Pluto is the farthest planet from the Sun (usually) and by far the smallest.

Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).

Pluto is 2/3 the size of Earth's moon but 1,200 times farther away, which makes viewing surface detail as difficult as trying to read the printing on a golf ball located thirty-three miles away.

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 knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.

• Facts:-

Sign:
Orbit:5,913,520,000 km (39.5 AU) from the Sun (average)
Diameter:2274 km (1,430 miles)
Mass:1.27e22 kg (0.002 x Earth's)
Density:1,750 kg/m^3
Minimum Distance from Sun:4.34 billion km (2.7 billion miles)
Maximum Distance from Sun: 7.4 billion km (4.6 billion miles)
Minimum Distance from Earth: 4.34 billion km (2.7 billion miles)
Revolution Period about the Sun:248 years
Tilt of Axis:122 Degrees
Surface Gravity:0.58 m/s^2 (0.06 x Earth's)
Average Surface Temperature:-229o C (-380o F) (44 K)
Satellites:1 known

Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface.

• Important Questions:-

• The 9th Planet or the 8th Planet?
Because of the eccentricity of Pluto's orbit, its orbit lies inside that of Neptune for the period from January 1979 thru February 11 1999. Thus, strictly, Pluto isn't even the most distant planet at present. Of course, we term it the 9th planet because its average separation from the Sun (length of semimajor axis) is greater than that for Neptune.

• What is Pluto?

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 (also known as Trans-Neptunian Objects). There is considerable merit to the latter position, but historically Pluto has been classified as a planet and it is very likely to remain so.

• Are There Planets Beyond Pluto?
From time to time there has been speculation of another planet beyond Pluto. As of this time, there is little conclusive evidence of such a planet.

• The surface and Interior:-

The Surface
The surface of Pluto is resolved for the first time in these NASA Hubble Space Telescope pictures, taken with the European Space Agency's (ESA) Faint Object Camera (FOC) in 1994. These images, which were made in blue light, show that Pluto has large-scale contrast than any planet except Earth.

The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). The "warmer" regions roughly correspond to the regions that appear darker in optical wavelengths.

Atmosphere and Interior

Atmosphere
Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane.
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 Pluto Express mission planners want to arrive at Pluto while the atmosphere is unfrozen.

Interior
We know essentially nothing about Pluto's interior at this point, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% 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, ethane 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.

# Charon

Charon ( "KAIR en" ) is Pluto's only known satellite

Orbit:19,640 km from Pluto
Diameter:1172 km
Mass:1.90e21 kg
Discovered in:1978 by Jim Christy

General Features:-
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). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon.

Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13, more than two percent. Its mass and density are also poorly known.

Orbit:-
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.)

The right image show Pluto and Charon.

Interiro:-
Charon's composition is unknown, but its low density (about 2 gm/cm3) indicates that it may be similar to Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with water ice. Interestingly, this is quite different from Pluto.

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.

• Difficulties:-
1-There is considerable uncertainty about the even most basic properties of mass, radii and density of Pluto and Charon.

2-Several Kuiper Belt objects apparently similar to Pluto have recently been discovered? Are there any as large as Pluto?

3-What are the dark areas on Pluto composed of?

4-What "geologic" features and processes exist on Pluto and Charon?

5-Will the Pluto Express mission be funded? If we miss this chance to observe Pluto near its perihelion our next chance won't come until the 23rd century.

6-What seasonal changes take place during Pluto's long 'year'?

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