For decades, planetary astronomers have postulated the existence of planets orbiting other stars. Only in the last several years, however, have specific extrasolar planets been inferentially identified by various techniques, primarily from observations of perturbations in the motions of stars. Although there is still disagreement among the scientific experts with respect to the exact nature of the conclusions that can be drawn from this indirect data, it is generally accepted that at least some of the observations do indeed confirm the existence and location of planets outside our solar system.

The discoveries that have been recently made, however, have already catalyzed a revolution in planetary astronomy. Existing models describing the formation and evolution of planetary systems, for example, are currently being reexamined in view of the large number of strange, or oddball, planetary entities that have been found. Based on the extrasolar planets discovered to date, it appears that the range of possible planetary systems is much broader than previously thought.

Confirmation of the existence of specific planets outside the solar system represents a major scientific breakthrough. The real goal in prospecting for extrasolar planets, however, is the unambiguous identification of Earth-like planets. The discovery of a planet comparable to Earth in size, orbiting a stable star at a distance that could allow thermal and other environmental conditions similar to those on Earth, would be one of the most important scientific results in human history. It is not hyperbole to say that such a discovery would have overwhelming significance for the human species. Just knowing for certain that there exists another planet similar to Earth will profoundly affect not only our present assessment of mankind’s place in the universe, but also the events of our future history.

The excitement engendered by the search for Earth-like planets is undoubtedly related to what some scientists have called an "Earth chauvinist" attitude. Many astrobiologists believe that the most likely location for the existence of other life forms, and possible intelligence, would be on a planet that is similar to Earth, both in size and orbital characteristics. Such a planet would exist in what has been called a "habitable zone," spending its entire orbital cycle at a distance from another star where conditions would not be inhospitable to the kinds of life that exist on this planet. In truth, that is a homocentric, or Earth chauvinist, attitude. We have not yet found incontrovertible proof that life has ever originated anywhere else. It is presumptuous to assume, absent any information, that the only paradigm for the development of life and the eventual evolution of intelligence is the one that occurred on Earth. Nevertheless, for most of us, the discovery of an Earth-like planet orbiting another star would dramatically increase the likelihood that we are not the only sentient species in the universe.

In the last few years several independent researchers have suggested observational techniques that appear to have the capability of identifying Earth-like planets in our stellar neighborhood. One of these methods, called differential photometry because the observational data set consists of measuring changes in stellar brightness, looks especially promising. Transits across the face of a star in our galaxy by an Earth-like planet produce fractional changes in stellar brightness of roughly one one-hundredth of 1 percent. These brightness changes last from two to 16 hours. Tracking the motion and duration of the brightness change gives information about the size and orbit of the transiting body. If a transiting planet can be repeatedly observed, and its data uniquely and correctly coupled, then its size and orbit can eventually be completely determined.

It is now well within the capability of advanced orbiting observational telescopes to observe the slight changes in stellar brightness that result from the transit of an Earth-like planet. Isolating that change from other possible sources that might cause the slight dimming, including normal stellar fluctuations, requires informed and sophisticated data processing algorithms that have been enabled by the massive increase in computational power of the last 10 years. But all the technologies necessary to identify Earth-like planets have now been demonstrated in the laboratory. What remains is to build the proper observational platform and launch it into space.

The COROT mission, currently under development by the French space agency CNES, will perform highly accurate differential photometry from space when it is deployed in the next year or two. However, its instrumentation and viewing protocols are not optimized strictly for the detection of extrasolar planets. COROT should be able to discover planets with short-period orbits down to Uranus' size or smaller, but will probably not discover Earth-sized planets orbiting other stars in the critical "habitable zone" discussed earlier.

Of course we would immediately start the development of techniques for observing these individual planets with remote sensors. Would we be able, for example, to identify the atmospheric constituents of these Earth-sized planets from a telescope in our solar system? What could we learn about any of these tantalizing other worlds from observatories in heliocentric space? Might we even build a robotic spacecraft and send it on a long interstellar journey to gather information for our children and grandchildren?