2005ApJ...627..974L

We present some new ideas about the possibility of life developing around subgiant and red giant stars. Our study concerns the temporal evolution of the habitable zone. The distance between the star and the habitable zone, as well as its width, increases with time as a consequence of stellar evolution. The habitable zone moves outward after the star leaves the main sequence, sweeping a wider range of distances from the star until the star reaches the tip of the asymptotic giant branch. Currently there is no clear evidence as to when life actually formed on the Earth, but recent isotopic data suggest life existed at least as early as 7x10⁸ yr after the Earth was formed. Thus, if life could form and evolve over time intervals from 5x10⁸ to 10⁹ yr, then there could be habitable planets with life around red giant stars. For a 1 M_☉star at the first stages of its post-main-sequence evolution, the temporal transit of the habitable zone is estimated to be several times 10⁹ yr at 2 AU and around 10⁸ yr at 9 AU. Under these circumstances life could develop at distances in the range 2-9 AU in the environment of subgiant or giant stars, and in the far distant future in the environment of our own solar system. After a star completes its first ascent along the red giant branch and the He flash takes place, there is an additional stable period of quiescent He core burning during which there is another opportunity for life to develop. For a 1 M_☉star there is an additional 10⁹ yr with a stable habitable zone in the region from 7 to 22 AU. Space astronomy missions, such as proposed for the Terrestrial Planet Finder (TPF) and Darwin, that focus on searches for signatures of life on extrasolar planets, should also consider the environments of subgiants and red giant stars as potentially interesting sites for understanding the development of life. We performed a preliminary evaluation of the difficulty of interferometric observations of planets around red giant stars compared to a main-sequence star environment. We show that pathfinder missions for TPF and Darwin, such as Eclipse and FKSI, have sufficient angular resolution and sensitivity to search for habitable planets around some of the closest evolved stars of the subgiant and red giant class.