Did We Just Find The Ninth Planet?

Researchers believe they may have found a ninth planet in our solar system, but why do they think that?

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Caltech Researchers Find Evidence of a Real Ninth Planet (CalTech)
"Caltech researchers have found evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of Earth."

The Kuiper Belt (NASA)
"In 1992, Dave Jewitt and Jane Luu at the University of Hawaii discovered a small object, designated 1992QB1, orbiting the Sun beyond Neptune at a distance of about 40 AU. Since then, more than 1,000 similar objects have been discovered beyond Neptune's orbit, and scientists estimate there are several hundred thousand objects bigger than 20 miles across waiting to be discovered in that vast region."