Massive Disk of Trans-Neptunian Objects Casts Doubt on Planet Nine | Astronomy - Sci-News.com

In 2016, Caltech astronomers showed that six distant solar system objects (2007 TG422, 2013 RF98, 2004 VN112, 2012 VP113, 2012 GB174 and a minor planet called Sedna) possessed orbital features indicating they were affected by the gravity of an as-yet-undetected planet, nicknamed Planet Nine. According to University of Cambridge researcher Antranik Sefilian and American University of Beirut’s Professor Jihad Touma, the strange orbits of those objects can instead be explained by the combined gravitational force of a disk of trans-Neptunian objects (TNOs) with a combined mass as much as 10 times that of Earth.

Beyond Neptune, there is a large disk of small objects, called the Kuiper Belt and beyond that is the Oort Cloud, home of the comets. This artwork shows a section of Kuiper Belt, crowded with the icy cores of potential comets. Image credit: M. Kornmesser / ESO.

“The Planet Nine hypothesis is a fascinating one, but if the hypothesised planet exists, it has so far avoided detection,” Sefilian said.

“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. 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 disk beyond the orbit of Neptune and see what it does for us?”

Sefilian and Professor Touma modeled the full spatial dynamics of TNOs with the combined action of the giant outer planets and a massive, extended disk beyond Neptune.

The duo’s 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 disk’s mass, its ‘roundness’ (or eccentricity), and forced gradual shifts in its orientations (or precession rate), which faithfully 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 said.

Earlier attempts to estimate the total mass of objects beyond Neptune have only added up to around one-tenth the mass of the Earth.

However, in order for the TNOs to have the observed orbits and for there to be no Planet Nine, the model put forward by the team requires the combined mass of the Kuiper Belt to be between a few to 10 times the mass of the Earth.

“When observing other systems, we often study the disk surrounding the host star to infer the properties of any planets in orbit around it,” Sefilian said.

“The problem is when you’re observing the disk from inside the system, it’s almost impossible to see the whole thing at once.”

“While we don’t have direct observational evidence for the disk, neither do we have it for Planet Nine, which is why we’re investigating other possibilities.”

“Nevertheless, it is interesting to note that observations of Kuiper belt analogues around other stars, as well as planet formation models, reveal massive remnant populations of debris.”

“It’s also possible that both things could be true — there could be a massive disk and a ninth planet. With the discovery of each new TNO, we gather more evidence that might help explain their behavior.”

The team’s results will be published in the Astronomical Journal.

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Antranik A. Sefilian & Jihad R. Touma. 2019. Shepherding in a Self-Gravitating Disk of Trans-Neptunian Objects. AJ, in press; arXiv: 1804.06859

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