Since the only forms of life we know about require liquid water, it was generally thought that the best chances for the emergence of life are on a planet that orbits a particular distance away from its star. Too close in and the heat boils away any oceans; too far out and all water freezes solid.

What’s needed is a planet that’s not too hot, not too cold, but just right – this range of habitable orbital distances has been dubbed the ‘Goldilocks Zone’. But stars get hotter and hotter as they age, and so a planet must orbit at a habitable distance long enough for life to develop, giving us the concept of the continuously habitable zone (CHZ) around a star. It has now become clear, however, that liquid water is possible much further out from the sun, such as in moons orbiting the gas giants Jupiter and Saturn.

In addition, some astrobiologists have been arguing that not only must a planet orbit its star at the right distance, but the star itself must orbit the galactic centre in a particular way. Rocky planets like Earth might only form around stars in certain regions of the galaxy, and astronomers can think of many potential hazards in the cosmos, including close-shaves with other stars, exploding stars, and great dark dust clouds. So for various reasons, it might turn out that life only has a fighting chance on planets within the galactic habitable zone (GHZ), especially more fragile life like complex, or even intelligent, animals.