Solar systems are comprised of planets, moons, asteroids, comets, gas, dust, and other particles orbiting one or more stars, usually called suns. It was once thought that solar systems were relatively uncommon in the universe, but newer information is changing that viewpoint. Better understanding of the formation of planets and stars implies that solar systems are not as difficult to form as was once thought; statistical analysis of observable stars reveals the improbability of our being the only solar system in the nearby galaxy; and improved instrumentation and observational techniques allow the search for planets around our stellar neighbors.
Our solar system, located on the Orion arm of the Milky Way galaxy, consists of nine planets (five "hard" planets and four gas giants) orbiting one middle-aged yellow star. The sequence of planets from the sun is as follows: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. This can be remembered with the following mnemonic device: My Very Educated Mother Just Served Us Nine Pizzas. Of these planets, Mercury, Venus, Earth, Mars, and Pluto are "hard" planets (although Pluto's status as a planet has been debated), and Jupiter, Saturn, Uranus, and Neptune are gas giants.
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This is a diagram of our solar system (not to scale). Note the crossing of the orbits of Neptune and Pluto - between 1979 and 1999, these two planets have actually switched places relative to the sun, so that Neptune is actually the outermost planet in our solar system. For more information, see the Planets and Moons Reference. |
Some stars do not stand alone like our sun, but instead orbit one or more companion stars to form a multiple-star system. Such systems are quite common in the universe, especially for young stars that have recently been formed from nebulae and have not yet had time to distance themselves from their star cluster. Such a system containing two stars is called a binary system, and containing three stars is called a tertiary system. Often binary or tertiary systems consist of stars of different sizes and magnitudes, resulting in the death of one long before the other. When a yellow star orbits a much hotter and shorter-lived blue star, the result is a middle-aged yellow star (much like our sun) in orbit around a blue or blue-white supergiant. Sometimes, the blue star collapses into a black hole, resulting in the yellow star being devoured by the inexorable gravity of the black hole.
For a long time, the search for other solar systems besides our own was virtually fruitless. However, in October of 1995, Michel Mayor and Didier Queloz of Geneva Observatory located the first extrasolar planet, (planet circling another star) in orbit around the star 51 Pegasi. (More information on this discovery and others like it can be found at Epistellar Jovian Exoplanets.) Then the official planet-hunt began. Since then, eight more extrasolar planets have been found, prompting cosmologists to redesign theories of the formation of both planets and solar systems.
There are basically two techniques for locating planets orbiting other stars: the stellar-motion technique and the Doppler technique. Both techniques rely on the fact that objects orbiting a star exert a tiny gravitational force on the star, causing it to "wobble" in its motion and in the light that it gives off. The stellar-motion technique takes advantage of changes in a star's motion due to orbiting bodies. Using the precise measurement of stellar location (known as astrometry), scientists can scan stars for such "wiggles." The Doppler technique, currently the leading technique in extrasolar-planet research, uses the fact that a star's spectrum of emitted light changes slightly and periodically due to the gravitational influence of orbiting bodies. By measuring these tiny changes, scientists can calculate the precise orbit and minimum mass of the orbiting bodies.
Created by Dan Corbett, Kate Stafford, and Patrick Wright for ThinkQuest.