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Date: Prior to 1993

Is Pluto a terrestrial planet or a gas planet?

Here is some info, mostly from the McGraw-Hill Encyclopedia of Science and Technology: Pluto's radius is approx. 715 mi (1150 km), so it is smaller than our moon (whose radius is 1080 mi, or 1738 km); its moon Charon has a radius of approx. 368 mi (593 km), so Pluto-Charon is the closest thing we have in our solar system to a double planet. The surface temperature is believed to be about 50 degrees K, or -370 degrees F. Though it is usually the 9th planet in terms of distance from the sun, Pluto has actually been closer to the Sun than Neptune since sometime in 1989 and will continue this until sometime in 1999. This is because of its eccentric orbit: its minimum and maximum distances from the sun are approx. 2.76 billion miles and 4.55 billion miles (4.43 billion km and 7.32 billion km) respectively. One Pluto "day" equals 6.4 Earth days and a Pluto"year" equals about 248 Earth years. According to the January issue of Sky and Telescope, Pluto's mass is about 1.3 x 10^25 gr or roughly 1/5 that of our moon, and Charon's mass is about 1/12 of Pluto's mass. This impliesthat Pluto's average density is around 2 gr/cc, which suggests that it is made up of about equal parts of ice and rock; Charon, on the other hand, has an average density of only 1.2 - 1.3 gr/cc, which suggests that it is mostly ice. If true, this makes it very unlikely that Pluto and Charon formed together. There is much more info out there, in encyclopedias and astronomy books. Do not be satisfied with the little bit written here!

RC Winther

Pluto is a terrestrial planet. Only Jupiter, Saturn, Uranus, and Neptune are gas planets.

John Hawley

Update - March 2011
As of 2011, Pluto has been listed as a dwaft planet. It is still terrestrial, however.


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