Some scientists are convinced that the orbits of Uranus and Neptune are being distorted by the gravitational pull of a planet beyond Pluto. The recent discovery of minor members beyond Pluto which could be responsible for this distortion makes the existence of a tenth planet unlikely. New planets are being discovered continually, however, not in our Solar System, but orbiting other stars.
Pluto has been downgraded from ‘ninth planet’ after we found the rest of the Kuiper belt, including Eris which is more massive, though with a slightly smaller volume. Even before that, the apparent need for an extra planet to explain deviations in the orbit of Neptune no longer applied. Voyager 2 made more accurate measurements and found nothing out of line. (And found that Neptune was more massive than Uranus, despite its smaller radius and volume.)
Anything large enough to count as a 10th Planet would have been expected to have caused disturbances in the orbits of known planets and in the Voyager and Pioneer probes as they moved outwards.
There is a small possibility of one or more Earth-sized bodies very far out. Under current rules these would probably not count because they would probably not have cleared their orbits of similar bodies.
Picture Credit : Google