Ask A Scientist© Astronomy Archive

Question: Is Pluto a terrestrial planet or a gas planet?
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Pluto is a terrestrial planet.  Only Jupiter, Saturn, Uranus, and 
Neptune are gas planets.
John Hawley
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Subject:     Facts About Pluto
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!
Ronald Winther
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