ULTRAVIOLET-RADIATION FROM F-STAR AND K-STAR AND IMPLICATIONS FOR PLANETARY HABITABILITY

Now that extrasolar planets have been found, it is timely to ask wheth er some of them might be suitable for life. Climatic constraints on pl anetary habitability indicate that a reasonably wide habitable zone ex ists around main sequence stars with spectral types in the early-F to mid-K range. However, it has not been demonstrated that planets orbiti ng such stars would be habitable when biologically-damaging energetic radiation is also considered. The large amounts of UV radiation emitte d by early-type stars have been suggested to pose a problem for evolvi ng life in their vicinity. But one might also argue that the real prob lem lies with late-type stars, which emit proportionally less radiatio n at the short wavelengths (lambda < 200 nm) required to split O-2 and initiate ozone formation. We show here that neither of these concerns is necessarily fatal to the evolution of advanced life: Earth-like pl anets orbiting F and K stars may well receive less harmful UV radiatio n at their surfaces than does the Earth itself.