Solar System

The Sun makes up 99.9 percent of the mass of the Solar System. The four planets closest to the Sun - Mercury, Venus, Earth and Mars - are small, rocky worlds. They have no rings and only Earth and Mars have moons. The four outer planets are Jupiter, Saturn, Uranus, and Neptune. These gas giants are considerably larger, and they have rings and many moons.

Image at right: The Sun and Planets (courtesy of NASA).

Asteroids, small solar system bodies composed of rock and metal, are left over from the early days of the Solar System. Most asteroids orbit around the Sun in a belt between Mars and Jupiter and travel in the same direction as the planets.

(Image Right: Asteroids, Courtesy of NASA)

Comets can be a spectacular sight. A mixture of rock and dust bound together with ice and frozen gases, comets usually stay in the deep freeze of space, orbiting the Sun in the Kuiper Belt beyond Neptune or at the edge of the Solar System in the Oort Cloud.

(Image Right: Comet, Courtesy of NASA)

Meteors are streaks of light formed when chunks of metallic or stony matter known as meteoroids enter the Earth's atmosphere at high speeds from space. Most meteoroids disintegrate before reaching Earth. Those that do strike our planet are called meteorites.

Image at right: Meteor (courtesy of NASA).

What is a moon? Funny thing is, scientists haven't decided yet! You might recall the IAU or International Astronomical Union, changed the definition of what a planet is recently recently and some scientists think the definition for Moon is next. For now a moon is simply defined as an object (not manufactured), in orbit around another object.

Image at right: Earth's moon, Calisto, Rhea, Mimas, Io (Original images courtesy of NASA.)