  The search for extrasolar Earth-like planets is underway. Over 100 extrasolar
giant planets are known to orbit nearby sun-like stars, including several in
multiple-planet systems. These planetary systems are stepping stones for the
search for Earth-like planets; the technology development, observational
strategies, and science results can all be applied to Earth-like planets. Stars
much less massive than the sun the most common stars in our Galaxy are being
monitored for the gravitational influence of Earth-like planets. Although
Earth-like planets orbiting sun-like stars are much more difficult to detect,
space missions are being built to detect them indirectly due to their effects
on the parent star and to quantify fundamental factors such as terrestrial
planet frequency, size distribution, and mass distribution. Extremely ambitious
space programs are being developed to directly detect Earth-like planets
orbiting sun-like stars, and must tackle the immense technological challenge of
blocking out the light of the parent star, which is brighter than the planet by
six to ten orders of magnitude. Direct detection of radiation from the planet
is necessary for the definitive goal of the search for Earth-like planets: the
study of atmospheric spectral signatures for signs of severe disequilibrium
chemistry that could be indicative of biological activity. In addition to
technological development, a growing flurry of scientific activity has begun
to: understand terrestrial planet formation and terrestrial planet frequency;
model terrestrial-like planet atmospheres and evolution; articulate the
biological signatures of our own Earth; and even to study Earth as an
extrasolar planet by observation and analysis of the spatially unresolved
Earth.
