PLUTO

in astronomy, ninth major planet of the solar system. It is named after the Greek and Roman god of the underworld. Pluto is normally the outermost of the known planets and the most distant from the Sun; its mean distance from the latter is calculated at about 5.9 billion km (3.7 billion miles). Pluto's orbit, however, is so eccentric (0.251) that the planet comes closer to the Sun than Neptune does at times around perihelion (about every 248 years), as for example between 1979 and 1999. During this 20-year interval Pluto will remain inside Neptune's orbit. Pluto's orbit has an inclination of 17.1 to the ecliptic, the highest of any of the major planets. Because of its great distance from the Earth, Pluto always appears relatively faint even when viewed with a telescopeno more than 14th magnitude. Pluto's brightness level varies regularly by about 12 percent in a period of 6.387 days, which indicates that the surface reflection properties of the planet are not uniform and that the planet rotates on its axis every 6.387 days. Having a diameter of approximately 2,300 km (1,430 miles), Pluto is roughly two-thirds the size of the Moon. Its mean density falls in the range from 1.8 to 2.2 grams per cubic centimetre. Because of this density value, many astronomers theorize that the planet consists largely of solid methane as well as a significant fraction of denser materials such as silicates. Infrared observations suggest strongly that Pluto has polar caps composed of methane ice. These ice caps sometimes appear to extend halfway to Pluto's equator. That the surface materials are frozen is supported by evidence of extremely low temperatures on the planet. It is estimated that the average surface temperature on Pluto ranges from 45 to 58 K. Temperature varies over the surface with local reflectivity and solar zenith angle and likely decreases by a factor of about three as the planet nears aphelion. During a stellar occultation in 1988, Pluto was observed to have a thin, distended atmosphere, composed of methane and most likely significant amounts of heavier gases, possibly nitrogen. Pluto was discovered in 1930 by the American astronomer Clyde W. Tombaugh during a systematic search for a trans-Neptunian planet predicted by Percival Lowell and William H. Pickering. In a series of photographs he had taken at the Lowell Observatory in Flagstaff, Ariz., Tombaugh recognized the new planet by its motion, which was much slower than that of numerous asteroids also recorded on the same photographs. Pluto has one satellite, an object of about half its size and one-sixth its mass. The satellite was discovered in 1978 by two American astronomers, James W. Christy and Robert S. Harrington, and was named Charon (q.v.) after the mythological boatman who ferried souls across the river Styx to Pluto for judgment. Pluto's origin and evolution are obscure. Pluto differs significantly from the outer planets in such physical properties as size, mass, axial rotation, orbital eccentricity, and inclination. Thus, it most likely formed and evolved in a different manner. Some investigators have proposed that Pluto may be a runaway satellite that was originally part of Neptune's system. Not only do Pluto's properties resemble those of Triton but its orbit almost intersects Neptune's periodically. The discovery of Charon makes this theory unlikely, however. Current models suggest that Pluto and its satellite were instead formed as two independent bodies in the solar nebula. Pluto may have collided with a proto-Charon, with the present satellite accreting from the resulting debris. The never-before-seen surface of the planet Pluto, resolved in these pictures taken with the normally the planet farthest from the Sun (see below). It is named for the god of the underworld in Roman mythology (Greek: Hades). Pluto, designated in astronomy, is so distant that sunlight traveling at 299,792 km per second takes more than five hours to reach the planet. An observer standing on the planet's surface would see the Sun as an extremely bright star in the dark sky, providing Pluto with only 1/1600 the amount of sunlight as that reaching the Earth. Pluto's surface temperature is therefore expected to be only 3550 above absolute zero, suggesting that its surface may consist largely of ices of nitrogen, methane, and carbon monoxide. Pluto has a single natural satellite, Charon. Their dimensions are sufficiently similar that it has become common to speak of the Pluto-Charon system as a double planet. Additional reading Clyde W. Tombaugh and Patrick Moore, Out of the Darkness: The Planet Pluto (1980), is an account of the planet's discovery by its discoverer (Tombaugh). Reviews of the search, discovery, and first 50 years of observations of Pluto are given in A.J. Whyte, The Planet Pluto (1980); and William Graves Hoyt, Planets X and Pluto (1980). The discovery of Charon is reported in articles by its discoverers, J.W. Christy and R.S. Harrington: The Satellite of Pluto, The Astronomical Journal, 83(8):100508 (August 1978), and The Discovery and Orbit of Charon, Icarus, 44(1):3840 (October 1980). A technical review is S. Alan Stern, The Pluto-Charon Systems, Annual Reviews of Astronomy and Astrophysics, 30:185234 (1992). Mark Littmann, Planets Beyond: Discovering the Outer Solar System, updated and rev. ed. (1990), chronicles the history of the discovery of Uranus, Neptune, and Pluto. Tobias Chant Owen