The Solar System consists of the Sun, the nine planets and their satellites; the comets, asteroids, meteoroids, and interplanetary dust and gas. It is composed of two systems, the inner solar system and the outer solar system. The inner solar system contains the Sun, Mercury, Venus, Earth and Mars. The outer solar system contains Jupiter, Saturn, Uranus, Neptune and Pluto.
The inner planets are relatively small and made primarily of rock and iron. The asteroids orbit the sun in a belt beyond the orbit of Mars, tumbling and sometimes colliding with one another. Made mostly of rock and iron, the asteroids may be the remnants of a planet that never formed. The outer planets, with the exception of Pluto, are much larger and made mainly of hydrogen, helium, and ice. Many astronomers believe that Pluto was and interstellar wanderer that was captured by the Sun's gravity and was not an original part of the solar system.
The orbits of the planets are ellipses with the sun at one focus, though all except mercury and Pluto are very nearly circular. The orbits of the planets are all more or less in the same plane that is called the ecliptic. The ecliptic is inclined only seven degrees from the plane of the ecliptic with and inclination of seventeen degrees. Again with the exception of Pluto, the planets all orbit the sun in almost the same plane.
The average distance of the earth to the sun is used as a standard for measuring distances in the solar system and is called an astronomical unit (AU). One AU is about 93 million miles or 150 million kilometers. Mercury the planet closest to the sun is at about 0.387 AU. Pluto is the outermost planet, and it is 39.44 AU from the sun. The heilopause is the boundary between the solar system and interstellar space, and it is about 100 AU from the sun. The comets, however, achieve the greatest distance from the Sun; they have highly eccentric orbits ranging out to 50,000 AU or more...
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