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N.J. Woolf (Steward Observatory, University of Arizona)
A major problem in defining TPF is the lack of information on the abundance of terrestrial planets. It is possible that they are around many nearer stars, or that even though there are a trillion of them in the universe, the nearest one is totally beyond our range for observation. We take as terrestrial planets, objects with a mass range from Mars to several Earth masses. These are the planets that are potentially habitable. Indirect detection does not seem practical for nearby planets, but can give an indication of the frequency, sizes, masses, and distances from their star. This information is inadequate to determine habitability. Direct detection of either visible or infrared radiation from the planet can give a good idea of habitability for those planets without overwhelming cloud cover. In fortunate cases, the likelihood of life being present can also be determined. Because the distance of the nearest such planets is indeterminate at present, we must be prepared for many possibilities. Early simple planet finders are needed to refine technology. They will at least tell us about larger planets. If we are fortunate, these devices will discover terrestrial planets. Larger missions should be in planning, but since the cost is likely to vary with a high power of distance to the nearest terrestrial planets, it is not appropriate to spend large resources there. If we are lucky, we might at similar cost be focussing on more advanced missions to study nearby objects.