The last two decades have seen an explosion of detections of exoplanets, as the sensitivity to smaller planets has dramatically improved thanks especially to the Kepler mission. These discoveries have found that the frequency of planets increases to smaller sizes: terrestrial planets are more common than gas giants. The significance of a universe rich in terrestrial sized planets naturally leads to the question about the “habitable zone (HZ)” – the region around a star where a suitable planet could sustain the conditions necessary for life. In this zone, the balance between stellar radiation onto the planet and radiative cooling from the planet allows water on the surface to be a liquid. (The definition also includes consideration of the planet’s atmosphere and solid surface.)