Terrestrial and Giant Planets
The planets are divided into two groups by their size and structurethe terrestrial planets and the giant planets.The innermost four planetsMercury, Venus, Earth, and Marsare known as terrestrial (Earth-like) planets, from the Latin word for Earth, terra. These planets, of which Earth is the largest, are rocky and comparatively small. The other Earth-like planets have 5.5 to 82 percent of Earth's mass and 38 to 95 percent of Earth's diameter. The densities of the terrestrial planets range from about four to about five and one-half times that of water. Diameters range from about 3,000 to about 8,000 miles (4,800 to 12,900 km). For the most part, the terrestrial planets are solid, being made up of stone and metals, with a relatively thin or nonexistent layer of atmosphere.
The outer four planetsJupiter, Saturn, Uranus, and Neptuneare called gas giants or Jovian (Jupiter-like) planets. These planets have no solid surfaces and consist mainly of gaseous atmospheres made up of hydrogen and helium. Their densities are low, ranging from about three-fourths to about one and one-half times that of water, and their diameters range from about 30,000 to about 89,000 miles (48,000 to 143,000 km). All but Neptune show a flattening at their poles and a bulge at their equators. In general, the giant planets seem to have small, dense cores surrounded by extremely thick layers of liquid and gas. Jupiter, Saturn, and Neptune give off more energy than they receive from the sun. Most of this extra energy takes the form of heat or infrared radiation, instead of visible light. According to some scientists, the source of some of the energy is probably the slow compression of the planets due to their own gravity.