planetary system definition of planetary system in the Free Online Encyclopedia.

planetary system, a star star, hot incandescent sphere of gas, held together by its own gravitation , and emitting light and other forms of electromagnetic radiation whose ultimate source is nuclear energy .
..... Click the link for more information. and all the celestial bodies bound to it by gravity, especially planets planet [Gr.,=wanderer], a large nonluminous ball of rock or gas that orbits a star. The term, once limited to any of the eight solid, nonluminous bodies (major planets) that revolve around the sun, has been extended to include similar bodies discovered revolving
..... Click the link for more information. and their natural satellites satellite, natural, celestial body orbiting a planet, dwarf planet, asteroid, or star of a larger size. The most familiar natural satellite is the earth's moon ; thus, satellites of other planets are often referred to as moons.
..... Click the link for more information. . Until the last decade of the 20th cent., the only planetary system known was the solar system solar system, the sun and the surrounding planets, natural satellites , dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass.
..... Click the link for more information. , which comprises the sun and the surrounding planets, natural satellites, asteroids asteroid, planetoid, or minor planet, small body orbiting the sun. More than 10,000 asteroids have orbits sufficiently well known to have been cataloged and named; thousands more exist.
..... Click the link for more information. , meteoroids (see meteor meteor, appearance of a small particle flying through space that interacts with the earth's upper atmosphere. While still outside the atmosphere, the particle is known as a meteoroid. Countless meteoroids of varying sizes are moving about the solar system at any time.
..... Click the link for more information. ), comets comet [Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun. Comets visible from the earth can be seen for periods ranging from a few days to several months.
..... Click the link for more information. , 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 pulsar, in astronomy, a neutron star that emits brief, sharp pulses of energy instead of the steady radiation associated with other natural sources. The study of pulsars began when Antony Hewish and his students at Cambridge Univ.
..... Click the link for more information. ; 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 in visible light. 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, 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. 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. Using photometric techniques it has been possible to photograph extrasolar planets in infrared light. 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. It is possible that smaller planets exist in the planetary systems already discovered or orbit stars that have been examined and found not to have planets, but they are undetectable with current instruments and techniques. It is also possible that the bodies that have been discovered stars are not planets in the solar-system sense but a new class of celestial bodies or even brown dwarfs brown dwarf, in astronomy, celestial body that is larger than a planet but does not have sufficient mass to convert hydrogen into helium via nuclear fusion as stars do.
..... Click the link for more information. .

Bibliography

See A. Boss, Looking for Earths: The Race to Find New Solar Systems (1998); J. K. Beatty, ed., The New Solar System (1999).