NASA Exoplanet Exploration Program missions represent a voyage of unprecedented scope and ambition, promising insight into humankind's most timeless questions:
Where did we come from? Are we alone?
The primary goal of these interrelated missions is to discover and characterize
planetary systems and Earth-like planets around nearby stars. The missions are designed to build on each other's success, each providing an essential step forward toward the
goal of discovering habitable planets and evidence of life beyond.
The evidence will be primarily in the form of detailed spectroscopic studies of the atmospheres of extrasolar planets. For a planet to host life, our expectations are that the planet would resemble Earth itself. It would lie in an orbit that is neither to close nor too far for from its star, so that liquid water could exist over geological timescales, and its atmosphere would contain the right balance of gasses that could support life. Moreover, the atmosphere of the planet would be altered by the presence of life, such that only the existence of living organisms could account for the unusually high levels of gasses in its atmosphere.
The volume of space that would be explored would be limited to closest stars. In this context "nearby" is understood to be stars that lie within approximately 20 parsecs (60 light-years) from our sun. This is roughly the distance that we can explore using technology available in the next decade.
Breaking through technology barriers
The technological challenge of the Exoplanet Exploration Program is essentially one of high dynamic range sensing coupled with high angular resolution imaging. This is true because extrasolar planets appear in the sky as extremely faint objects located in extreme close proximity to their host stars. The light from the planet must first be resolved separately from the starlight, and the glare of the starlight must be then be suppressed to allow atmospheric spectroscopy of extrasolar planets.
Optical/infrared interferometry is a key technology to this task being developed through the Exoplanet Exploration Program. Traditional telescope designs have an achievable resolution that is limited by the diameter of the telescope's primary mirror. When telescopes are combined in an array, the achievable resolution is limited instead by the separation of the telescopes. In principle, arrays can be built that would have the resolution of a single telescope whose primary mirror was several hundred meters in diameter.
When implemented in space, free from the distortions of the turbulent atmosphere, such interferometers hold the promise of resolving the planets around nearby stars.
Mirror technology, wavefront control, and coronagraphy are other key approaches being developed in the Exoplanet Exploration Program. High-dynamic range imaging requires near perfect wavefronts so that speckles of starlight, created by subtle aberrations in the telescope optics, won't create a glare that would obscure the presence of faint planets.
The Terrestrial Planet Finder observatories are being designed with this approach to detect planets at visible and near-infrared wavelengths.
