Something incredibly massive appears to be lurking in the outer reaches of the solar system, way beyond Pluto, and scientists are exciting about the possibility of making a major discovery. Scientists at the University of Arizona’s Lunar and Planetary Laboratory believe that a planet about the size of Earth or Mars is floating out in the Kuiper Belt, an icy region beyond Neptune filled with asteroids and comets.

It was back in January 2016 when a group of scientists first posited the existence of a massive planet orbiting 25 times farther from the sun than even distant Pluto called “Planet Nine.” But that’s a separate potential discovery, meaning that there may be two more planets orbiting the sun that we don’t know about, with this latest hypothesis being a 10th planet.

This “planetary mass object” as it is being described appears to be having an impact on the orbits of space rocks that are floating in the Kuiper Belt. While some Kuiper Belt objects (KBOs) have tilted orbits around the sun because of the invariable plane of the solar system, some of the most distant KBOs tilt away from that by an average of 8 degrees, suggesting a massive object interfering with them.

“The most likely explanation for our results is that there is some unseen mass,” Kat Volk of the University of Arizona’s Lunar and Planetary Laboratory, or LPL, said in a statement along with fellow researcher Renu Malhotra. “According to our calculations, something as massive as Mars would be needed to cause the warp that we measured.”

“Imagine you have lots and lots of fast-spinning tops, and you give each one a slight nudge,” Malhotra said. “If you then take a snapshot of them, you will find that their spin axes will be at different orientations, but on average, they will be pointing to the local gravitational field of Earth. We expect each of the KBOs’ orbital tilt angle to be at a different orientation, but on average, they will be pointing perpendicular to the plane determined by the sun and the big planets.”