The mysterious “Planet Nine,” which is theorized to be 10 times larger than Earth and lies somewhere in the outer reaches of our solar system, might not be a planet at all, says a new study.
It may really be a gigantic disk made up of smaller objects lying just beyond Neptune exerting the same gravitational force as a super-Earth-sized planet, according to researchers at the University of Cambridge and the American University of Beirut.
“Planet Nine” is thought to be located so far from the Sun that it reflects very little light, and scientists have so far hypothesized its existence based on the strange clustered orbits of a group of trans-Neptunian Objects (TNOs), a class of objects that includes Pluto. Some TNOs exhibit orbits that can’t be explained by the eight-planet model of our solar system—there must be a ninth, and we just can’t visually observe it.
In a statement, study co-author and PhD student Antranik Sefilian said that the alternative disk explanation side-steps the thorny issue of explaining how a massive planet formed on the fringes of the solar system, while accounts for the clustered orbits of TNOs attributed to Planet Nine. The study has been accepted for publication in Astronomical Journal.
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“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.
The researchers are keeping an open mind about what lies beyond Neptune, however, and note in the paper that a disk of smaller objects may work in conjunction with an undiscovered planet à la Planet Nine to exert the observed gravitational force.
The possibility of a Planet Nine alternative comes with its own eyebrow-raising questions. For example, Caltech astronomer Mike Brown—a prominent champion of the Planet Nine hypothesis—questioned how a disk of small objects has stayed intact for billions of years in an interview with Popular Science.
“Interestingly, a ring of objects would be much much easier to find than a singular planet, yet there is no evidence that such a thing exists,” Brown told Popular Science.
The search for whatever is holding objects at the edges of the solar system in its gravitational thrall continues—be it a massive planet, a disk, or both.
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