gliese 581 c may be tidally locked to its star. just as one side of the moon always faces earth, the length of gliese 581 c's day would then match the length of its year. tidal locking also means the planet would have no axial tilt and that means the planet would not experience any seasons. even with a permanent day one one side, and a permanent night on one side, a sliver of land between the two zones may be habitable.