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A Mysterious Mars-Sized Object Could Be Hiding at the Edge of Our Solar System

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It’s been about 11 years since Pluto was demoted to dwarf planet status, leaving a 1,473 mile (2,370 kilometers)-size void in our hearts. Since then, the hunt for Planet X—aptly renamed Planet 9—has grown into an international movement to find such an object in the Kuiper Belt beyond Neptune’s orbit. Now, scientists Kat Volk and Renu Malhotra from the University of Arizona’s Lunar and Planetary Laboratory are upping the ante—they suggest that a completely different, tenth planetary-mass object is hiding somewhere in the Kuiper Belt as well. Is someone keeping track of all these goddamn hypothetical planets?

Here’s what we know about this undiscovered, maybe-planet: According to the researchers’ work, which has been published in the Astronomical Journal, this so-called “Planet 10" would be located at about 55 astronomical units (AU), if it’s real. That’s significantly closer than the estimated location of Planet 9, which is thought to be in the Kuiper Belt as well, but at about 700 AU. The team posits Planet 10's mass is somewhere between Mars’ and Earth’s, which they determined by studying the orbital tilts of roughly 600 icy bodies in the region, known as Kuiper Belt Objects (KBOs). KBOs orbit the Sun at an incline that’s different from the eight known planets in our solar system, but according to the researchers’ calculations, the orbital tilts of these particular KBOs were so unusual that something else would have to be at work. The researchers assert the gravitational tug of a tenth planet with roughly the mass of Mars could be responsible for their unusual orbital plane.

“The most likely explanation for our results is that there is some unseen mass,” Volk, a postdoctoral fellow at University of Arizona’s Lunar and Planetary Laboratory and the lead author of the study, said in a statement. “According to our calculations, something as massive as Mars would be needed to cause the warp that we measured.”


While we’d all love to ride the Planet 10 hype train, Caltech astronomer Konstantin Batygin—who co-authored a paper last year proposing the existence of Planet 9—says we should pump the brakes, at least for now.


“At the end of the day, the solar system has quite a bit of real estate, so you can stuff a lot of small things in it,” he told Gizmodo. “What [these researchers] are talking about is not a massive body, so is it plausible for a Mars-like body to exist at 100 AU or so? Yes.” Although, it’s important to note that because the object is pretty small, it might not even fit the official definition of a planet, as it would have to clear its orbit of other objects—in this case, KBOs.

“What problem does that explanation run into?” Batygin continued. “It runs into the problem that it hasn’t been observed yet.”


Batygin explained that the Catalina and Pan-STARSS surveys should have had the sensitivity to discover a Mars-sized object somewhere in the Kuiper Belt. But searching for a tenth planetary object as small as this one in a region as vast as the Kuiper belt may not yield any definitive answers soon.

“Of course, no survey is complete,” Batygin said. “You have issues come up all the time and you miss certain parts of the sky. If an object is sitting right in front of the galaxy, it could be missed simply from a point of view that nobody ever observes, because so many stars in the background [mean] it’s hard to conduct any meaningful search.”


Ideally, we’ll find planets nine and 10 relatively soon, before the void of Pluto’s loss consumes us all. Seriously, science, we need this.