Since 1999, when Pluto moved outside of Neptune's' orbit, it has been the farthest planet from the sun as it usually is. For a period of time (approximately 20 years) it is only second farthest. Little is known about this tiny planet, and no Earthly spaceships have yet visited it. Pluto is unusual for several reasons. Besides being smaller than seven of our system's moons its orbit is highly eccentric and highly inclined and its axis is tipped far from the perpendicular. It does not fit tidily into the categories of terrestrial or jovian worlds. It could be that Pluto is an asteroid or an object from the Kuiper Belt, but traditionally it is still recognized as a planet. Because its moon Charon is quite large by comparison (approximately one half the size of Pluto), the Pluto/Charon pair are often considered a double planet.

Most of what we know about Pluto has been from observations in 1978 when Charon was discovered. At that time the orbit of Charon and Pluto around each other was such that they alternately passed in front of each other and measurements were taken from their mutually occulting brightnesses. From this data, astronomers have been able to map light and dark areas of both bodies.

In 1930 calculations were made that predicted the existence of a ninth planet. These calculations were based on the orbits of Uranus and Neptune. Clyde W. Tombaugh is credited with discovering the planet after surveying the sky from Lowell observatory in Arizona. The calculations however, were later discovered to be erroneous and Voyagers' measurements of Neptune's mass actually accounted for the perturbations that Pluto's existence was to account for. Nevertheless, Pluto exists as the ninth and last planet in our solar system.

Very little is known about the distant planet but estimates of its density (2g/cm³) suggests that it is perhaps 70% rock and 30% water ice. The bright areas, have been postulated to consist of frozen nitrogen, solid methane and carbon dioxide. Its thin atmosphere is likely a combination of nitrogen, carbon monoxide and methane. Its eccentric orbit though, means that the atmosphere would only exist at perihelion and would freeze as it moves away from the sun.

Charon's size is considerable when compared to Pluto and both bodies are incredibly close to one another. Astronomers have been able to determine the combined mass of Pluto and Charon, but neither one has been determined with sufficient accuracy by itself. Both bodies orbit each other at a distance of about 19640 km (12275 mi) and each is synchronous with the other; that is, each keeps the same face pointing towards its neighbour throughout their mutual revolution. Charon is approximately one half the size of Pluto making it the largest moon relative to its primary.