Tombaugh's find was in fact an accidental, he had in fact been trying to locate the mysterious "Planet X" which had erroneously been predicted to exist by several eminent experts, in an attempt to account for a perceived "missing mass" within our Solar System.

How far is the Planet Pluto away from the Sun ?
Pluto is the furthest planet in our Solar system from the Sun, the average distance being 5913 million Km (39.5 Au), Earth by comparison is only a fortieth this distance away from the sun.

How big is it? Interesting Facts about the Planet Pluto
The Planet Pluto is the smallest Planet in our Solar System, it only has a diameter of about 2320km which is only about a fifth of the size of the Earth, it has the second smallest mass of all the Planets in our Solar System, approximately 1.29e22 kg.

What is it like on the surface of the Planet Pluto ?
Cold, very cold, there is a relatively small range of surface temperature on this Planet, varying from  approximately -235 to -210 C, these very cold temperatures reflect the Planet's great distance from the Sun. We know little of the make up of the Planet, and will not know much until it is visited by a Space Probe, but it is probable than it is two thirds rock and one third ice.

Does the Planet Pluto have any Moons ?
Although most of the Planets in the Solar System posses multiple moons Pluto is one of the few planets which has only one, Charon, nearly as big as the Planet Pluto it's self ! Charon was discovered in 1978 by Jim Christy, and is named after the mythical ferryman who was said in Roman mythology to ferry the dead across the River Styx into the Underworld.