Over the last few years, the term "Goldilocks zone" has come to be widely used to describe the range of distances from a star where its planets would be warm enough to have liquid surface water, but not too hot.

Obviously, our planet lies in the Sun's Goldilocks zone, with Mars orbiting at its outer edge. Planets lying in their stars' Goldilocks zones are prime candidates in our searches for life.

However, things have become much more complicated. There are other ways planets or moons can have oceans of liquid water that don't involve being at a precise distance range from a star.

If a moon orbits its host planet very closely, the gravitational pull of the planet on the side of the moon closer to the planet is larger than the gravitational pull on the other side of the moon. The result is a force trying to stretch the moon along a line joining it to the planet, so the moon gets stretched.

If that moon rotates, or if its distance from the planet changes, the........

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