How Many Earths Are Enough?

The goals of NASA’s Terrestrial Planet Finder (TPF) and ESA’s Darwin missions are to find and characterize terrestrial planets in the habitable zones of other stars, and to search for evidence of life in the atmospheres of any planets found. A key issue that must be addressed is the size of the sample of stars that must be searched before the scientific community, the funding agencies, and the public at large will be satisfied that an expensive space observatory will have a high probability of success. This question lies at the heart of the definition of TPF/Darwin. In this paper, I discuss some of the parameters that bound the size of the TPF/Darwin sample and outline a science program to improve our knowledge so that we can make timely decisions about the scope and expense of TPF/Darwin.