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), comets, and other celestial bodies. 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, or exoplanet, around a normal star, 51 Pegasi, was made in 1995. This was quickly followed by the detection of a number of single exoplanets orbiting normal stars, and in 1999 the first discovery of multiple exoplanets orbiting a sunlike star, Upsilon Andromedae, was announced.
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. Most of the extrasolar planets discovered so far, however, are larger than earth, and many of those are 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—in one system found by the Kepler space telescope, five planets orbit a star more closely than Mercury does the sun; and many have elliptical rather than circular orbits. Planets have also been found orbiting binary stars. 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.
An increasing number of planets with masses between one and seven times the earth's have been found in the 21st cent. In 2010 the discovery of a planet located in its star's habitable zone with a diameter 1.2–1.4 times that of the earth was reported (though the planet rotates so that one side always faces the star, as the moon does with respect to the earth). In 2013 it was announced that astronomers reviewing the data from the Kepler space telescope had identified a planetary system (Kepler 37) that included a planet (Kepler 37c) somewhat smaller than the earth and another (Kepler 37b) that was smaller than Mercury. Kepler 37b is the smallest extrasolar planet discovered so far; neither of the two planets was in the habitable zone. It is possible that some of the bodies that have been discovered are not planets in the solar-system sense but a new class of celestial bodies or even brown dwarfs.
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 in visible light. Several 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, also called the transit method, is to measure the distinct dimming of light from the star as the planet's orbit brings it between the star and the earth. Using photometric techniques it also has been possible to photograph extrasolar planets in infrared light. Doppler spectroscopy is based on the fact that a planet periodically pulls its star closer to and farther from the earth as it orbits the star; this motion has a measurable effect on the spectrum of light coming from the star. In pulsar timing, planets orbiting a pulsar can be detected by measuring the periodic variation in the pulse arrival time; however, because the planets are orbiting a pulsar, a "dead" star, rather than a main-sequence star like the sun, this tends to be of less interest in the search for an earthlike extrasolar planet.
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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