A remarkable planetary system has been found where there is not one, not two, but three separate disks capable of forming planets. It is the first time such a system has ever been found.
The system is called IRS 43, and it is a binary system – where two stars are orbiting each other – located about 400 light-years from Earth. Being just 200,000 years old, each star has its own disk of dust and gas that will likely give rise to planets.
But the most bizarre thing is the third disk, surrounding both stars and misaligned in such a way to make its origins confusing. The finding could shake our understanding of exoplanets.
The discovery was made by astronomers led by Christian Brinch from the Niels Bohr Institute at the University of Copenhagen in Denmark, and published in the Astrophysical Journal Letters. The team used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile in late August 2015 to observe the system, which had been known about for 25 years.
Never before, though, had the system been studied in such detail. Closer examination revealed that the two stars each had a Solar System-sized disk of dust and gas around them, each roughly 50 AU in diameter (1 AU, astronomical unit, is the distance from Earth to the Sun).
The third disk, meanwhile, surrounds the two stars, extending about 500 AU away from the system’s center.
“The system here may fill out a hole in our statistics on planets in binary star systems,” Brinch told IFLScience. “That makes it important, of course.”
And the orientation of the system makes it even weirder. We would expect the disk to be aligned with the orbits of the two stars, just like how a disk around a single star is flattened to follow that star’s rotation.
That’s not the case here, though, with the two stars orbiting almost perpendicular – about 60 degrees – to the plane of the outer disk.
Why this is so isn’t clear. It’s possible that this is just an anomaly left over from the formation of the system, which was not long ago in cosmic terms and may have been especially turbulent.
A more enticing scenario, though, is that a third star might have once orbited here too, before being flung out by the other two stars and leaving only this larger planet-forming disk as its calling card. That third star may still be locatable somewhere in the vicinity of this system, if it exists.
Above is ALMA's view of the system. Christian Brinch/NBI/KU
We also aren’t sure yet if all three of the disks will actually give rise to planets. The exact process of planet formation remains a bit of a mystery, and it’s not known if there is sufficient material in each to form planets.
If that is the case, though, planets in the outer disk would be in a wide, sweeping orbit around the two inner stars. This might be a feature that is common to exoplanetary systems, and could explain how some planets are found in such far-flung orbits around stars. A similar process may even explain some characteristics of our own Solar System, such as how Pluto orbits at such a high inclination.
Despite the misalignment of the outer disk, we would expect the IRS 43 system to eventually line up and become flat, which means it is a race against time to see if planets can form here.
“We know the time scale for that would be on the order of a couple of million years,” said Brinch, “and that happens to be the same time scale as the planet formation process.”
If the planets form before it aligns, then the system will be frozen as it is and be misaligned forever. But if planet formation happens slightly slower, then it’s possible the entire system will align and give rise to a more traditional flat planetary system.
What is clear, though, is that this system is unlike anything we have seen before. Brinch said they are planning to run computer simulations on how long the alignment might take, and other astronomers are now eyeing similar systems, with the possibility that this behavior is not that unusual.
“It’s a system that will teach us about the potentially turbulent process that goes into star and planet formation,” Brinch said.