Astronomers use ESO’s ‘Very Large Telescope’ to identify a new exoplanet that defies expectations

Lead image and all other images courtesy of ESO

The European Southern Observatory’s Very Large Telescope has discovered a new star in the night sky. Astronomers have photographed a planet orbiting b Centauri, a two-star system that you can see with the naked eye. The observation is significant because it’s the hottest and most massive planet-hosting star system ever found. The planet orbits its star at about 100 times the distance that Jupiter orbits the Sun.

Researchers have published their research in Nature. The research paper’s first author, Markus Janson, an astronomer and professor at Stockholm University, explains, ‘Finding a planet around b Centauri was very exciting since it completely changes the picture about massive stars as planet hosts.’

The discovered planet, named b Centauri (AB)b or b Centauri b, is as extreme as its environment. The planet is 10 times as massive as Jupiter, making it one of the most massive known planets. As mentioned earlier, its orbital distance is also exceedingly wide. At a distance 100 times greater than the distance of Jupiter from the Sun, b Centauri b’s orbit is one of the widest discovered. Researchers believe that the large distance between the planet and the pair of stars could be why the planet is able to survive in the harsh environment that typically prevents planet formation.

The b Centauri b system, as captured by the ESO’s VLT and SPHERE instrument, planet b Centauri b (noted by the arrow). Image credit: ESO/Janson et al.

The b Centauri two-star system, also known as HIP 71865, is about 325 light-years away from the constellation Centaurus. With a mass at least six times that of the Sun, the b Centauri system is the most massive around which a planet has been confirmed. Before the discovery, the most massive known system with an orbiting planet was about three times as massive as the Sun.

Massive stars are typically very hot, and the b Centauri system follows the trend. The main star is a B-type star and is more than three times hotter than the Sun. Due to its high temperature, the main star emits a lot of ultraviolet and X-ray radiation.

The surrounding gas is affected by the large mass and high heat. The surrounding material evaporates faster, which typically works against planet formation. ‘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,’ says Janson.

‘This large distance from the central pair of stars and the associated reduced irradiation could have been the key to the planet’s survival,’ says Lucio Mayer, professor of computational astrophysics at the University of Zurich and study co-author. ‘Some evidence suggests that the planet did not form by the popular accretion mechanism, in which – to put it simply – solid bodies grow into a planet over time by gradual accumulation. But we cannot yet say whether it was formed by the so-called gravitational instability mechanism, in which clouds of matter collapse under their own gravity, or in some other way.’

The new discovery bucks typical trends and exhibits that stars can form in severe star systems. Co-author Gayathri Viswanath, PhD student at Stockholm University, says, ‘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. 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.’

Sascha Quanz, Professor of Exoplanets and Habitability at the ETH Zurich, co-author and member of the NCCR PlanetS adds, ‘So far, comparatively few planets have been discovered around stars with more than two solar masses. Earlier studies thus concluded that planet formation is hindered near more massive stars and that giant planets may even be impossible near stars with more than tree solar masses.’

The discovery was made using the Spectro-Polarimetric High-contrast Exoplanet Research instrument (SPHERE), mounted on the ESO’s VLT in Chile. This isn’t SPHERE’s first time imaging planets outside our Solar System. It was also used to capture the first image of two planets orbiting a Sun-like star.

Illustration of b Centauri b. Illustration credit: ESO/L. Calçada

However, SPHERE wasn’t actually the first instrument to photograph the newly-discovered planet. In fact, more than 20 years ago, researchers imaged it using the ESO 3.6-m telescope, but it wasn’t recognized as a planet at the time.

Researchers hope that with the ESO’s planned Extremely Large Telescope (ELT) slated to begin observations later this decade and upgrades to the existing VLT, more can be learned about b Centauri b. Janson adds, ‘It will be an intriguing task to try to figure out how it might have formed, which is a mystery at the moment.’

For more information, visit ESO, ETZ Zurich, and Nature.

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