Planet 10 - Orange Blue Final
Scientists think there could be a planet the size of Mars in the Kuiper belt, a ring of material in the outer solar system beyond Neptune. Heather Roper/LPL

Astronomers find new exoplanets all the time in the far reaches of the galaxy and the universe, but there is still much to be discovered at home in our own solar system. Some scientists think one of those objects is a tenth planet.

Sorry, Pluto lovers, but the ninth planet coming before this 10th mystery object in the planetary order would not refer to the beloved orb that was demoted to dwarf planet about 10 years ago. Instead, that spot in the lineup would belong to another suspected planet in the outer solar system that many scientists are searching for but have yet to confirm. It has been dubbed Planet Nine, because it would be beyond eighth planet Neptune. But the additional planetary object scientists are now searching for near the Kuiper belt -- although still in the far reaches of the outer solar system -- would be much closer than Planet Nine.

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The Kuiper belt is a disc of icy chunks that forms a ring around the planets. It is leftover material from the formation of the solar system and is now classified as containing Pluto, as well as other dwarf planets. According to a study in the Astronomical Journal, the orbital planes of some of the objects in the Kuiper belt are quite skewed, specifically the ones that are in the outer part of the disc, and that could be because there is a “planetary mass object” — whose gravity would affect the rocks’ positions — lurking among them, waiting to be discovered.

For comparison, Planet Nine is projected to be an object 10 times the mass of Earth orbiting beyond the Kuiper belt, at hundreds of times the distance between the Earth and the sun.

The scientists are saying that the Kuiper belt planet would have a mass somewhere between those of Earth and Mars in order to tilt the orbital planes of these objects as much as has been observed — about 8 degrees on average.

“The most likely explanation for our results is that there is some unseen mass,” researcher and lead author Kat Volk said in a statement from the University of Arizona. “According to our calculations, something as massive as Mars would be needed to cause the warp that we measured.”

The Kuiper belt has a diameter that is dozens of times the distance between Earth and the sun and is not to be confused with the asteroid belt, which lies between Mars and Jupiter separating the inner from the outer solar system. It also contains small icy rocks and dwarf planets.

In their research, the astronomers looked at the orbital planes of upward of 600 Kuiper belt objects and analyzed how much they “wobbled” like spinning tops during their rotations, according to the university.

“Imagine you have lots and lots of fast-spinning tops, and you give each one a slight nudge,” researcher Renu Malhotra explained.

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That far out in the solar system, they should be pretty perpendicular to the average orbital plane around the sun, but a large object could put them on an angle.

Seeing that angle in the outer Kuiper belt, Volk and Malhotra suggest an undiscovered planetary object whose gravitational force is pushing them around. They say the planet would be about 60 astronomical units from the sun, with a single AU representing the distance between Earth and our star, and massive enough to affect objects as far as 10 AU in each direction.

If the culprit behind these tilted planes is not a Mars or Earth-size planet -- or even more than one planetary object — the researchers suggest a star could have swept through the area previously and warped them.

“A passing star would draw all the ‘spinning tops’ in one direction” if it got close enough, Malhotra said. However, “the warp would be erased within 10 million years, so we don’t consider this a likely scenario.”

Planetary Object
An undiscovered object the size of a planet could be lurking among the icy rocks in the Kuiper belt in the outer solar system, even farther out than dwarf planet Pluto. Heather Roper/LPL