  We assess the possibility that planets Gliese 581 c and d are within the
habitable zone. In analogy with our solar system, we use an empirical
definition of the habitable zone. We include assumptions such as planetary
climates, and atmospheric circulation on gravitationally locked synchronous
rotation. Based on the different scenarios, we argue that both planets in
Gliese 581 could develop conditions for a habitable zone. In an Earth-like
environment planet d could be within a habitable zone, if an atmosphere
producing greenhouse effect of 100K could have developed. If the planets are
gravitationally locked-in, planet c could develop atmospheric circulation that
would allow it to reach temperatures consistent with the existence of surface
liquid water, which in turn could support life.
