Is it Planet Nine or a massive disc?

So, are those mystery orbits in outermost reaches of solar system caused by an unknown ninth planet? There’s another group of astronomers suggesting the answer is “no”.

They believe it can all be explained by the combined gravitational force of small objects orbiting the Sun beyond Neptune.

“The Planet Nine hypothesis is a fascinating one, but if the hypothesised ninth planet exists, it has so far avoided detection,” says Antranik Sefilian, from the Department of Applied Mathematics and Theoretical Physics at the University of Cambridge in the UK.

In a paper published in in The Astronomical Journal, Sefilian and colleagues suggest there’s a disc made up of small icy bodies with a combined mass as much as 10 times that of Earth. 

When combined with a simplified model of the solar system, the gravitational forces of the disc can account for the unusual orbital architecture exhibited by some objects at the outer reaches of the solar system.

Theirs is not, they concede, the first theory involving such a disc, but it is the first, they say, which explains the significant features of the observed orbits while accounting for the mass and gravity of the other eight planets in our solar system. 

Beyond the orbit of Neptune lies the Kuiper Belt, which is made up of small bodies left over from the formation of the solar system. Neptune and the other giant planets gravitationally influence the objects in the Kuiper Belt and beyond, collectively known as Trans-Neptunian Objects (TNOs), which encircle the Sun on nearly-circular paths from almost all directions.

However, astronomers have discovered some mysterious outliers. Since 2003, around 30 TNOs on highly elliptical orbits have been spotted: they stand out from the rest by sharing, on average, the same spatial orientation.{%recommended 6699%} 

The Planet Nine hypothesis suggests that to account for the unusual orbits of these TNOs, there would have to be another planet, about 10 times bigger than Earth, lurking in the distant reaches of the solar system and shepherding the TNOs in the same direction through the combined effect of its gravity and that of the rest of the solar system.

“We wanted to see whether there could be another, less dramatic and perhaps more natural, cause for the unusual orbits we see in some TNOs,” Sefilian says.

“We thought, rather than allowing for a ninth planet, and then worry about its formation and unusual orbit, why not simply account for the gravity of small objects constituting a disc beyond the orbit of Neptune and see what it does for us?”

He and Jihad Touma, from the American University of Beirut, modelled the full spatial dynamics of TNOs with the combined action of the giant outer planets and a massive, extended disc beyond Neptune. 

Their calculations revealed that such a model can explain the perplexing spatially clustered orbits of some TNOs. In the process, they were able to identify ranges in the disc’s mass, its “roundness” (or eccentricity) and forced gradual shifts in its orientations (or precession rate), which reproduced the outlier TNO orbits.

“If you remove planet nine from the model and instead allow for lots of small objects scattered across a wide area, collective attractions between those objects could just as easily account for the eccentric orbits we see in some TNOs,” Sefilian says.

Of course, he adds, it’s possible that both things are true: there could be a massive disc and a ninth planet. “With the discovery of each new TNO, we gather more evidence that might help explain their behaviour.”