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Two large planets, Jupiter and Saturn, were the first to form. They are mostly gas. These massive planets probably had a great influence on how the rest of the solar system evolved. For some reason they were in almost perfect circular orbits. This is unusual. In the few other planetary systems studied so far, most of the giant planets are in oval (elliptical) orbits.

Later four small rocky planets formed near the Sun: Mercury, Venus, Earth and Mars. As we shall see, life later evolved on one of these (Earth). These planets too had almost circular orbits. If they had oval orbits, life could not have evolved.

Another planet started to form but broke up into pieces because of the gravity of Jupiter. It formed a ring of small rocks where the planet should have been. We call them the asteroids.
Beyond Saturn there are two gassy planets, Uranus and Neptune and a little icy planet Pluto. Far out beyond Pluto there are many icy objects like the snowballs which formed the planets. They are the comets, and sometimes one of them wanders in near the Sun.