Since the discovery of Pluto by Clyde Thombaugh on February 18, 1930, astronomers have searched for additional planets beyond the orbit of Pluto. In the 1980's there was a new flurry of interest in this when Bob Harrington at the US Naval Observatory tried to account for some irregularities in the orbit of a comet as indicating a large new planet orbiting the Sun at a very high inclination angle to the ecliptic plane. The prediction of where this object might be was pretty imprecise and covered a chunk of sky about 10 times the diameter of the Full Moon on a square side. I collaborated with Bob in 1985 when the infrared data from the Infrared Astronomical Satellite became available, but after sifting through several thousand infrared sources, I found several hundred that under various circumstances might be candidates for Planet X. A further study of these revealed that there were other equally plausible explanations for these infrared sources such as known stars, galaxies and the like.

Because sunlight is so weak at these distances, only very large telescopes would be able to detect planets the size of the Earth or larger, well beyond the present limits to the solar system. Still, astronomers have found over 400 trans-Neptunian minor planets smaller than Pluto. The figure below shows the locations of these objects as of March 2004:

The orbits of Jupiter, Saturn, Uranus and Neptune are shown....can you spot Pluto?

There are three known objects that are over 500 miles in diameter! The latest of these called 2004 DW is at least as big as Pluto's satellite Charon! So who knows. In 10 years a real big object may be detected further out! Right now, PLuto is where the line is drawn for defining the smallest planet, due to its history and 'fame'.