In case you missed it, yesterday was a chaotic day in astronomy. In rapid succession, we had a solar eclipse, the Geminids meteor shower, and the Hayabusa2 team found sample chamber A contained a nice quantity of black grains and chunks from the asteroid Ryugu. While all these events took place, science also rolled on, with one of the weirdest new papers taking a look at an 11 Jupiter mass planet on a wild orbit around a binary star system.

The two central stars are both hotter and larger than our Sun, and they are in a tight orbit such that they appear as a single star unless looked at spectroscopically. Surrounding the stars is a massive debris disk that caught astronomers’ eyes and, starting in 2005, their telescope time. 

During a multi-year study, a faint spec was seen to co-move with the stars, leading a team led by Vanessa Baily to publish the discovery of a planet in a several hundred AU orbit. Now, with an even longer observing baseline, and the combined efforts of Hubble and several ground-based telescopes, this world has been charted to have an elliptical orbit that is highly inclined relative to the debris disk, and that orbit is actually stirring up the debris disk with every passage around the twin suns. 

According to team member Robert de Rosa: The idea is that every time the planet comes to its closest approach to the binary star, it stirs up the material in the disk. So every time the planet comes through, it truncates the disk and pushes it up on one side. This scenario has been tested with simulations of this system with the planet on a similar orbit — this was before we knew what the orbit of the planet was.

We have never seen a planet in this kind of an orbit before; this is largely because our planetary searches aren’t sensitive to this kind of a world, so we really have no idea how common this kind of a planet might be. It could be very common, as we think our system may have a ninth planet out in the distant reaches of the solar system with an orbit not too different from what we’re seeing here. 

In both cases, an encounter with two massive objects may be the reason for their weirdness. In the case of HD 106906 b, the migration was driven by the stars. According to a NASA press release: The prevailing theory is that it formed much closer to its stars, about three times the distance that Earth is from the Sun. But drag within the system’s gas disk caused the planet’s orbit to decay, forcing it to migrate inward toward its stellar pair. The gravitational effects from the whirling twin stars then kicked it out onto an eccentric orbit that almost threw it out of the system and into the void of interstellar space. Then a passing star from outside the system stabilized the exoplanet’s orbit and prevented it from leaving its home system.

For our own planet 9, interactions with Jupiter and Saturn may be to blame; it’s really hard to say until we find it.

So far no other exoplanets have been found orbiting with HD 106906 b. The star system is about 15 million years old, and other worlds are possible. When or if the JWST is able to launch, this system is a prime candidate for its infrared observing powers that would be able to detect other worlds the size of Saturn or larger. For now, our current understanding of this system is summarized in a paper in The Astrophysical Journal with first author Meiji Nguyen.

More Information

NASA press release 

UC Berkeley press release 

Hubblesite press release 

“First Detection of Orbital Motion for HD 106906 b: A Wide-separation Exoplanet on a Planet Nine–like Orbit,” Meiji M. Nguyen, Robert J. De Rosa, and Paul Kalas, 2020 Dec 10, The Astronomical Journal