Planet 10 is a hypothetical planet that might be 30-55 AUs from the Sun, and somewhere in the Kuiper Belt. It is
predicted that it is a rocky planet with a similar mass to Mars. It is the ninth planet from the sun.
Planets typically orbit around a completely horizontal orbit around their stars, but objects in the Kuiper Belt (TNOs) that orbit around the same semi-major axis as Planet 10 seems to have their orbits tilted around 8 degrees of the invariable plane (Its typical inclination of its orbit). Only something as massive as Mars would be able to tilt an orbit like that. Planet 9 couldn't possibly be the culprit of this, due to its ridiculously distant orbit.
Discovery Attempt Edit
The Large Synoptic Survey Telescope (LSST), which is currently under construction in Chile, may be able to find Planet 10 (if it exists, of course), other dwarf planets, and other KBOs. Scientists think that LSSt will probably be able to discover 2,000 to 40,000 objects and hypothetical planets.