The ninth, and perhaps the most distant planet from
the Sun, Pluto has a variable orbital distance of about 30–40
astronomical units (between 2.7 billion miles, or 4.4–6 billion
km) from the Sun, circling once every 249 Earth years, and
has a retrograde rotation period of 6.4 Earth days. It is a
small planet with a mass of 0.003 Earth masses, and a diameter
of 1,400 miles (2,250 km; only 20 percent that of Earth),
and a density of 2.3 grams/cubic centimeter. It has one
known moon, Charon. Some scientists debate whether Pluto
should be considered a planet or a captured asteroid, supported
by the large, 17.2° inclination of its orbital plane with
respect to the ecliptic plane.
The physical properties of the Pluton-Charon system
suggest that it is an icy planet system similar to some of the
Jovian moons, being most similar to Neptune’s moon Triton.
Models for the origin of Pluto range from it being a captured
icy asteroid, an escaped moon, to being a remnant of material
left over from the formation of the solar system. The great
distance and small size of the system make it difficult to
observe, and certainly as deep planetary probes explore the
outer reaches of the solar system, new theories and models
for the origin and evolution of this system will emerge. In
2005 scientists announced that they may have discovered a
tenth planet orbiting the sun, beyond the orbit of Pluto.
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