Heavyweight trio with exoplanet

Researchers discover an object around the most massive stellar couple to date

December 08, 2021

A group of astronomers, including scientists from the Max Planck Institute for Astronomy (MPIA), has captured an image of a planet orbiting b Centauri, a binary star visible with the naked eye. It is the hottest and most massive planet-hosting star system found to date. The team spotted the planet orbiting the stellar couple at 100 times the distance from Jupiter to the Sun. Some astronomers believed planets could not exist around stars this massive and this hot — until now.

Looking at an exotic world: This image shows b Centauri, and its giant planet b Centauri b (arrow), by the SPHERE instrument on ESO’s Very Large Telescope and using a coronagraph. It blocked the immensely bright light from the massive star system (top left) and allowed astronomers to detect the faint planet.

Located approximately 325 light-years away in the Centaurus constellation, astronomers have detected a giant planet in orbit around a young massive binary star called b Centauri. “Finding a planet around b Centauri was very exciting since it completely changes the picture about massive stars hosting planets,” explains Markus Janson, an astronomer at Stockholm University, Sweden. He is the main author of the new study published in Nature today.

The only 15 million years old b Centauri binary star has at least six times the mass of the Sun. This property makes it by far the most massive stellar system around which astronomers have found a planet. Until now, previous studies had failed to detect any such object around a star more than three times as massive as the Sun.

Most massive stars are also very hot, and this system is no exception: its primary star is a so-called B-type star that is over three times hotter than the Sun. Due to its high temperature, it emits large amounts of UV and X-ray radiation. 

The large mass and heat from this type of star strongly impact the surrounding gas, which should counteract planet formation. In particular, the hotter a star is, the more high-energy radiation it produces. This property causes the surrounding material to evaporate more efficiently. “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.

Now, the latest discovery demonstrates planets can, in fact, form in such extreme stellar environments. “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.”

Zoom on an exoplanet: This artist’s impression shows a close up of the planet b Centauri b, which orbits a binary system with mass at least six times that of the Sun. This is the most massive and hottest planet-hosting star system found to date. The planet is ten times as massive as Jupiter and orbits the two-star system at 100 times the distance Jupiter orbits the Sun.

Indeed, the planet discovered, named b Centauri (AB)b or b Centauri b, is an alien world experiencing conditions completely different from what we face here on Earth and in our Solar System. It is ten times more massive than Jupiter, making it one of the most massive planets ever found. Moreover, it revolves around the binary star at a staggering 100 times greater distance than Jupiter does from the Sun, one of the widest orbits discovered yet. This large distance from the central pair of stars could be key to the planet’s survival.

These results were made possible thanks to the sophisticated Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) mounted on the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in Chile. A consortium of several astronomical institutions, of which MPIA is an essential member, has constructed and built this highly successful instrument. It has managed to image several planets orbiting stars other than the Sun before, including the first image of a growing infant planet and a potentially moon-forming disk.

“Combining the innovative technique of adaptive optics, the performance of the ten metre-class telescopes in Chile and sophisticated data reduction tools made this amazing discovery possible,” Thomas Henning from MPIA in Heidelberg, Co-I of the SPHERE instrument and co-author of the study, emphasises.

However, SPHERE was not the first instrument to take a picture of this planet. As part of their study, the team looked into past data on b Centauri and discovered that the planet had actually been imaged more than 20 years ago by the ESO 3.6-metre telescope, although it was not recognised as a planet at the time.

With ESO’s Extremely Large Telescope (ELT), due to start observations later this decade, and further technical progress, 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.

MN / HOR

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