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 discovered so far are larger than earth, 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. 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. Although no extrasolar planet as small as earth has been detected, a number of planets with masses between two and seven times the earth's have been found, and improved detection techniques may lead to the discovery of smaller planets. It is also 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.
See A. Boss, Looking for Earths: The Race to Find New Solar Systems (1998); J. K. Beatty, ed., The New Solar System (1999).
A planetary system consists of the various non-stellar objects orbiting a star such as planets, moons, asteroids, meteoroids, comets, and cosmic dust. The Sun together with its planetary system, which includes Earth, is known as the Solar System.
Planetary systems are generally believed to form as part of the same process which results in star formation. Some early theories involved another star passing extremely close to the Sun, drawing material out from it which then coalesced to form the planets. However, the probability of such a near collision is now known to be far too low to make this a viable model. Accepted theories today argue that a protoplanetary disk forms by gravitational collapse of a molecular cloud and then evolves into a planetary system by collisions and gravitational capture.
Some planetary systems may form differently, however. Planets orbiting pulsars—stars which emit periodic bursts of electromagnetic radiation—have been discovered by the slight variations they cause in the timing of these bursts. Pulsars are formed in violent supernova explosions, and a normal planetary system could not possibly survive such a blast—planets would either evaporate, or the sudden loss of most of the mass of the central star would see them escape the gravitational hold of the star. One theory is that existing stellar companions were almost entirely evaporated by the supernova blast, leaving behind planet-sized bodies. Alternatively, planets may somehow form in the accretion disk surrounding pulsars.