Published online by Cambridge University Press: 24 September 2020
Here, I consider other factors than distance from a star that may affect a planet’s habitability. These include its atmosphere, its magnetic field, and whether it has any moons. However, I emphasize that it is important not to draw up a list of all the Earth’s specific features, for example its unusually large Moon (which helps to stabilize its axial tilt) and make the assumption that all of these are necessary for another planet to support life. Making such an assumption leads to the Rare Earth hypothesis, which I regard as flawed. For life to originate on a planet, there must be places where conditions favour the biochemical evolution that leads to proto-cells and hence to life. For life to continue and diversify, there must be places where organisms can survive. Even if conditions are normally benign, all planets are subject to occasional major threats, such as impacts and glaciations. I examine the mass extinctions on our own planet, some of which were caused by asteroids impacting our surface. Finally, I examine a problem that Earth is not subject to – tidal locking. This may be a major problem for planets in the habitable zones of red dwarfs.