The search for extraterrestrial habitable planets will require long observation times and the intelligent selection of appropriate parent stars and target biosignatures. While life can certainly develop in the absence of photosynthesis, such life forms on earth exhibit metabolic rates several orders of magnitude less than the activity accompanying a photosynthetic-driven ecosystem. The most accessible spectral biosignatures are those accompanying a system driven away from thermodynamic equilibrium by photosynthetic activity. For example, the co-existence in a planetary atmosphere of significant amounts of ozone, oxygen, and methane would be a strong indication of biotic activity. Investigating the issue of the Habitable Zone from the standpoint of the constraints inherent in photosynthesis it appears that the absorption characteristics of photosynthetic microorganisms on this planet make it likely that photosynthetic activity can exist on planets orbiting stars to red-ward of the Sun on the H-R diagram. Such a possibility is encouraging for terrestrial planet finder efforts since stars classified red-ward of our sun (G3 to K7) account for more than 55% of our nearest neighbors.
Photosynthetic Constraints on the Habitable Zone
