There are eight major planets in the solar system. The inner planets are Mercury, Venus, Earth and Mars. These planets are made mostly of silica-based rocks and minerals and have solid surfaces. Jupiter, Saturn, Uranus and Neptune are gas giants. They are made mostly of gases surrounding solid cores.
The International Astronomical Union defines a planet as being a body in orbit around the sun, not another planet. A planet must have enough mass for its gravity to cause it to assume a relatively round shape and must have either swept up or scattered other objects from its orbital path.
The inner planets are called terrestrial or earth-like planets, but there is no evidence that either Mercury or Venus have ever supported life. Mars may have supported life in the past. Gas giants are at least 10 times larger than Earth, and all four of the gas giants have ring systems. Uranus and Neptune are sometimes called ice giants because they are composed mostly of substances that were in ice form when these planets formed.
Researchers have found many planets orbiting other stars. The National Aeronautics and Space Administration estimates that there may be billions of these exoplanets in the Milky Way Galaxy alone.