Pluto is the farthest planet from the Sun. It is also the smallest in mass and size, and for this reason it is 250 times dimmer to our eyes than Neptune. Neptune itself is so dim that it was only discovered after the invention of the telescope, and then only after accurate calculations of other planets' motion indirectly had hinted to scientists where to aim their telescopes. But dim as it is, Pluto's own discovery had much more to do with chance: it resulted from a broad, blind search of the sky, with the simple goal of finding any undiscovered planet in the solar system.

Pluto's orbit is very eccentric, and its plane forms a large angle with the plane of the ecliptic. The orbit is so elongated that for about 20 years, between 1980 and 1999, the planet was actually closer to the Sun than Neptune. Pluto's period of rotation was recently found by imaging the planet with the powerful Hubble Space Telescope. Pluto has a retrograde rotation, whose axis lies sideways, along the plane of the orbit.

Pluto has one moon, Charon, orbiting it in close proximity. Charon has a mass representing quite a large fraction of Pluto's, and it has a very similar density and composition. Moreover, Pluto's period of rotation is the same as Charon's, and it equals the orbital period of that moon. The harmony of all these periods may result from the large tidal forces these bodies exert on each other. The Pluto-Charon system is part of a large population of many small, mostly asteroid-sized, bodies in orbit at distances beyond 40 AU's from the Sun. The search for these objects is still in its initial phase.