Pluto is the farthest planet from the Sun (usually) and by far the smallest.
Pluto is
smaller than seven of the solar system's moons (the Moon, Io, Europa,
Ganymede,
Callisto, Titan and Triton).
orbit:
5,913,520,000 km (39.5 AU) from the Sun (average)
diameter: 2274 km
mass:
1.27e22 kg
In Roman mythology, Pluto (Greek: Hades) is the god of the underworld.
The
planet received this name (after many other suggestions) perhaps because
it's so far from the Sun that it is in perpetual darkness and perhaps because
"PL" are the initials of Percival Lowell.
Pluto is the only planet that has not been visited by a spacecraft.
Even the
Hubble Space Telescope can resolve only the largest features
on its surface
Fortunately, Pluto has a satellite, Charon. By good fortune, Charon
was
discovered (in 1978) just before its orbital plane moved edge-on toward
the inner solar system. It was therefore possible to observe many transits
of Pluto over Charon and vice versa. By carefully calculating which portions
of which body would be covered at what times, and watching brightness curves,
astronomers were able to construct a rough map of light and dark areas
on both bodies.