10 - Habitable Zones

Summary

Here, I investigate the concept of a habitable zone. Given that all life on Earth – and possibly all life in general – needs water, the defining of a habitable zone as the belt around a star in which liquid water could exist on a planet’s surface seems sensible. However, habitable zones may shift in position over time. One reason for this is the gradual increase in energy output from a star as it progresses through its main-sequence phase. To take account of this, we can define a continuously habitable zone (CHZ) in which liquid water could exist on a planet’s surface throughout its history. Inevitably, the CHZ will be narrower than its ‘instantaneous’ counterpart. I consider the question of whether life might exist, or might have existed, on Mars – a planet that is near the outer edge of the Sun’s habitable zone. I also look at whether life might exist on the moons Europa and Enceladus. Although these are far outside the habitable zone, they are thought to have sub-surface oceans. Finally, I ask whether in addition to there being stellar habitable zones there may also be larger-scale habitable zones – galactic ones. The answer is a qualified ‘no’.