THE SEARCH FOR EXTRASOLAR EARTH-LIKE PLANETS: SPECTRAL SIGNATURE OF ATMOSPHERIC EVOLUTION

(Abstract) We report the calculation of spectra for various extrasolar Earth-like planets, which characterize their temperatures and atmospheres for evidence of habitability and life. Future space missions such as Darwin and Terrestrial Planet Finder (TPF) will ultimately offer the opportunity to obtain spectra of extrasolar planets situated within the habitable zones of stars and search for signs of life. The atmospheric biosignatures such as ozone, carbon dioxide, and methane, which are abundant in the Earth, could be attributable to primitive life in extrasolar terrestrial planets. In order to explore the possibility of diagnosing the existence of life from spectra, we have calculated synthesized global spectra for hypothetical terrestrial planets. We performed calculations of radiative-convective equilibrium and radiative transfer, taking into account the global cloud distribution, viewing angles, seasonal variation in solar insolation and surface temperatures, and consequently, the infrared radiation emitted to space. The Earth is our only example of a planet whose atmospheric composition is the consequence of the supply of gas from the presence of life. We examine characteristics for the anoxic / low-oxygen atmosphere of the Archean / Proterozoic Earth with the evolution of oxygenic photosynthesis using the photochemical models coupled to radiative-convective equilibrium code. We compare the derived atmospheric spectral feature of extrasolar Earth-like planets with different age and discuss the detectability of atmospheric constituents of extrasolar terrestrial planets.