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The @stro object for the week of 01/24/2000

image of Pluto

image courtesy of STSI/HST

Pluto. Pluto is our solar system's ninth planet and is named after the Roman god of the underworld. Pluto is a small rocky body whose planetary status in recent years has been questioned. Some astronomers wish to classify it as a large minor planet (or asteroid) rather than a planet. They have a case, but it was considered a planet since its discovery and will likely remain so. Pluto, which has a diameter of 2274km, is orbited by its own moon named Charon. Pluto is the only planet in the solar system that has not yet been visited by space probes. It rotates in about 6.3 days, has an orbital distance from the Sun of 39 A.U. and takes about 248 years to complete one orbit.

Discovered by the late Clyde Tombaugh at the Lowell observatory in 1930, Pluto is by far smaller than any of the other planets and is even smaller than Earth's Moon. The search for Pluto resulted from a discovery (which was later proven errant) that Neptune's orbit had perturbations that couldn't be accounted for and a planetary mass even farther than Neptune had to exist. This led to the search that eventually turned up Pluto, even though Pluto's mass was later found to be too small to account for the error. The real error came from the original calculations for Neptune's mass, and if new figures from Voyager 2 are used, the error in Neptune's orbit is non-existent.

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