Pluto
Pluto's Basic Properties
* Controversial "ninth planet"
* No longer recognized by many astronomers as a planet
* Smaller and lower in mass than the other eight planets
* average distance from Sun 39.5 AU
* orbital period 248.54 yr
* mass = 0.0022 EM
* Radius = 0.182 ER
* 1 large moon (Charon)
* period of rotation 6.39 days
* eccentricity (elliptical) 0.248
* inclination of orbit 17.15°
For many decades Pluto has been known as the ninth planet. Discovered by Clyde Tombaugh in 1930, this object does not seem to fit any of the traditional catagories of the other members of the Solar System. It is far too low in density and distant from the Sun to be considered a terrestrial planet. It is much to small to be grouped with the jovian planets and has an orbit that is more eccentric and inclined to the plane of the Solar System than any of the other planets. It is too large to be a comet or and too icy to be an asteroid.
Since no spacecraft has ever visited Pluto the best photographs of the "planet" we have come from the Hubble Space Telescope (shown above). However, because of its similar properties (size, composition and atmosphere) to Neptune's moon Triton, Pluto is probably very close in appearance. Below is a rough computer rendering of Pluto using the data from the Hubble Space Telescope and the Voyager photographs of Triton.
The mass of Pluto is only 4% that of the next lightest planet Mercury, and 0.22% that of the Earth (its extremely low mass has been the basis for many astronomers to deny its status as a planet). The radius of Pluto is 18% that of Earth (2/3 that of the moon) and it has 165 times less volume than the Earth.
Pluto's orbit is highly eccentric (elliptical), moving closest to the Sun than Neptune during its perihelion and almost 50 times further from the Sun than Earth during its aphelion (furthest distance from...