| Pluto |
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The ninth planet of the solar system, discovered as a fifteenth magnitude object on 18 February 1930 from the Lowell Observatory by Clyde Tombaugh. Searches for a planet beyond Neptune had started in 1905, stimulated by apparent discrepancies between the calculated and observed orbits of Uranus and Neptune. However, it is now known that the mass of Pluto is less than one-fifth that of the Moon, insufficient to have any gravitational effect on Uranus and Neptune. Pluto's orbit is more highly inclined to the ecliptic and more eccentric than that of any other planet. Its distance from the Sun ranges between 30 and 50 AU. Perihelion occurred in 1989 and, between 1979 and 1999, Pluto's orbit brings it nearer the Sun than Neptune. The discovery of Pluto's satellite, Charon, in 1978, made it possible to obtain improved values for the planet's diameter and mass. The diameter is 2,300 ± 40 kilometres. Pluto's overall density is approximately twice that of water and it is thought likely to consist of a thick layer of water ice overlying a core of partially hydrated rock. Charon and Pluto are locked in synchronous rotation with a period of 6.39 days. Pluto's rotation axis is inclined at 122° to the plane of the ecliptic so that, like Uranus, it rotates in a retrograde sense, "lying on its side". A rare series of mutual occultations and transits took place between 1985 and 1990. Such events, as viewed from Earth, take place only twice in the planet's 248-year orbital period. They made it possible to distinguish the spectral signatures of Pluto and Charon and to construct the first approximate albedo maps of Pluto's surface. These confirmed previous suspicions of a highly non-uniform and variable surface based on the change of brightness over the rotational period, and in the longer term. In contrast with Charon, which is grey, Pluto's surface is reddish in colour. Methane ice was detected on Pluto in 1976 by infrared spectroscopy. The occultation of a star by Pluto in 1988 revealed the presence of an extended tenuous atmosphere. Nitrogen and carbon monoxide ices were discovered on the surface in 1992. The surface temperature is about 40 K. In 1996 observations with the Hubble Space Telescope resolved broad light and dark features on Pluto's surface for the first time. |