2. The Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago.

3. The four smaller inner planets; Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal.

4. The four outer planets, Jupiter, Saturn, Uranus and Neptune, also called the gas giants, are composed largely of hydrogen and helium and are far more massive than the terrestrials.

5. Planet Sizes In Equatorial Diameter (km), the Sun is the largest at 1,392,000km. Mercury is 4,878km and Venus is 12,104km. Earth is almost the same size as Venus at 12,756 km. Mars is 6,787km. Jupiter and Saturn are both large planets at 142,800km and 120,000km respectively.