Solar System Distance Model The planets nearest the Sun are very different from the planets farther out in composition and structure.

Mercury, Venus, Earth and Mars are called the “inner” planets because they occupy the inner part of the solar system. They are also called the “terrestrial” planets, which means “earthlike”. They are all small and rocky with iron cores.

Between the “inner” and “outer” planets is the asteroid belt. Within the asteroid belt is Ceres, the largest asteroid, now classified as a “dwarf” planet. Ceres is about 1/3 rd the size of the Moon.

Jupiter, Saturn, Uranus and Neptune are the “outer” planets because they are in the outer part of the solar system. Jupiter and Saturn are sometimes called the “gas giants” and Uranus and Neptune are sometimes called the “ice giants”.

Past the outer planets is the Kuiper Belt. The Kuiper belt is composed of millions of icy bodies. At least 5 have been recently discovered that are about half the size of the Moon. Pluto is a “dwarf” planet and Kuiper Belt Object.

Beyond the Kuiper Belt is the “Scattered Disk”, made of Kuiper Belt Objects that have been flung out by gravity or collisions into more elliptical and larger orbits. Eris, which is bigger than Pluto, is a “dwarf” planet and a “Scattered Disk Object”.