The closest exoplanet to us, if we include only single stars like the Sun, is the planet around Barnard’s Star, Barnard’s Star-b (“BSb”). (The planet Promixa Centauri-b is closer, but Proxima Cen is part of a triple-star system with Alpha and Beta Centauri, and understanding the evolutionary development of the planet is more complicated.) BSb orbits at a distance similar to that of Mercury around the Sun, but Barnard’s Star is a cool M-dwarf star and so despite the planet being close to the star it probably resides near the snow line – the distance where stellar irradiation is weak enough to allow volatile elements to condense onto the planet’s surface. This makes BSb an especially interesting planet and possibly a keystone for future progress understanding planet formation and atmospheric evolution.