Gravitational lock, also known as captured rotation, is the tying of the orbital period of a planet or moon to its axial period through a tidal effect caused by the gravitational pull of the primary. The gravitational lock experienced by the Moon, for example, explains why it always keeps the same face directed toward the Earth. Likewise, the five inner satellites of Jupiter complete one orbit for every axial rotation. Mercury is locked in a 3:2 resonance with the Sun so that it completes 3 orbits of the Sun (each lasting 88 days) in the time it takes to spin twice on its axis (see Mercury, rotation of). The opportunity for life to evolve on planets around red dwarf stars may be compromised by this phenomenon. So small is the habitable zone around a red dwarf that any planets within this zone may be forced into a 1:1 lock thus preventing the kind of climatic variations, such as wetting and drying, that may have been conducive to early biological development on Earth. It may be possible, however, to envisage alternative pathways leading to life that do not require a day-night cycle.