But many astronomers believe the discoveries represent oddballs, planetlike bodies that were easily found precisely because of their unusual characteristics. Consider the planet with a mass similar to Jupiter's that a Swiss team found around the sunlike star 51 Pegasi. And to find a planet similar to Earth that lies neither too close to nor too far away from its parent star to support life will probably require an infrared observatory in space. Most proposed surveys focus on the nearest 1,000 stars, those within about 42 light-years of the solar system. Despite their variety, nearly all strategies fall into one of two categories, notes David C. Black, director of the Lunar and Planetary Institute in Houston.
Indirect methods focus exclusively on the parent star.
Telltale wobbles in its motion betray the gravitational tug of an unseen planet, and periodic dimming of its light could signify a planet crossing in front of it. In contrast, direct methods aim to produce images and spectra of the planet itself. Among indirect methods, astrometry and spectroscopy record different facets of the motion of stars, while photometry records changes in brightness. A fourth technique, which relies on the phenomenon of gravitational lensing, holds the promise of detecting Earth-mass planets but does not enable observers to study them. As a planet circles its parent star, moving from one side to another, it pulls the parent to and fro, creating a wobble in the star's motion.