Most of the more than 4,300 confirmed exoplanets discovered so far share something in common – a relatively close orbit to their host star.
Now, astronomers have found something extraordinary – a giant exoplanet in a bizarre 15,000-year orbit around a binary star. It is the first time that scientists have been able to characterize such a huge orbit.
The exoplanet is called HD 106906 b, with 11 times the mass of Jupiter. It surrounds a pair of warm main-sequence yellow-white stars called HD 106906; these stars are only 15 million years old and orbit each other in just 100 days. The entire system is 336 light-years away.
Although the location as a whole is very different from our own Solar System, the huge orbit of HD 106906 b is reminiscent of worn-out objects that astronomers hope to find closer to home – the hypothetical extremely wide orbit Planet Nine.
“This system makes perhaps a unique comparison with our Solar System,” said astronomer Meiji Nguyen of the University of California at Berkeley.
“It is very widely separated from its host stars on an eccentric and highly misaligned orbit, as is the prediction for Planet Nine. [raises] the question of how these planets formed and evolved to end in their current configuration. “
The reason that most exoplanets we find are relatively close to their stars is quite simple, and it relates to how we usually find and confirm exoplanets.
Two methods are the most commonly used – the transition method, where telescopes look for faint dives in the light of a star as an orbiting exoplanet passes in front of it, known as transit; and the staggering method, very small changes in the wavelengths of stellar light, as it is drawn by the gravity of the exoplanet.
For both of these methods, a single dive into stellar light or a single wobble could be anything; generally astronomers want a number of dives or oscillations, at regular intervals, to confirm an exoplanet.
So you can see why something on a larger orbit would be harder to confirm; Jupiter, for example, is in 12-year orbit. So you should look at the sky for a while.
But HD 106906 b, first discovered in 2013, is a rare animal: an exoplanet that was imaged directly. Most often exoplanets are too weak and too close to their host star, but the distance of HD 106906 b means that it does not disappear in the brightness of its binary stars.
However, elaborating the orbit of the exoplanet was not easy. For this, a team of researchers needed data from the Hubble Space Telescope. Going back 14 years in the archives, they managed to get more data on HD 106906 b slowly changing position at a distance of 737 astronomical units from its stars.
As you can imagine, an exoplanet on a 15,000-year-old orbit won’t seem to move much after 14 years, but it was enough that astronomers could put the orbit together.
The 15,000-year orbital period is just one part of what they learned. The other more confusing part is that the exoplanet has a high orbital inclination – at a pronounced angle to the debris disk of material surrounding the two stars.
“To highlight why this is strange, we can just look at our own Solar System and see that all the planets lie approximately in the same plane,” Nguyen said.
“It would be strange if, for example, Jupiter only tilted 30 degrees relative to the plane in which every other planet orbits. This raises all sorts of questions about how HD 106906 b has come so far on such a sloping orbit.”
One possibility is that HD 106906 b was not born around the binary stars. In this scenario, the exoplanet would be a rogue, wandering space unbound until it has passed close enough to a star (or binary) to be trapped in orbit.
Another scenario – and to which the team is leaning – is that HD 106906 b formed in the garbage disk of the binary. Initially, dragging in the disk would cause the exoplanet’s orbit to decay to the star. There, at a closer orbit, the gravitational interactions between the two stars could drive the exoplanet to a much more distant, very sloping orbit that has stabilized from a passing star.
Both scenarios could lead to a strange, eccentric orbit that produces an irregularity observed in the debris disk orbiting around the binary.
“It’s like getting to the scene of a car accident, and you’re trying to reconstruct what happened,” said UC Berkeley astronomer Paul Kalas.
“Did passing stars worry the planet, then the planet worried the disk? Did the binary in the middle first worry the planet, and then it worried the disk? Or did passing stars bother both the planet and the disk at the same time? Time? Here’s an astronomical detective.” a work that gathers the evidence we need to come up with some credible stories about what happened here. “
And both scenarios were proposed for Planet Nine (although in the case of Planet Nine, Jupiter made the kick), a hypothetical solar system predicted to be about 5 to 10 times the mass of Earth, in orbit about 300 to 700 astronomical units of the sun .
There is significant doubt that there is Planet Nine, but HD 106906 b shows not only that such a strange orbit is possible, but that it can occur relatively early in the life of a planetary system.
But there is more work to be done HD 106906 b.
“There are still a lot of open questions about this system,” said astronomer Robert De Rosa of the European Southern Observatory in Chile.
“Surely both observers and theorists will study HD 106906 over the next few years, revealing the many mysteries of this remarkable planetary system.”
The research was published in The Astronomical Magazine.