The Zone

Three rocky planets orbit our Sun in the region where liquid water could exist, but only one of them is alive. For fifty years, astronomers have chased a single elegant idea about what makes a world habitable — a range of distances from a star where the temperature is just right. In this episode, we trace the "habitable zone" from a thought experiment to the calculation that still defines the field today, and find a warning hiding in plain sight. Even the scientists who built the concept knew it wasn't quite the answer everyone thinks it is.