How Close Can a Planet Get to a Star and Still Be Habitable?

In exoplanetology, the ring around the star is often called the “Goldilocks zone,” in reference to the 19th-century fairy tale Goldilocks and the Three Bears. In that story, Goldilocks encounters sets of three objects that are either too extreme for her liking or just right. In the case of a bowl of porridge, the three …