The Solar System

The Solar System is composed of the Sun and the material that orbits around it, as well as the area containing these objects. Objects in the Solar System include the Sun, planets, asteroids, moons, comets, meteoroids, and other interplanetary material. More than 99% of the Solar System's mass is in the Sun, with the planets accounting for only about 0.14%. The Solar System also contains interplanetary material, such as meteoroids (mostly the dust left over by comets), dust, and gas.

There are eight known planets in the Solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. (Pluto was considered a planet until August 2006 when the International Astonomical Union approved a definition of planet that disqualified the icy world.) Compared to the Sun, the planets are relatively small and cold. They fall into two groups: the rocky Terrestrial Planets (Merc ury, Venus, Earth, and Mars) and the gaseous jovian planets (Jupiter, Saturn, Uranus, and Neptune). Most of the planets have nearly circular orbits around the Sun. They all orbit counter-clockwise as if you could view them from ab ove the Sun's north pole.

Most asteroids are located in a zone called the Asteroid Belt between the orbits of Mars and Jupiter. However, some, called Apollo asteroids, actually cross the orbit of Earth! There are asteroids in other locations as well. About 6,000 asteroids are currently known, but hundreds more are being discovered every year. Ceres is by far the largest asteroid, at about 914 km in diameter. Most asteroids, however, are less than 50 km across. One asteroid, named Chiron, orbits between Saturn and Uranus, but it is not yet known whether this is just a straying asteroid or the first of a whole new family of asteroids outside of the Asteroid Belt. Despite their large numbers, if you were to put all of them together, their combined mass would likely be less than that of the Moon.

There are currently more than 60 moons known in the solar system. Mercury and Venus have none; Earth has one moon; Mars has two; Neptune has eight; Uranus has 15; Jupiter holds 16; and Saturn has 21 moons, the most of all. In late 1995, three new moons (1995 S3, 1995 S4, and 1995 S5) were discovered orbiting Saturn, bringing its total from the previous 18 to 21. The International Astronomical Union (the IAU) will assign them official names. There are probably more small moons to be dis covered, but it is assumed that essentially all of the large moons have been detected. Some moons like Titan and Ganymede are even larger than the planets Mercury and Pluto, but they are still called moons because they orbit planets rather than directly around the Sun.

Comets are icy bodies that orbit the Sun with highly elliptical paths. Short-period comets are thought to come from a zone of icy bodies outside the orbit of Pluto called the Kuiper Belt. In fact, Pluto itself may have been captured from this zone. Lo ng-period comets, on the other hand, are thought to originate from a more distant realm of ice bodies called the Oort Cloud. While the Kuiper Belt is believed to span a zone 35 to several hundred astronomical units from the Sun (an astronomical unit, or A.U., is the distance from Earth to the Sun), the Oort Cloud is estimated to extend some 20,000 to 100,000 A.U. from the Sun.* Comets are like "dirty snowballs" made of frozen gases and debris, and they can be spectacular upon nearing the Sun. The Su n's heat melts the outer layers of ice, releasing gas and dust that can form a bright tail. As the comet goes away from the Sun, the comet gets colder and its tail disappears.

* Flamsteed, Sam. "Where Comets Come From," Discover, November 1995, p. 83.

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Revised 08/06