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A massive, unknown planet may be hiding in our solar system. These astronomers explain.

Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

Some 20 times farther away than Neptune — which is currently the last planet in the solar system — astronomers at the California Institute of Technology say they have found evidence of a new ninth planet.

For now, they're calling it…

…wait for it…

Planet Nine.

The astronomers haven't observed the planet directly with a telescope. But they argue there's a big gravitational presence in the outer solar system that's throwing off the orbits of dwarf planets and other objects in the Kuiper belt.

"Its estimated mass would make it about two to four times the diameter of the Earth, distinguishing it as the fifth-largest planet after Jupiter, Saturn, Uranus and Neptune," the Washington Post reports on the finding. "But at such extreme distances, it would reflect so little sunlight that it could evade even the most powerful telescopes."

In the video above, the Caltech astronomers who published the findings explain why they think a new ninth planet may be lurking out there.

"The only way we could get them [the Kuiper belt objects] to swing in one direction is if there is a massive planet, also very distant in the solar system, keeping them in place while they all go around the sun," Mike Brown, one of the co-authors, says in the video. (Brown's previous work led to the demotion of Pluto to a dwarf planet.)

According to the Washington Post, the pair originally set out to disprove the theory that a planet was throwing off orbits in the Kuiper belt. But they came away more convinced of its probable existence.

"History shows us that it is a bad idea to consistently say, 'We have now reached the end of the solar system, and there is nothing beyond what we already know,'" Konstantin Batygin, the other co-author, says in the video.

As Washington Post science reporter Rachel Feltman writes, the existence of Planet Nine is far from proven.

The telescopic hunt for Planet Nine is on.