The Surprising Ways Mars is Hostile to Life

One thing I haven't seen much discussion of is the Moon's role in plate tectonics and the Earth's magnetic field. The tidal stresses induced in the Earth's interior by the moon may play a significant role in keeping the outer core liquid, creating the magnetic field. wich is essential for life. This movement of the liquid core also plays a role in plate tectonics. Platectonics plays a role in sequestering carbon and replenishing atmospheric nitrogen, and thus is also essential for life.

The only other terrestrial planet with a magnetic field is Mercury, whose field is much weaker than Earth's. In Mercury's case, the tidal stresses caused by its proximity to the Sun would account for the continued liquefaction of the core.

If this is the case, then a requirement for life on extrasolar planets would be that they must be under some form of tidal stress, either from having a large moon or from being Earth-sized moons of a close-orbit gas giant. However, close-orbit gas giants are invariably migrating inward and will become hot Jupiters, whose moons would then be to hot to sustain life. Thus such moons would not be long-enough lasting to harbour complex life.

This leads me to think that while planets are common, the circumstances that gave rise to the Earth-Moon system are correspondingly rare, meaning that life - particularly complex life - is also most likely very rare.