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When Michel Mayor and Didier Queloz of Switzerland discovered a planet orbiting the star 51 Pegasi in 1995, many skeptics probably shook their heads in disbelief. But when the planet's existence was confirmed by Geoff Marcy and Paul Butler later that year, there was little room for skepticism. Since then, Marcy and Butler have discovered more than 30 planets orbitng other stars. And there are countless others yet to be discovered. What could be so important to make Jack Lissauer call the discovery of extrasolar planets "One of the great scientific advances of the 1990s"? And how are these discoveries made?

The discovery of extrasolar planets is important for many reasons. The first concerns our knowledge of the universe. Before 1995, we knew that planets orbited remnants of large stars after supernova explosions, called pulsars. As far as we knew, there were no extrasolar planets orbiting stars like we orbit our sun. Knowing of the reality of extrasolar planets changes our understanding and our way of looking at the universe. So far we've not yet discovered a terrestrial planet. Just as we once couldn't detect planets orbiting stars, we now cannot detect smaller sized and possible terrestrial planets with the methods available. But because we've yet to detect them doesn't mean they're not there. Our ability to discover planets outside our own solar system may lead to the discovery of an earth like planet somewhere else in the universe. The potential benefits to that happening are innumerable. Because other planets may help us better understand our own, discovering planetary companionship to stars is an important step for the field of astronomy. The methods by which to discover planets are just as important as the discoveries themselves.

So far planets orbiting stars have been detected by one of two methods. The textbook Universe calls one the astrometric method, involving a measurement of a star's position in the sky relative to other stars. A star with a potential orbiting planet will change position in the sky, likely in a cyclic pattern. The limits to this method is that it not only requires strict precision, but also requires the observer to trace a star for a long period of time.

More likely, astronomers use the radial velocity method. When a planet orbits a star, it gravitational pull affects the orbit of the star. It's important to remember that both the planet and star rotate around a center of mass. If a planet is large enough, its pull produces a change

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