Pluto was discovered in 1930 by Clyde Tombaugh
Pluto is the only solar system planet not yet visited by a spacecraft. A flyby is proposed early in the next century.
Pluto is approximately 2300 km in diameter, about two-thirds the size of Earth's Moon, and 1,200 times farther away.
Pluto has the most eccentric orbit of any planet. It averages about 39.4 AU from the sun, varying from about 29.7 to 49.2 AU. (e = 0.248).
Because of its great eccentricity, Pluto is closer to the sun at perihelion than Neptune. Pluto's orbit crosses the orbit of Neptune, and it will remain closer to the sun than Neptune for a period of about 20 years, until 1999. Pluto was at its closest point to the sun in late 1989.
Pluto's orbit is tilted at 17 degrees to the ecliptic, and hence, Pluto and Neptune will never collide.
Its orbital period (year) is 248.6 years long.
Pluto's day is 6.39 Earth days long.
Pluto has one moon, Charon, which was not detected until 1978.
Pluto is named for the mythological Roman god of the underworld; Charon was the mythological boatman who ferried the souls of the dead across the river Styx to the underworld.
For the first time, astronomers have seen details of Pluto's surface using the Faint Object Camera aboard the Hubble. Pictures cover nearly the entire surface of Pluto, taken over 6.4 days.
These images were made in Late June and early July 1994, and have just been released (March 7, 1996).
Pluto is a complex planet with more large-scale contrast than any planet, except Earth. Various light and dark areas have been seen on Pluto's surface, including a ragged northern polar cap cut by a dark strip, a bright spot, a cluster of dark spots, and a bright linear marking.
Images of Pluto Credit: Alan Stern (Southwest Research Institute), Marc Buie (Lowell Observatory), NASA, and ESA.
The smaller images at the top are actual images from Hubble, taken when Pluto was 3 billion miles from Earth. Each square pixel is more than 100 miles across.
The larger images are from a global map constructed from computer image processing of the Hubble data.
Surface Map of Pluto; see also gif (111 kB) of Pluto's surface.
Some of these features may be caused by topographic features, such as basins or imapct craters.
Most of these surface features are likely produced by frost that migrates across its surface with orbital and seasonal cycles. Pluto is so cold that nitrogen, carbon monoxide, and methane gases would partially freeze on its surface during the part of its year when it is farthest from the sun.
During Pluto's closest approach to the sun in 1989, the warm season, the frozen gases would sublimate from ice directly to gas, increasing its atmospheric pressure.
At present, Pluto has a relatively warm temperature of -350 degrees F in the dark areas, and a cooler -380 F in the bright colored icy areas.
These temperature diffreences probably cause high winds on Pluto's surface.
The light regions on the planet are bright, and are probably due to ice (frozen gases).
The dark areas may be hydrocarbon residues on the ice, resulting from the effects of ultraviolet radiation from sunlight and cosmic rays on Pluto's ice.
Pluto is unique. It is neither a terrestrial planet nor a jovian planet.
It behaves somewhat like a comet by periodically warming and losing its atmosphere to space, but Pluto is far too large to be a comet.
Pluto undergoes strong seasonal changes as a result of its elliptical orbit. "Pluto may be the last survivor of a lost population of objects called ice dwarfs that inhabited the primeval solar system."
It may be a "distant cousin" Triton, Neptune's moon.
Or it may have other "relatives" in the Kuiper Belt, a disk of icy debris left over from the birth of the solar system.
Pluto and Triton survive because they have found gravitational positions in the solar system where they remain in stable orbits. Interestingly, Pluto and Neptune are in resonance orbits; Neptune circles the sun three times for every two orbits of Pluto. So, "Pluto never gets close enough to Neptune to be thrown out of the solar system." Triton was captured by Neptune's gravity. The other ice dwarfs that formed within 50 AU from the sun are believed to have been ejected by gravitational interactions with the Jovian planets.
Pluto and Charon are an example of a "double planet". The are reasonably close in mass and orbit around a common center of gravity.
Charon may have formed as a result of a collision between Pluto and another object, in much the same way at Earth's Moon is believed to have formed.
