Most of the more than 4,300 confirmed exoplanets discovered so far have one thing in common – relatively short orbits around their host stars.
Now astronomers have found something extraordinary: a gigantic 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 and is 11 times heavier than Jupiter. It circles around a pair of hot, yellow and white main 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.
While the place is generally very different from our own solar system, the massive orbit of HD 106906 b is reminiscent of an elusive object that astronomers hope to find closer to home – the hypothetical planet Nine with an extremely wide orbit.
“This system draws a potentially unique comparison to our solar system,” said astronomer Meiji Nguyen of the University of California, Berkeley.
“It is very far from its guest stars in an eccentric and highly aligned orbit, just like the prediction for Planet Nine. [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 quite simple, and it has to do with how we usually find and confirm exoplanets.
Two methods are most commonly used: the transit method, in which telescopes search for faint valleys in the light of a star as an extrasolar planet passes in orbit in front of it, also known as a passageway; and the wobble method, very small changes in the wavelengths of a star’s light as it is pulled by the exoplanet’s gravity.
For either method, it can be a single dip in the starlight or a single wobble of everything; In general, astronomers want to descend or wobble a few times, at regular intervals, to confirm an exoplanet.
So you can see why something is harder to fix in a bigger job; For example, Jupiter is in orbit for 12 years. So you should be staring at the sky for a while.
But HD 106906 b, first discovered in 2013, is a rare beast: an exoplanet directly imaged. Usually, exoplanets are too faint and too close to their host star for this, but the distance of HD 106906 b means it won’t disappear in the bright glow of its binary stars.
Yet it was not easy to work out the orbit of the exoplanet. For that, a team of researchers needed data from the Hubble Space Telescope. Going back 14 years in the archives, they managed to obtain more data on the slowly changing position of the HD 106906 b at a distance of 737 astronomical units from its stars.
As you can imagine, an extrasolar planet orbiting 15,000 years doesn’t seem to move much in 14 years, but it was enough that astronomers could determine its orbit together.
The 15,000-year orbital period is only part of what they have learned. The other, more baffling part is that the exoplanet is in a high orbital slope – at a pronounced angle to the debris disk of material surrounding the two stars.
“To emphasize why this is weird, we can just look at our own solar system and see that all the planets are roughly in the same plane,” Nguyen said.
“It would be bizarre if, say, Jupiter happened to be inclined 30 degrees from the plane around which every other planet orbits. This raises all sorts of questions about how HD 106906 b so far into such an inclined orbit. ended up. “
One possibility is that HD 106906 b was not born in orbit around the binary stars. In this scenario, the exoplanet would have been a rogue, wandering unbound until it got close enough to a star (or binary) to be trapped in orbit.
Another scenario – and what the team is leaning towards – is that HD 106906 b is formed in the debris disk of the binary. Initially, the resistance in the disk would have caused the extrasolar orbit in the direction of the star to disappear. There, in a shorter orbit, the gravitational interactions between the two stars could have kicked the exoplanet into a much more distant, steeply sloping orbit stabilized by a passing star.
Both scenarios can lead to the strange eccentric orbit that causes an irregularity seen in the debris disk orbiting the binary star.
“It’s like arriving at the scene of a car accident and trying to reconstruct what happened,” said UC Berkeley astronomer Paul Kalas.
‘If the passing stars disrupted the planet, then the planet disrupted the disk? Is it the binary in the middle that disrupted the planet first and then the disk? Or did passing stars disrupt both the planet and the disk at the same time? time? This is astronomical detective work, where we gather the evidence we need to come up with some plausible storylines about what happened here. ‘
And both scenarios have been proposed for Planet Nine (although in the case of Planet Nine, Jupiter did the shovel), a hypothetical planet in the solar system predicted to be about 5 to 10 times the mass of Earth, orbiting 300 to 700 astronomical units from the sun.
There is considerable doubt about the existence of Planet Nine, but HD 106906 b not only shows that such a strange orbit is possible, but that it can occur relatively early in the life of a planetary system.
But there is still more work to be done on HD 106906 b.
“There are still many open questions about this system,” said astronomer Robert De Rosa of the European Southern Observatory in Chile.
“It is likely that over the next few years observers and theorists alike will study HD 106906 and unravel the many mysteries of this remarkable planetary system.”
The research is published in The Astronomical Journal.