Pluto

Orbit: 5,913,520,000 km (3,674,661,000 miles) or (39.5 AU) from Sun
Diameter: 2274 km or 1,413 miles (?)
Mass: 1.27 x 10²² kg or 2.8022 lbs (?)  

Pluto is the farthest Solar System body from the sun (usually). Pluto is smaller than seven of the solar system's moons (the moon, Io, Europa, Ganymede, Callisto, Titan and Triton).

In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it's so far from the sun that it is in perpetual darkness.

Pluto was discovered in 1930 by a “fortunate accident”. Calculations, which later turned out to be in error, had predicted a planet beyond Neptune based on the motions of Uranus and Neptune. Not aware of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.

After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued, but nothing was found at that time.

Pluto has a large satellite, or moon, named Charon. It was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies.

Pluto's radius is not well known. JPL's value of 706.5 is given with an error of +/-8, almost one percent.

Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon's orbit and Kepler's Third Law), the individual masses of Pluto and Charon are difficult to determine. This would require determining their mutual motions around the center of mass of the system, which requires much finer measurements. They're so small and far away that even HST has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157. More observations are underway.

Pluto's orbit is highly eccentric. At times it is closer to the sun than Neptune (the last time was from 1979 to 1999). Pluto rotates in the opposite direction from most of the other planets.

Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly 1.5 times longer than Neptune's. Its orbital inclination is also much higher than the other planets. Thus, though it appears that Pluto's orbit crosses Neptune's, it really doesn't; and they will never collide.

Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit.  The surface temperature on Pluto is not well known but is probably between 35 and 45 Kelvins (-378 to -396 F).

Pluto's composition is unknown, but its density indicates that it is probably a mixture of 70 percent rock and 30 percent water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays.

Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon.

The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular theory is that Triton, like Pluto, once moved in an independent orbit around the sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud. Like the Earth's moon, Charon may be the result of a collision between Pluto and another body.

Pluto can be seen with an amateur telescope, but it is not easy. Mike Harvey's planet finder charts show the current position of Pluto (and the other planets) in the sky, but much more detailed charts and careful observations over several months will be required to actually find it. Suitable charts can be created with many planetarium programs such as Starry Night.

Charon Charon ("KAIR en") is Pluto's largest satellite. Orbit: 19,640 km (12,204 miles) from Pluto Diameter: 1172 km or 728 miles (?) Mass: 1.90 x 1021 or 3.821 lb.. (?) Charon is named for the mythological figure who ferried the dead across the River Styx intolly find it. Suitable charts can be created with many planetarium programs such as Starry Night.

Jim Christy discovered Charon in 1978. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together.

Charon is unusual in that it is the largest moon with respect to its primary planet in the solar system (a distinction once held by Earth's moon).

Charon's radius is not well known. JPL's value of 364 has an error margin of +/-13, more than two percent. Its mass and density are also poorly known.

Pluto and Charon are also unique in that not only does Charon rotate synchronously, but Pluto does, too. They both keep the same face toward one another. (This makes the phases of Charon as seen from Pluto very interesting.)

Charon's composition is unknown, but its low density indicates that it may be similar to Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with water ice.

Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved.

It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's moon.  It is doubtful that Charon has a significant atmosphere.

In May 2005, an important discovery was made.  By studying many images sent by the Hubble Space Telescope, astronomers concluded that Pluto actually has 3 moons.  The 2 additional moons were previously unknown.  Later named Hydra and Nix, their diameters are close to 100 miles and are on the same orbital plane as Charon, though much farther away from Pluto.  Observations of Hydra and Nix continue, with the on-going assistance of the Hubble Space Telescope, to gain further knowledge of these two distant moons of Pluto.

Pluto is the only planet that has not yet been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface.  So, NASA designed the first mission to explore Pluto, called New Horizons.  The spacecraft of the same name was launched in January of 2006, destined to reach Pluto and it’s 3 moons by 2015.  New Horizons will unlock many secrets of Pluto and its three satellites. NASA has 7 scientific instruments on board the probe.  The instruments will send back much information about the surface properties, geology, interior makeup and atmosphere.

There are some who think Pluto would be better classified as a large asteroid or comet rather than as a planet. Some consider it to be the largest of the Kuiper Belt objects.

In August 2006, the International Astronomical Union passed a resolution and created 3 new categories to classify planets and other Solar System bodies:

1. A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
2. A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit and (d) is not a satellite.
3. All other objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

This reclassifies Pluto as “dwarf plant”, based on this new criteria.  Some astronomers, educators, students and the public do not agree with this change and believe our Solar System should stay the same as it has been.  Others understand that Pluto and it’s 3 satellites haven’t changed; our understanding of the physical features and attributes have changed, due to advanced technology and new discoveries. This recent information allows scientists to recognize differences and see our Solar System in a new way.