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.
Galileo was the first person to see that the planets were not tiny points of light, like the stars, but were instead round disks. From this observation, Galileo began to think that the planets had to be much closer to us than the stars. He also wondered if planets like Jupiter, Saturn, and Mars might be more similar to Earth than anyone had previously believed.
Image at right: The Sun and Planets (courtesy of NASA).
