Powell John Wesley

its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

• 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 neighborhood around its orbit, and (d) is not a satellite.

• All other objects, except satellites, orbiting the sun shall be referred to collectively as "Small Solar System Bodies."

The International Astronomical Union made some further footnotes to their revised definition of a planet.

• The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

• An IAU process will be established to assign borderline objects into dwarf planet and other categories.

• These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

The IAU further resolved that "Pluto is a 'dwarf planet' by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects." Thus, after 100 years of searching for planet "X," and 76 years of considering it to be a planet since Pluto's discovery in 1930, the formerly most distant planet is now regarded as the second largest known dwarf planet in the solar system, and as just another large object in the Kuiper belt of the outer solar system.

The dwarf planet Pluto has a variable orbital distance of about 30-40 astronomical units (2.7-3.7 billion miles, or 4.4-6 billion km) from the Sun, circling once every 249 Earth years, and has a retrograde rotation period of 6.4 Earth days. It is a small body with a mass of 0.003 Earth masses, a diameter of 1,400 miles (2,250 km; only 20 percent that of Earth), and a density of 2.3 grams/cubic centimeter. It has one moon known as Charon, and it closely resembles other asteroids of the outer solar system. The large 17.2° inclination of its orbital plane with respect to the ecliptic plane supports the contention that Pluto is a captured asteroid.

The physical properties of the Pluton-Charon system suggest that it is an icy dual-asteroid system similar to some of the Jovian moons, being most similar to Neptune's moon Triton. Models for the origin of Pluto range from its being a captured icy asteroid or an escaped moon to being a remnant of material left over from the formation of the solar system. The great distance and small size of the system make it difficult to observe, and certainly as deep planetary probes explore the outer reaches of the solar system, new theories and models for the origin and evolution of this system will emerge.

See also Earth; Jupiter; Mars; Mercury; Neptune; Saturn; solar system; Uranus; Venus.

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