|Kerrold, Robin. The Heavens. Chicago: Fetzer, 1991: 143.||"Mass of Pluto (earth = 1), 0.002(?)"||0.002 earth masses|
|The Dorling-Kindersley Science Encyclopedia. New York: Dorling-Kindersley, 1993: 418.||"Mass (earth = 1) Pluto = 0.002"||0.002 earth masses|
|Hoyt, William Graves. Planets X and Pluto. Tucson, AZ: University of Arizona, 1980: 221.||"Mass of Pluto <0.7 (1930)
(Published by Crommelin in 1931)"
|0.7 earth masses|
|Cook, A. H. Physics of The Earth and Planets. New York: Wiley, 1973: 260.||"Pluto = 6.6 × 1023 kg"||0.01 earth masses|
|Sagan, Carl & Leonard, Jonathan N. Planets. New York: Time, 1966: 193.||"Pluto = 0.8 (approx.) (mass relative to earth)"||0.8 earth masses|
Pluto is the smallest, furthest, and lightest planet in the solar system. It is so small and faint that it is only distinguished from a star by its motion. It is so light that it is unlikely that much of an atmosphere can exist around the planet. Because of this, observations of Pluto are subject to considerable inaccuracy. It was revealed by more recent sources that Pluto has a mass of 0.002 times that of the Earth. The latest estimates of Pluto's mass are based on observations of the orbital motion of Charon (Pluto's moon). Previous measurements based on the deviations of Uranus and Neptune from their orbital paths were much larger (up to 0.8 earth masses). From these observations, it is difficult to say what the mass of Pluto really is. Even to estimate it from an average would be difficult because the masses found in different sources are so different from each other.
Allison Mak -- 1999