Earth is the fifth largest of the planets in the solar system. It's smaller than the four gas giants — Jupiter, Saturn, Uranus and Neptune — but larger than the three other rocky planets, Mercury, Mars and Venus.
Earth has a diameter of roughly 8,000 miles (13,000 kilometers) and is round because gravity pulls matter into a ball. But, it's not perfectly round. Earth is really an "oblate spheroid," because its spin causes it to be squashed at its poles and swollen at the equator.
Water covers roughly 71 percent of Earth's surface, and most of that is in the oceans. About a fifth of Earth's atmosphere consists of oxygen, produced by plants. While scientists have been studying our planet for centuries, much has been learned in recent decades by studying pictures of Earth from space.