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How far from the Sun is Pluto?

Here is some information, mostly from the McGraw-Hill Encyclopedia of Science and Technology: Pluto's radius is approximately 715 miles (1150 km), so it is smaller than our Moon (whose radius is 1080 miles, or 1738 km); its moon Charon has a radius of approximately 368 miles (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 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 1979 and will continue this until sometime in 1999. This is because of its eccentric orbit: its minimum and maximum distances from the Sun are approximately 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 grams or roughly 1/5 that of our Moon, and Charon's mass is about 1/12 of Pluto's mass. This implies that Pluto's average density is around 2 g/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 g/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 information out there, in encyclopedias and astronomy books. Do not be satisfied with the little bit written here!

RC Winther

July 2008
Mercury is currently the smallest planet, as of Summer 2008. There are several historical reasons for the the Mercury vs. Pluto confusion.

First, even after Pluto was well-known, early estimates of it's mass were grossly inaccurate and indicated it was larger than Mercury, making Mercury the smallest planet. After several reductions in the estimate over the decades, a study in 1976* showed it to be quite small, making Pluto the smallest planet. This was a bit complicated by Pluto's having a moon that many felt was actually a binary planetary system*, thus potentially making it's moon Charon the smallest planet. In 2006, the IAU (Int'l Astronomical Union) demoted Pluto to the status of "Dwarf Planet"***, making Mercury again the smallest planet.

As of June 2008, the IAU reclassified Pluto as a "Plutoid"****, although this is just a special type of dwarf planet (in particular, having an orbit beyond Neptune), so Mercury remains as the smallest true planet.

* "Pluto's albedo is 1.3–2.0 times greater than that of Earth". Pluto Fact Sheet. NASA ** B. Sicardy et al. (2006). "Charon's size and an upper limit on its atmosphere from a stellar occultation" ** "IAU 2006 General Assembly: Resolutions 5 and 6", IAU, 2006-08-24.

P. Bridges

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