from this type of star have a strong impact on the surrounding gas that should work against planet formation. In particular, the hotter a star is, the more high-energy radiation it produces, which causes the surrounding material to evaporate faster. “B-type stars are generally considered as quite destructive and dangerous environments, so it was believed that it should be exceedingly difficult to form large planets around them,” Janson says.

Mystery of How the Massive System and Planet Formed

“Up until now it was believed from previous trends in exoplanet and host star demographics that stars more massive than 3 solar masses cannot host planets,” explains Viswanath in reply to an email from The Daily Galaxy asking how such a massive planet and star system could have formed. “This is because massive stellar systems, such a b Cen (AB), are extreme environments that emit a lot of radiation. As a result, the disk around such stars will evaporate/dissipate faster, and planet formation at closer distances to such stars becomes a losing race against time, leaving insufficient material for a gaseous giant planet like b Cen (AB)b to form. This dissipation is, however, slower the farther away we go from the central star. The large distance of b Cen (AB)b from its central stars possibly played a key role in giving it enough time to accrete sufficient material from the disk before it dissipates.” 

“We are not sure how the planet might have formed,” wrote Markus Janson In an email to The Daily Galaxy: “Most planets are believed to have formed through gradual merging and buildup of dust and ice particles around the star,” he explained.. “However, it is not clear if this process happens fast enough to form planets around a massive star like b Centauri. Perhaps it might have formed through an alternative route, where material around the star undergoes a gravitational collapse, potentially forming a planet in a much faster time. We don’t know yet, but we hope to figure out more details of how it could have formed in the near future.”

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“These wide giant planets (farther away than the Kuiper belt in our Solar System) detected with direct imaging are indeed puzzling,” replied Cornell University astronomer Laetitia Rodet in response to an email from The Daily Galaxy asking how the massive planet orbiting the b Centauri star system formed. “They challenge the traditional formation mechanism in which giant planets slowly accrete their material in the protoplanetary disk close to the snow line, at a few astronomical units,” she explained. “This discovery is even more surprising as planets were thought to be scarce around massive stars. b Cen b does look like a planet though, both from the point of view of its mass and its mass ratio with the stars. The low eccentricity of its orbit does not favor a violent dynamical past involving interactions with other planets or stars. Everything seems to indicate that the planet formed quickly near its current location, from the gravitational fragmentation of the supposedly massive disk (that has since evaporated). The previously observed dearth of planets around massive stars could be valid only when looking at the closer planets, those targeted by the radial velocity surveys. Massive stars do not appear to prevent the formation of wider planets like b Cen b.”

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Gigantic! Forty Times More  Massive than Jupiter

The planet discovered, named b Centauri (AB)b or b Centauri b, is also extreme. It is 10 times as massive as Jupiter, making it one of the most massive planets ever found. Moreover, it moves around the star system in one of the widest orbits yet discovered, at a distance a staggering 100 times greater than the distance of Jupiter from the Sun. This large distance from the central pair of stars could be key to the planet’s survival.

Actually Imaged More than 20 years Ago

SPHERE was not the first instrument to image this planet. As part of their study, the team looked into archival data on the b Centauri system and discovered that the planet had actually been imaged more than 20 years ago by the ESO 3.6-m telescope, though it was not recognized as a planet at the time.

With ESO’s Extremely Large Telescope (ELT), due to start observations later this decade, and with upgrades to the VLT, astronomers may be able to unveil more about this planet’s formation and features. “It will be an intriguing task to try to figure out how it might have formed, which is a mystery at the moment,” concludes Janson.

Source: “A wide-orbit giant planet in the high-mass b Centauri binary system”, Nature (DOI: 10.1038/s41586-021-04124-8).

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Gayathri Viswanath, Markus Janson, Laetitia Rodet, and ESO 

Image credit top of page: shows the most massive planet-hosting star pair to date, b Centauri, and its giant planet b Centauri b. (Photo by ESO/Janson et al.)

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