First There Was Planet Nine. Now There Might Be A "Planet Ten"

Artist's impression of Planet Ten. Heather Roper/LPL

Last year we learned that there may be a hidden Planet Nine in the outer reaches of our Solar System. Now it turns out there may be a Planet Ten out there too.

The study, to be published in the Astrophysical Journal, was carried out by Kat Volk and Renu Malhotra from the University of Arizona's Lunar and Planetary Laboratory (LPL). A pre-print is available on arXiv.

In their study, the researchers monitored the motion of space rocks in the Kuiper Belt, a region of bodies beyond the orbit of Neptune. The motion of some of these bodies suggests an unseen planet may be affecting their orbits.

"The most likely explanation for our results is that there is some unseen mass," said Volk, the study’s lead author, in a statement.

The planet would be somewhere between the Earth and Mars in mass. It would orbit much closer than the theorized Planet Nine, about 60 times further from the Sun than Earth (60 AU, or astronomical units). Planet Nine is thought to be between 500 and 700 AU.

Planet Ten’s existence is based on the study of 600 Kuiper Belt Objects (KBOs). Some of these appear to have a “warped” orbit, which tilts them at eight degrees to the flat orbital plane of the major planets in the Solar System.

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The possible orbit of Planet Ten. Heather Roper/LPL

According to the researchers, the best way to describe this warping is if there is another planet out there. They say Planet Nine, which is thought to be 10 times the mass of Earth or around half the mass of Neptune, cannot account for the anomaly.

"[Planet Nine] is too far away to influence these KBOs," Volk said. "It certainly has to be much closer than 100 AU to substantially affect the KBOs in that range."

That’s not to say Planet Ten definitely exists. Speaking to New Scientist, Alessandro Morbidelli at the Côte d’Azur Observatory in Nice, France, said he was “dubious that a planet so close and so bright would have remained unnoticed.”

Volk and Malhotra, though, say we haven’t seen it yet because it might be located towards the dense central portion of the Milky Way. From our location, this region is full of stars, which makes it hard to study.

Another possibility is that a passing star could have caused this warping. However, that would need to have happened in the last 10 million years, which seems unlikely.

It’s theorized that there could be many hidden worlds at the edge of the Solar System. Planet Ten, and indeed Planet Nine, could turn out to be just two of a whole host of planets we haven’t found yet. The Solar System might soon be getting a lot busier.

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