The anomalies in the KBOs are the key to their evidence.  They meticulously examined the data from the region:

Well, when you’re suggesting that something as large as Planet Nine – which is estimated to be 10 times more massive than Earth and four times the size – exists within a fairly crowded patch of the Universe, you’d expect to see its considerable gravitational forces affect the smaller things floating around nearby. 

And that’s exactly what Brown and his colleagues have been identifying. 

“We saw a strange signal in the data that meant something odd was going on in the outer Solar System,” Brown told Ian Sample at The Guardian back in January.

If and when they do discover this planet what do you think this will tell us?  And do you there would be a high likelihood of extraterrestrials on it?

thanks to shockingscience.com for the great info