Ask A Scientist© Astronomy Archive

Name:      brian
Status:    stduent
Grade:     6-8

Question:  What is known about Pluto?
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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!

rcwinther
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Update:July 2008
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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.

http://www.iau.org/public_press/news/release/iau0804/

P. Bridges
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