A list of potentially habitable alien planets has been compiled, with Gliese 581g ranked as a top candidate. The number of planets on the list is reported to be at least 5, but some sources suggest there may be more, with some outlets listing 6 or 10 exoplanets that could potentially harbor life. The criteria for determining habitability may be under review, as some research suggests that the habitable zones near certain stars may be smaller than previously thought.