Anyone familiar with the search for alien life will have heard of the "Goldilocks Zone" around a star. This is defined as the orbital band where the temperature is just right for liquid water to pool on a rocky planet's surface—a good approximation for what we thought of as the early conditions for life on Earth. But what happens if that life doesn't stay on an Earth analog? If they, like we, start to move toward their neighboring planets, the idea of a habitable zone becomes much more complicat...