Searching for extrasolar planets and life

The emerging possibility of searching for life outside the solar system creates a challenging situation for today's civilization. Maybe it is somewhat analogous to that of our Paleolithic ancestors when, roaming along a shoreline, they saw inaccessible off-shore islands and wondered what life forms they could carry. Today's inaccessible islands, which may also carry life forms, are the extrasolar planets, dubbed as “exoplanets,” and man's curiosity about them has pushed NASA and ESA to encourage efforts toward observing exo-Earths. When images of exoplanets are obtained with large telescopes in space, a subsequent step will be the construction of even larger versions designed to obtain images with enough resolved detail to search for life signatures. There are so many possible answers to the question of where life might be sustainable, and therefore where to look for such signs, that it is necessary to define what is called a “habitable zone”: for the purposes of present-day efforts this is considered to be the region where liquid water can exist for at least some part of the time. This defines a rather small shell around any star, which in the case of our solar system includes the planets Venus, Earth and Mars. Of course this definition excludes the possibility of life which has evolved independently of water, but somewhere a line has to be drawn in order to contain the investigation.