A planet is a large space body which reflects the light of a star around which it revolves. The planets in our solar system are classified as inner planets and outer planets. The inner planets, the closest to the Sun, are solid spheres of rock and include Mercury, Venus, Earth, and Mars. The inner planets were constantly bombarded by asteroids and meteorites during their first 600 million years in existence. Consequently, you will find craters of varying sizes on the inner planets and their satellites. The outer planets are large gaseous spheres with rings and include Jupiter, Saturn, Uranus, and Neptune. Between the inner and outer planets is an asteroid belt. Every planet, except for Earth, was named for an ancient Roman god or goddess. Some of the planets have naturally occurring satellites, or moons, while others do not. All eight planets orbit the Sun in their own unique way.
The International Astronomical Union (IAU) said that the definition for a planet is now officially known as "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 neighborhood around its orbit." At the same time, new moons are also being discovered, both around existing planets and within these mysterious new worlds. Once the existence of a moon is confirmed and its orbit determined, the moon is given a final name by the International Astronomical Union (IAU), the organization that assumed this task since 1919.
Source: NASA
|