|Lodders, Katharina & Bruce Fegley, Jr. The Planetary Scientists Companion. New York: Oxford University Press, 1998: 237-240.||"the surface temperature of 40 K implies a surface pressure of approximately 58 millibar"||40 K|
|"Pluto." The World Book Encyclopedia. Vol. 15, Chicago: World Book, 1999: 580-589.||"Astronomers believe the temperature on Pluto may be about −390 to −370 degrees F (−230 to −220 degrees C)"||43–53 K|
|Hamilton, Rosanna L. Pluto. Views of the Solar System.||"Due to its great distance from the sun, Pluto's surface is believed to reach temperatures as low as −240 degrees C (−400 degrees F)."||33 K|
|The Heavens. Chicago: World Book, 1989: 120-121.||"Its estimated temperature: −369 degrees F (−223 degrees C) also makes the presence of a gaseous atmosphere unlikely."||50 K|
Pluto was the last planet to be discovered. Astronomers noticed discrepancies in the orbital paths of Uranus and Neptune. From this observation they hypothesized that there was some other planet whose gravitational pull was exerting an influence on the motions of Uranus and Neptune. After many years of searching, this ninth planet was discovered by accident in 1930 by Clyde Tombaugh, who named it Pluto. In 1980 Tombaugh was only able to describe the planet as being "a huge iceberg", but by 1990 Lodders and Fegley were able to state more definitively "Pluto's surface is dominantly nitrogen (N2) ice with smaller amounts of methane (CH4) and carbon monoxide (CO)."
Since Pluto is so far from the sun its temperature is estimated to be as low as 33 kelvin. Because it is so cold, it is unlikely for there to be a gaseous atmosphere: however, it may have an unstable atmosphere containing methane gas because methane has such a low sublimation point (79 to 89 K). Pluto has a density of less than 1.3 times that of water, which makes it one of the lightest planets in the solar system. Since there is some sort of methane atmosphere around Pluto, some heavier elements must be present, possibly in the planet's core which exert gravitational force on the methane gas.
Paul Stengel -- 2000