A census of the closest stars to the Sun (within a distance of 13 light-years - see Table 16.1 on page 405) shows that 3 out of 34 stars are in the right mass range for the development of life. Two of these are paired in the Alpha Centauri binary system: a planet orbiting either of the stars would have two "suns" in the sky (see Fig. 16.6 on page 406).

Gonzalez, Brownlee, and Ward (October 2001, Scientific American) suggest that there is a Galactic Habitable Zone which is defined by the distance from the center of the Milky Way Galaxy. The GHZ depends on:

• Availability of atoms heavier than H, He to make rocky planets. If "metal" abundance is too low (in outer galaxy), then not enough material to make planets. If too high, then planets may have too much water.
• Seclusion from threats. Cometary bombardment (more dangerous in inner galaxy where more stars exit) and radiation ( supermassive black hole in the center of the galaxy, supernovae, gamma-ray bursts, nearby massive stars).