There are five methods currently in use or in development for finding planets in planetary systems beyond our own. Because extrasolar planets are so difficult to distinguish in the wash of brightness from their nearby stars, most methods look for the secondary effects (indirect) of a planet's presence to locate the body rather than searching for the planet itself (direct).
Radial Velocity (indirect): This technique tracks minuscule movements of distant suns as they're affected by the pulls of orbiting planets. The regular shifting of a star toward, and then away from, a ground-based telescope is revealed through extraordinarily precise measurements of its spectrum. Although it can detect only large, Saturn- or Jupiter-sized planets, this method has been used to identify some 50 extrasolar bodies.
Precision Astrometry (indirect): Like radial velocity, this system tracks the repeated movement - this time, side to side - of stars responding to planets' gravity. NASA's Space Interferometry Mission (SIM), planned for launch in 2006, may be able to use precision astrometry to identify planets smaller than Jupiter, though still not as small as Earth.
Direct Imaging (direct): To capture the light from an extrasolar planet directly requires both a large telescope and a way of excluding light from the parent star. One option is to use a very large space telescope with a coronagraph, a gadget that blocks the light from the star. Another is to combine the light from several variously positioned space telescopes such that the images of the star cancel each other out, leaving only the light from the planet. NASA's planned Terrestrial Planet Finder (TPF) will take one of these billion-dollar approaches, as will the European Space Agency's Darwin. The two will most likely be combined in a single project - which probably won't be ready for an additional 10 years.
Transit Photometry (semidirect): This method, proposed by the Kepler project, detects the passage of planets between their parent stars and Earth. Kepler's orbiting telescope, if trained for years on numerous stars, could locate planets as small as Earth by measuring the barely detectable dimming of a sun's light at regular intervals (planetary "years").
Microlensing (semidirect): According to the general theory of relativity, gravitational fields bend light. Thus, a planet moving between Earth and a far-off star (not its parent star) can act as a "gravitational lens" and increase the brilliance of that very far-off star. Such microlensing techniques, which require a specially designed camera and telescope, are not especially informative about the body doing the lensing, nor can they identify the distant star around which it orbits. But they are the only means of detecting the possibly billions of planet-sized objects that do not orbit a star.
