Introductory Astronomy: The Planet PLUTO

Basic Facts:
Distance from Sun: 29.7 - 49.3 AU
Period of revolution about Sun: 248.6 years
Period of rotation: 6 days, 9.3 hours
Radius: 0.18 Earth radii
Mass: 0.0025 Earth masses
Average Density: 2.3 g/cm^3
Magnetic Field: not detected
Surface features: methane ice, cratered surface
Atmosphere: thin, mostly methane
Temperature: -370 degrees Farenheit

Pluto is not a gaseous planet because it is not made mostly of gas. It is not a terrestrial planet, either, because it is not made mostly of rock and metal. It is smaller than earth and travels on a highly elliptical orbit; so elliptical, in fact, that it periodically crosses the orbit of Neptune. (At these times, Pluto is actually the 8th planet from the Sun and Neptune the 9th! In fact, Pluto is currently the 8th planet and will be until 1999.) Pluto and Neptune will never collide, though, because Pluto's orbit is inclined at 17 degrees to the plane of the other planets.

Pluto has a very thin atmosphere, made mostly of gaseous methane. It has solid methane ice on its surface. (Pluto is so cold that elements like methane (which are generally in gas form) freeze into solids.) In fact, the entire planet is an ice-rock mixture.

Pluto has one moon, Charon , which orbits it once every 6 days. Charon is about half the size of Pluto. (This means that, if we were standing on Pluto, Charon would appear 14 times larger than our Moon does from earth.) However, Charon is much less dense than Pluto, which means that it must contain more ice and less rock.

Some astronomers believe that Pluto and Charon (and possibly Triton, one of Neptune's moons) are remnants of an icy "dwarf" planet population in the outer Solar System. If there used to be thousands of these small, icy bodies, collisions would have been much more common in the early days of the Solar System. This could explain the irregularities in the orbits of Neptune's moons and the tilted axis of rotation of both Uranus and Pluto. Other astronomers believe that Pluto may be an escaped planetary moon. The fact the Pluto has a moon of its own makes this harder to believe, however. It could be that Pluto is a normal planet which formed as the planets did.

To learn more about Pluto and Charon, click here.