Pluto, the most distant known planet, was discovered in 1930 by Clyde Tombaugh as a result of a systematic search for a trans-Neptunian planet initiated by Perival Lowell. The planet's orbit is inclined and eccentric. While it orbits the Sun once every 248 years. In 1989 Pluto will pass through perihelion and between 1979 and 1999 it will be closer to the sun than Neptune. The average surface temperature must be near -220 degrees but should vary by about 15 degrees from apehelion to perihelion, a significant effect at such low temperatures.
Until recently very little was known about the planet's physical characteristics. Periodic fluctuations in the brightness indicated that the planet has at most a tenuous atmosphere, a spotted surface, and a spin period of 6.4 days. In 1978, a satellite, Charon, was discovered by American astronomer James W. Christy, making it possible to obtain an accurate mass for the Pluto system. Pluto is 1.7 times brighter than its satellite and its mass is only 0.2 percent that of the Earth. Pluto is an icy object that probably contains frozen methane. Spectral evidence of methane ice on the surface was reported in 1976. Small amounts of methane gas have since been reported. Pluto is the smallest of the planets. In fact, it is slightly smaller and much less massive than our moon.
Charon orbits Pluto every 6.4 days at a distance of 20,000 km. The orbit is highly inclined to the ecliptic and the observed light variations of Pluto cannot be attributed to mutual eclipses. The satellite is about 1,400 km across, making it larger relative to its planet than any other moon in the solar system. The provenance of this tiny "double planet" remains a mystery, but the idea that Pluto is an escaped satellite of Neptune seems unlikely given the double nature of the system.
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