Scientists hypothesize that, in addition to being in a star’s habitable zone, an Earthlike planet’s chances of supporting life are greatly improved if it is also far enough away from its star that it could have day-night cycles.

Currently, at least two planets with masses somewhat close to that of Earth have been discovered orbiting in habitable zones where they could have day-night cycles: one around the star Kepler-22, and another around the star HD 40307. Their discoveries were respectively announced in December 2011 and November 2012.