Most of the 4,300 confirmed exoplanets discovered to date have something in common – a relatively close orbit to their host star.
Now astronomers have found something out of the ordinary – a giant exoplanet in a bizarre 15,000-year-old orbit around a binary star. It is the first time that scientists have been able to characterize such a large orbit.
The exoplanet is called HD 106906 b and reaches 11 times the mass of Jupiter. It orbits a pair of hot yellow and white main sequence stars called HD 106906; These stars are only 15 million years old and orbit them in just 100 days. The whole system is 336 light years away.
Though overall the location is very different from our own solar system, HD 106906b’s vast orbit is reminiscent of an elusive object that astronomers want to find closer to home – the hypothetical extremely large orbit planet Nine.
“This system makes a potentially unique comparison to our solar system,” said astronomer Meiji Nguyen of the University of California at Berkeley.
“It is very far separated from its host stars in an eccentric and severely misaligned orbit, just like the prediction for Planet Nine. This [raises] the question of how these planets formed and evolved to end up in their current configuration. “
The reason most of the exoplanets we find are relatively close to their stars is pretty simple and has to do with how we usually find and confirm exoplanets.
Two methods are most commonly used: the transit method, in which telescopes look for faint dip in the light of a star as an orbiting exoplanet passes in front of it, known as transit. and the wobble method, very small changes in the wavelengths of a star’s light as it is dragged by the exoplanet’s gravity.
With either method, a single immersion in the starlight or a single wiggle can be everything. In general, astronomers want a few dips or wobbles on a regular basis to confirm an exoplanet.
So you can see why it is more difficult to confirm something in a larger orbit. For example, Jupiter is in a 12-year orbit. So you’d have to stare at the sky for a while.
But HD 106906 b, which was first discovered in 2013, is a rare animal: an exoplanet that was imaged directly. Most of the time, exoplanets are too weak and too close to their host star, but the distance from HD 106906 b means they won’t disappear in the bright glow of their binary stars.
Even so, it was not easy to determine the exoplanet’s orbit. To do this, a team of researchers needed data from the Hubble space telescope. After 14 years in the archives, they managed to get more data on the slowly changing position of HD 106906b at a distance of 737 astronomical units from its stars.
As you can imagine, in a 15,000 year orbit, an exoplanet doesn’t seem to move much in 14 years, but it was enough for astronomers to piece the orbit together.
The 15,000 year orbital period is only part of what they learned. The other, more confusing part is that the exoplanet is at a high orbit incline – at a pronounced angle to the debris disk of material that surrounds the two stars.
“To highlight why this is strange, we can just look at our own solar system and find that all the planets are roughly in the same plane,” said Nguyen.
“It would be bizarre if, for example, Jupiter were only tilted 30 degrees from the plane in which every other planet orbits. This begs all sorts of questions about how HD 106906b ended up so far out in such an inclined orbit.”
One possibility is that HD 106906 b was not born in the orbit of the binary stars. In this scenario, the exoplanet would have been a villain, wandering unbound through space until it got close enough to a star (or binary) to be trapped in orbit.
Another scenario – and one the team leans towards – is the HD 106906b that formed in the debris disk of the binary file. Initially, pulling the disk would have caused the exoplanet’s orbit to disintegrate towards the star. There, in a closer orbit, the gravitational interactions between the two stars could have brought the exoplanet into a much more distant, highly inclined orbit that was stabilized by a passing star.
Either of these scenarios could result in an odd, eccentric orbit resulting in irregularities in the debris disk orbiting the binary.
“It’s like arriving at the site of a car accident and trying to reconstruct what happened,” said UC Berkeley astronomer Paul Kalas.
“Did passing stars disturb the planet, then did the planet disturb the disk? Is it the binary in the middle that first disturbed the planet and then it disturbed the disk? Or passing stars have both the planet and the disk disturbed at the same time? time? this is a detective work of astronomy that gathers the evidence we need to develop plausible storylines about what happened here. “
And both scenarios have been proposed for Planet Nine (though Jupiter kicked it in the case of Planet Nine), a hypothetical solar system planet that is said to predict roughly 5 to 10 times the mass of the Earth in an orbit of 300 to 700 astronomical units The sun.
There is considerable doubt that Planet Nine exists, but HD 106906b not only shows that such a strange orbit is possible, but that it can occur relatively early in the life of a planetary system.
However, there is still a lot to be done with the HD 106906 b.
“There are still many unanswered questions about this system,” said astronomer Robert De Rosa from the European Southern Observatory in Chile.
“It is likely that both observers and theorists will study HD 106906 in the years ahead to unravel the many mysteries of this remarkable planetary system.”
The research was published in The Astronomical Journal.