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


Replies:
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.

RC Winther

Update - March 2010
Based on the 2008 IAU classification of what is a planet, Pluto has been downgraded from a planet to a dwarf planet.

Mercury is now the smallest planet in our solar system.

PG

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