The eight planets can be divided into two distinct categories on the basis of their densities (mass per unit volume). The four inner, or terrestrial, planets—Mercury, Venus, Earth, and Mars—have rocky compositions and densities greater than 3 grams per cubic cm (g/cm3; 1.73 oz/in3). (For comparison purposes, water has a density of 1 g/cm3.) In contrast, the four outer planets, also called the Jovian, or giant, planets—Jupiter, Saturn, Uranus, and Neptune—are large objects with densities less than 2 g/cm3 (1.16 oz/in3).
The eight planets of the solar system and dwarf planet Pluto, in a montage of images scaled to show the approximate sizes of the bodies relative to one another. Outward from the Sun, which is represented to scale, are the four rocky terrestrial planets (Mercury, Venus, Earth, and Mars), the four hydrogen-rich giant planets (Jupiter, Saturn, Uranus, and Neptune), and Pluto. NASA/Lunar and Planetary Laboratory.
The relatively small inner planets have solid surfaces, lack ring systems, and have few or no moons. The atmospheres of Venus, Earth, and Mars are composed of a significant percentage of oxidized compounds such as carbon dioxide. Among the inner planets, only Earth has a strong magnetic field, which shields it from the interplanetary medium. The magnetic field traps some of the electrically charged particles of the interplanetary medium inside a region around Earth known as the magnetosphere. Heavy concentrations of these high-energy particles occur in the Van Allen belts in the inner part of the magnetosphere. The moons of Earth and Mars move around their respective planets in the same direction in which those planets orbit the Sun.
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