Collection of exoplanets with measurements of Mg II in the NUV. The XUV irradiation (90–912 Å) for each planet is plotted vs. the planet’s escape velocity. Small gray points show the background population of known planets. We then run a logistical regression to constrain the median location of the “shoreline” (blue line) and the 1σ and 2σ confidence regions (blue shaded region). We assumed a scaled solar spectrum to calculate the XUV irradiation, but for planets around stars where T_eff > 8250 K, we show the XUV flux assuming a photosphere at the stellar T_eff. While KELT-9b would be expected to show escaping Mg II in either scenario, WASP-178b’s nondetection can be explained through a lack of XUV irradiation due to T_eff > 8250 K.