Note: Kuiper was a Dutch-born American astronomer best known for his study of the surface of the Moon. He discovered Miranda and Nereid, satellites of Neptune, and found an atmosphere on Titan. Dr.Kuiper was solidly Americanized: his name is pronounced to rhyme with viper.
Pluto to date [2006] has not been photographed at close quarters. Low resolution images remain interesting. Pluto is mostly brown. The map at right captures the true colors of Pluto as well as the highest surface resolution so far recovered. The map was created by tracking brightness changes during times when the 'planet' was partially eclipsed by its moon Charon. The map therefore shows the hemisphere of Pluto that faces Charon. Pluto's brown color is thought to be due to frozen methane deposits altered by faint sunlight. The dark band below Pluto's equator is seen to have rather complex coloring, indicating that some unknown mechanism has affected the surface. The map is here blurred to better represent a low resolution photograph.
Pluto has one large moon called Charon. The hemisphere of Charon that faces Pluto has been imaged in the same way in this reconstruction. Pluto is in an eccentric orbit that for one extended period brings it closer the the Sun than Uranus. It has long been suspected that Pluto is a planet in the sense of a wanderer, but not a true planet in the sense of its derivation from the inner proto-planetary disc that is believed to form during the initial stages of a star's condensation. In 2006 Pluto was finally officially classified as a Pluton, a Kuiper belt object, rather than a regular planet.
The Kuiper belt is a disc in the plane of the ecliptic that extends maybe five or ten times the diameter of the orbit of Neptune. The Kuiper belt was - until the 1990's a purely conjectural entity, proposed by Kuiper in 1951.
| The Kuiper belt should not be confused with the Oort cloud that is the conjectural (1950) source of comets. The Oort cloud is very much larger in diameter and more nearly spherical. |
Several large dark bodies have recently been found in the Kuiper belt. One appears to be two-thirds the size of Pluto and the second is about twice the size, in a 560 year elliptical orbit that takes it from twice the distance of Pluto to inside the orbit of Neptune at an inclination of 45°. The speculation is that all these bodies, including Pluto and its moon, are members of a class of dark, methane surfaced bodies, that orbit in the Kuiper region.
A few asteroids, called Centaurs, are known to orbit in the outer solar system. They include 5335 Damocles that ranges from near Mars to beyond Uranus and 5145 Pholus that ranges from the orbit of Saturn to beyond Neptune. The orbits of these objects are more like the orbits of Plutons than those of ordinary asteroids. We can expect a long succession of announcements over the next century, as more observations become available.