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Beyond Neptune, in the farthest reaches of our solar system, is Planet Nine.
At least, that is what two Caltech professors believe. This isn't a dwarf planet, like the recently downgraded Pluto. It's up to 10 times as massive as Earth. Also, it's far away — about 20 times as distant as Neptune, with an orbit around the Sun that could take as long as 20,000 years to complete.
It seems like an incredible discovery. The problem? Nobody has actually seen the planet.
"We see how it has pushed around objects in the distant solar system, so we know it's there," Mike Brown, one of the Caltech researchers behind the discovery, told NBC News. "We haven't spotted precisely where it is yet."
How to find a planet
It all started 13 years ago with the discovery of Sedna in the Kuiper Belt, the distant region of our solar system beyond Neptune.
Smaller than Pluto, Sedna had a strange orbit. It was affected by the gravitational pull of something large — but that something clearly wasn't Neptune. As scientists began discovering more objects in the Kuiper Belt, a pattern started to emerge.
"There was no 'ah-ha!' moment," Brown said. "We kept trying to prove it was something else, because a planet seemed ridiculous."
Brown and Konstantin Batygin, whose findings where published Wednesday in The Astronomical Journal, weren't the only people noticing these odd, far-flung orbits.
Two years ago, at the Carnegie Institution for Science in Washington, D.C., astronomer Scott Sheppard also found objects in the Kuiper Belt that seemed to be clustering around some unknown body. He thought it might be a planet, but couldn't be sure.
"We expected people either to debunk those results or dive deeper," Sheppard told NBC News.
Nobody has debunked his research. Instead, the new paper puts his work "on a much more stable foundation," he said.
The turning point for Brown was when he and Batygin started predicting the presence of objects, and then found them.
Five of those objects were entering the plane of the solar system at a 90-degree angle, which is very hard to explain without the presence of a large planet, he said.
"In my head, that was the moment where I thought, 'Oh my god, this is real,'" Brown said. He also found six objects in the Kuiper Belt whose clustering was only given a 0.007 percent chance of being random.
The evidence of Planet Nine was mounting. It seemed fitting that Brown would make the discovery, since he was the one who found Eris in 2005, creating the category of "dwarf planet" that would eventually be used to define the former ninth planet of the solar system, Pluto.
But why did it take so long to find a planet that — in the grand, cosmic scale — is basically in our backyard?
Rapidly evolving technology
Our solar system is a speck in the universe, but by human standards, it's still really big: around 9 billion miles across, according to NASA.
In the last few years, scientists have been able to analyze more of it than ever. While telescopes are getting bigger, that really isn't what is pushing the change. Construction of the Samuel Oschin telescope, which was used to find Sedna, was finished at the Palomar Observatory near San Diego in 1948.
The driver of change has been the cameras that are attached to the telescopes. Compare the clunky digital cameras of the 1990s, which could capture around 1 megabyte of data, to the 12-megapixel camera on the iPhone 6S. A similar shift happened in astronomy.
"The cameras are getting bigger and better," Sheppard said. That allows researchers to not only spot fainter objects, but to cover a wider area of space more quickly.
For example, a swath of sky that would take the Hubble Space Telescope 500 images to capture could take only a single image for a telescope connected to a new digital camera.
"We're looking for a needle in a haystack," Sheppard said. "In the past, we were basically looking through a straw for that needle. Now it's more like looking through a big pipe. We're able to find things much more efficiently."
In a single night, he said, astronomers can get up to a terabyte of data from a telescope. Sifting through that data for signs of moving objects takes a lot of computing power. Over the past decade, that process has also become a lot more efficient.
"Something that took a day to complete five years ago could probably be done in few hours now," Sheppard said.
What does that mean for Planet Nine? Now that astronomers know its rough orbit, it should only take a few years to nail down where, exactly, Pluto's replacement is hiding — if it exists. Brown is fairly confident that it does.
"We're pretty convinced it's there, or else we wouldn't have published the paper and potentially made ourselves look like idiots," he said. He conceded, however, that "until it's actually imaged, nobody is going to believe for sure that it's out there."