Disk beyond Neptune could explain odd orbits

The Planet Nine hypothesis²  arose after observations of 23 ‘trans-Neptunian objects’ (TNOs), following unusual orbits that couldn’t be explained using conventional eight-planet solar system models. A ninth planet, around ten times the mass of Earth and twenty times further from the Sun than Neptune, would shepherd these orbits, although astronomers acknowledge that this is only one possible explanation.

When the Planet Nine work was published, Jihad Touma at the American University of Beirut and his then master’s degree student Antranik Sefilian were studying planet formation in disks surrounding binary stars (a system of two stars that revolve around each other or around a common centre). This work gave them a unique perspective to tackle the TNO puzzle.

“We asked: what would happen if we took Planet Nine and spread it out in a disk?” says Touma. “The answer to this question could be quickly explored with a slight modification of our work on massive disks around stellar binaries.”

Sefilian and Touma performed detailed simulations of disks with various plausible sizes and masses, showing how the action of a massive disk, together with the known planets, can recover the essential features of TNO orbits. 

“Our hypothesized disc extends radially from around 40 to 750 AU (where 1 AU represents the mean distance of Earth from the Sun) and contains, in total, a few to ten times the mass of Earth,” says Sefilian, who is now based at the University of Cambridge. 

However, one of the original proponents of Planet Nine, Scott Sheppard at the Carnegie Institution for Science in Washington DC, USA, points out some limitations of the disk theory.

“Neither a massive planet nor a disk of smaller objects have been seen, but the planet hypothesis requires just one object, which would be faint and thus unlikely to have been seen yet,” says Sheppard. “The massive disk would contain even fainter objects, but there would be a lot of them and we likely should have seen some.”  

Despite these concerns, Sefilian and Touma believe there is no reason to discount either theory yet. “Our hypothesised disc may actually complement the action of an additional planet, while naturally forcing refinements in the mass and kinematics of the combined planet–disc system. In fact this is our preferred scenario, the details of which we will work out in the future,” Sefilian says.

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