The planets in our solar system can be divided into two groups: the Inner Solar System and the Outer Solar System. The Inner Solar System consists of Mercury, Venus, Earth, and Mars; the Outer Solar System consists of Jupiter, Saturn, Uranus, and Neptune.

Here's a good way to remember the names of the eight planets in proper order:

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The four worlds of the Inner Solar System (including Earth) are relatively small, rocky bodies. The four worlds of the Outer Solar System are large gassy spheres and don't have solid surfaces.

But beginning around the orbit of Neptune, a very large number of very small, very cold "dwarf planets" wander in elongated eccentric orbits. Pluto is one of the largest of these. Astronomers are discovering new members all the time.

Planets closer to the sun move faster than those farther away. Mercury, the closest planet makes a complete trip around the sun in 88 days, whereas Neptune takes a whopping 165 years. The Earth takes about 365 days—the definition of a year.