In planning a targeted search, either for exoplanets of potential biological interest or signals from an extraterrestrial intelligence (see SETI), two major factors to be considered are: (1) the likelihood that a given star could support life on a world of suitable size orbiting within the habitable zone, and (2) the star's proximity to the Sun. Since the only known examples of life have evolved on a world circling around a solitary, 5-billion-year-old G star, this type of star, or ones similar to it, appear to offer the best chances of success. Targeted searches for other planets and targeted SETI programs, therefore, have tended to focus on Sunlike stars within a few tens of light-years of the Earth (see stars, nearest).

 

Of the stars within a radius of 12 light-years (3.7 pc), only three are solitary and reasonably similar to the Sun in temperature and brightness: Epsilon Eridani (slightly cooler and dimmer, at 10.7 light-years), epsilon Indi (slightly cooler and significantly dimmer, at 11.2 light-years), and tau Ceti (slightly cooler and about the same brightness, at 11.9 light-years). The nearest star system of all is the triple-sunned Alpha Centauri, but it remains unclear how likely life is to evolve on a world subject to the complex gravitational perturbations of a binary or multiple star.

 

Of course, for all we know, it might be that a solitary star very different from the Sun, with a compliment of orbiting worlds, could nurture the evolution of some form of life. Inhabited planets around red dwarfs, for example, the commonest type of star, cannot be ruled out.