Astronomers have identified the biggest star known to have exoplanets yet, upsetting standard planetary formation models. The exoplanet, catalogued as b Centauri (AB)b, orbits its hot binary star at a distance 14 times that between the Sun and Pluto, and was directly imaged using using the ESO’s Very Large Telescope (VLT).

To date, b Centauri is the biggest star known to host an exoplanet. Estimates of its mass put it at between two and four times the previous record holder, meaning it could be up to ten times bigger than our own Sun.

“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.

Giant exoplanet orbiting a massive star

It was discovered by astronomers working on the B-star Exoplanet Abundance Study (BEAST), a project to find and study exoplanets around hot, massive stars. BEAST focuses on B type stars in the Scorpius/Centaurus young stellar association, stars that have been mostly ignored in the hunt for exoplanets.

Astronomers used the VLT to image these stars, looking for extremely faint objects in their immediate neighbourhoods. Whenever they saw something with a proper motion matching the target star, they flagged it as a potential exoplanet.

They then verified the exoplanets by first measuring their colour (objects shining brightest in near-infrared light are too cold to be stars), and by repeating the observation a few years later. If it’s position has changed correctly, it’s an exoplanet.

The host star, b Centauri, is actually a close binary — two stars orbiting each other so closely that we cannot study them individually. In fact we only know that there is a secondary star from the way they wobble as they orbit each other. The exoplanet orbits at a distance of 550 Astronomical Units (AU), far enough that it circles both stars as if they were one. This means that the distance between b Centaurus b and its stars is a full 100 times more than the distance between Jupiter and the Sun.

When the BEAST astronomers measured the brightness b Centauri b, they compared this to the amount of light that they knew it must be receiving from it’s stars, and fitted those numbers into various models of planet size and formation. Their conclusion? This exoplanet is a Super-Jupiter, an enormous gas giant similar to Jupiter, but fully ten times larger. This happens to match another directly imaged exoplanet named HIP 6542 b quite closely!

Interestingly, a search of previous surveys that included this star showed that b Centauri b had actually been imaged before, in the year 2000. It was so faint in that earlier image that researchers wrote it off as just another bit of photographic noise. But by combining it with the newer observations, researchers how have a full 20 years worth of orbital data, allowing them to calculate its orbit quite accurately. It’s orbit has a very low eccentricity, meaning that it moves in an almost perfect circle, and is inclined at a (very rough) 45º angle against our line of sight.

This observation challenges some of what we thought we knew about how planets form. Up until recently, it was generally accepted that such hot, massive stars cannot form exoplanets. “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,” according to Janson

The bigger picture

B-type stars are hot and energetic enough that they put out significant radiation pressure on anything in their vicinity. It had generally been assumed that this radiation pressure, and their strong stellar winds, would blow any remaining dust and gas away before it had a chance to form into planets.

Previous detections, using both the radial velocity method and direct imaging, have shown that as stars get bigger, so do their planets. This is presumably because larger stars form from larger proto-stellar clouds, and so have more spare material for building planets. However, the number of exoplanet detections drops sharply when we look at stars more than about three times the Sun’s mass.

In fact, going by radial velocity detections alone, such large stars seem to almost never have exoplanets, which supports the idea that hot stars are simply too violent to allow planetary formation. But since radial velocity detections work by detecting the wobble caused by a planet’s orbit, they are biased towards heavy objects closer to smaller stars. Distant planets orbiting larger stars are much harder to detect with this method simply because they cause much smaller and slower wobbles.

Direct Imaging has the opposite bias – it is extremely difficult to image exoplanets against the glare of an entire star, so you’re more likely to spot something that’s large, and orbiting at a great distance.

Put together, these observations seem to suggest that large B type stars do evaporate their protoplanetary disks too rapidly to form planets at close orbits, but since this evaporation happens from the inside out, the outer regions will last longer, and so have more time to form planets.

“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 Gayathri Viswanath, a PhD student 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.”

For a detailed summary of this discovery, you can read the original paper here: A wide-orbit giant planet in the high-mass b Centauri
2 binary system

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