Instead, University of Arizona astronomers Kat Volk and Renu Malhotra say it's a Mars-sized body in the Kuiper belt, a swarm of small icy objects that extends beyond the orbit of Pluto. If both the Arizona and Caltech researchers are right, then these proposed bodies could bring the total number of planets in our solar system to 10.

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Volk and Malhotra haven't seen their new planet, but they say they can sense its presence. In a new paper due to be published in the Astronomical Journal, they describe an odd distortion in the orbits of objects in the outer part of the Kuiper belt, ones that are between 50 and 80 AU away (AU stands for astronomical unit, or the distance from the sun to Earth, about 92 million miles).

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Although most of the nearer bodies in the solar system circle the sun in the same plane, largely thanks to Jupiter's steadying heft, these faraway Kuiper belt objects (KBOs) orbit at all kinds of wonky angles.

That in itself wouldn't raise too many questions. But when Volk and Malhotra analyzed these orbits in search of the average plane, they found that it was offset by about 8 degrees.

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“It's significant,” Volk said. “And the most likely explanation is this object on the outer solar system.”

If there is a planet out there with roughly the same mass as Mars, its gravity could pull on the orbits of small KBOs, dragging them out of the “invariable plane” that Earth, Jupiter and the rest of the planets inhabit.

The Caltech researchers used similar logic to infer the presence of Planet Nine, arguing that this “massive perturber” is responsible for peculiarities in the point at which KBOs are closest to the sun.

“It's the same idea of indirectly detecting a planet by its effects,” Volk said.

For their study, Volk and Malhotra examined the orbits of about 600 KBOs. Scientists know of roughly 2,000 KBOs right now, but they think there may be as many as 100,000 of significant size.

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“It would be useful to have more Kuiper belt objects to make sure this is a real signal,” Volk acknowledged. But even so, their analysis suggests there's only a 1 to 2 percent chance that the results are because of a fluke in the data.

Alessandro Morbidelli at the Côte d’Azur Observatory in Nice, France, told New Scientist he found it hard to believe that astronomers could have missed something as big as a planet so nearby. (The Caltech scientists' “Planet Nine” is said to be 10 times as distant, which explains why it's been so hard to track down).

“I am dubious that a planet so close and so bright would have remained unnoticed,” Morbidelli said.

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Adding an extra planet to the solar system has long been astronomers' favorite way to explain any orbital weirdness. Scientists spent more than a century searching in vain for a fabled “Planet X” that was believed to be disrupting the orbit of Uranus. It wasn't until the Voyager 2 spacecraft flew by in the 1990s that they realized the “disturbance” was actually a result of a miscalculation of Neptune's size. So the astronomy community is understandably skeptical when anyone claims to have proof of a new planet.

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But none of the potential alternate explanations seem any more suitable, Volk said. Another force capable of creating this effect, a large passing star, would be even more unlikely because the disturbance would be erased within 100,000 years — an astronomical blink of an eye.

Besides, humans don't know nearly enough about our celestial back yard to rule out the possibility of finding new neighbors, she said. Because they are so small and far away, KBOs look like faint stars even to the most powerful telescopes. We discover new bodies in this region only by detecting their motion against the background of stars — a process that requires long nights of patient observation. Scientists have been accumulating new KBOs since they detected the first one, in 1992, and even still, we haven't surveyed the whole sky.

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The most likely hiding spot for the proposed 10th planet, according to Volk, is the galactic plane — the region of the Kuiper belt that lines up with the center of the Milky Way galaxy. Because this region is so crowded with stars, it would be nearly impossible to detect even a Mars-sized object moving across the background. If an ordinary search for a KBO is like a celestial game of “Where's Waldo,” this is like trying to find Waldo when an additional billion background characters have been added to the page.

“We have a good sense of the outer solar system,” Volk said, “but it would not surprise me at all if there are very distant things we have missed.”