Still a Mystery
For ten years after he discovered Pluto, Clyde Tombaugh continued his search, which included observing nearly 45 million stars. If there was another planet out there, even much dimmer than Pluto, Tombaugh should have seen it. Still, planets have been observed before and have gone unrecognized for what they were.
Some astronomers believe that the deviations in Uranus and Neptune are really observational errors. Before about 1910, sightings of the planets were not precise enough -- by today's standards -- to predict planetary motion. Discarding early data leaves only minute discrepancies in the orbits, which can still be explained away as observational errors. Based on this, there is little need to postulate a tenth planet as the cause of these orbital discrepancies.
Other astronomers feel differently. One theory has the outer solar system populated by hundreds of rock-ice bodies about 1000 to 2000 kilometers in diameter, orbiting at 40 astronomical units from the Sun. If more of these are found we might learn that, rather than having nine planets, the solar system has hundreds. Pluto would finally be in good company. By sheer number, these small bodies would be considered the "normal" planets, and the other eight planets would be the oddities.