Planets

    Planet is any of nine large objects that orbit the sun. A planet may also be a similar body that revolves about a star other than the sun. Earth is a planet that travels around the sun once a year. The sun, the planets that orbit it, and the planets' satellites (moons) are parts of the solar system. Going outward from the sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Pluto, and Neptune. 

    The sun and other stars that shine are giant balls of hot gases. The planets of the solar system are much smaller than the sun and these other stars, and they are either solid or probably have solid cores. Planets do not produce their own visible light. The planets of the solar system can be seen only because they reflect sunlight. Mercury, Venus, Mars, Jupiter, Saturn, and Uranus are visible from Earth without a telescope. 

    Planets and stars look much alike in the night sky, but there are two ways to tell them apart. First, the planets shine steadily, but the stars seem to twinkle. Second, the planets move in relation to the stars. The word planet comes from a Greek word meaning to wander. 

    The planets differ greatly in size and in distance from the sun. All the planets together weigh less than a hundredth as much as the sun. The diameter of Jupiter, the largest planet, is about a tenth of the sun's diameter. Yet Jupiter is more than 60 times as large as Pluto, the smallest planet. Earth and the three other planets nearest the sun are somewhat similar in size. They are called the terrestrial (earthlike) planets. The four largest planets--the giant planets--are much farther from the sun. Astronomers know little about Pluto, and do not put it in either group.

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