Ask A Scientist© Astronomy Archive

Author:      honest abe
Some of our resources have said that Mercury is the smallest planet others
say that it is Pluto.  What is correct?  

Response #:  1 of 1
Author:      rcwinther
My sources differ on the specifics, but they agree that Pluto is by far the
smallest planet.  The Encyclopedia Britannica (1990) says that Pluto's
diameter is 3500 km (about the size of our Moon); my 1994 almanac says that,
based on further observations this number has been reduced to only 2300 km. 
Mercury, on the other hand, has a diameter of about 5000 km.  Because Pluto
is so far away, accurate observation is difficult even with our largest
telescopes.  If the proposed Pluto flyby mission does occur, much better
measurements will be obtained.