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planetary system, a star and all the celestial bodies bound to it by gravity, especially planets and their natural satellites. Until the last decade of the 20th cent., the only planetary system known was the solar system, which comprises the sun and the surrounding planets, natural satellites, asteroids, meteoroids (see meteor), and comets. Speculation that other planetary systems exist dates back to antiquity, and through the years ever increasing numbers of astronomers searched for earthlike planets circling sunlike stars. The breakthrough came in 1992, when radio astronomers detected three planets orbiting a pulsar; however, because pulsars are not normal stars, this was not considered a true planetary system. The first detection of an extrasolar planet around a normal star, 51 Pegasi, was made in 1995. This was quickly followed by the detection of a number of single planets orbiting normal stars, and in 1999 the first discovery of multiple planets orbiting a sunlike star, Upsilon Andromedae, was announced.
These discoveries have given planetary scientists pause. Because the solar system was the only planetary system known, all models of planetary systems were based on its characteristics : several small planets close to the star, several large planets at greater distances, and circular planetary orbits. However, all of the extrasolar planets are large, many much larger than Jupiter, the largest of the solar planets; many orbit their star at distances less than that of Mercury, the solar planet closest to the sun; and many have elliptical rather than circular orbits. All of this has caused planetary scientists to revisit accepted theories of planetary formation. Future theories will be measured against stars surrounded by a ring of gas and dust, such as Beta Pictoris, which are thought to be young adult stars with a planetary system forming around them.
Because stars are so distant and bright and an extrasolar planet, no matter how large, is relatively small and dim, it cannot be seen or photographed directly. Three techniques have been used to infer the presence of such planets. Astrometry is based on the slight gravitational disturbance, or wobble, that the planet causes in the motion of the star. Photometry is to measure the distinct dimming of light from the star as the planet's orbit brings it between the star and the earth. Doppler spectroscopy is based on the planet periodically pulling the star closer to and farther away from the earth as it orbits the star; this motion has a measurable effect on the spectrum of light coming from the star. It is possible that smaller planets exist in the planetary systems so far discovered or in systems examined and eliminated, but they are undetectable with current instruments and techniques. It is also possible that the bodies orbiting these stars are not planets in the solar-system sense, but a new class of celestial bodies or even brown dwarfs. The Hubble Space Telescope, the European Space Agency space probe Corot, to be launched from Russia in 2005, and the James Webb Space Telescope, tentatively scheduled for 2011, will monitor the light from distant stars looking for the telltale wobble. The American Space Interferometry Mission, to be launched in 2009, will look for extrasolar planets as small as Neptune, and the Terrestrial Planet Finder probe, to be launched in 2012, will be capable of detecting earth-sized planets orbiting distant stars.
Bibliography :
See A. Boss, Looking for Earths: The Race to Find New Solar Systems (1998); J. K. Beatty, ed., The New Solar System (1999).
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