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Bocs, a Wikipedia volt kéznél...
Ez a Nasa honlapjáról van:
A planet is a large, round heavenly body that orbits a star and shines with light reflected from the star. We know of nine planets that orbit the sun in our solar system. Since 1992, astronomers have also discovered many planets orbiting other stars.
Pluto is not always the farthest planet from the sun, however. Its orbit is such a long oval that Pluto moves inside the path of Neptune for about 20 years every 248 years. One such 20-year period lasted from Jan. 23, 1979, to Feb. 11, 1999.
The planets of our solar system can be divided into two groups, except for Pluto. The innermost four planets -- Mercury, Venus, Earth, and Mars -- are small, rocky worlds. They are called the terrestrial (earthlike) planets, from the Latin word for Earth, terra. Earth is the largest terrestrial planet. The other Earthlike planets have from 38 to 95 percent of Earth's diameter and from 5.5 to 82 percent of Earth's mass (amount of matter).
The next four planets -- Jupiter, Saturn, Uranus, and Neptune -- are called gas giants or Jovian (Jupiterlike) planets. They have gaseous atmospheres and no solid surfaces. All four Jovian planets consist mainly of hydrogen and helium. Smaller amounts of other materials also occur, including traces of ammonia and methane in their atmospheres. They range from 3.9 times to 11.2 times Earth's diameter and from 15 times to 318 times Earth's mass. Jupiter, Saturn, and Neptune give off more energy than they receive from the sun. Most of this extra energy takes the form of infrared radiation, which is felt as heat, instead of visible light. Scientists think the source of some of the energy is probably the slow compression of the planets by their own gravity.
The ninth planet, Pluto, is only 19 percent the diameter of Earth and 1/500 of its mass. As a small, rocky planet with a larger orbit than the gas giants, it does not fit in either group. Some astronomers think that Pluto may not be a major planet at all.
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