Illustration of the imagined view from the surface of the exoplanet Barnard's Star b. Discovered in 2018, this rocky planet orbits Barnard's Star - a red dwarf only 20 percent of the diameter of the Sun - at a distance comparable to that of Mercury around the Sun. Barnard's star is the fourth closest star to the Sun (after the triple system of Alpha Centauri A and B, and Proxima Centauri). This makes the planet the second closest known exoplanet. It is estimated to be three times the mass of the Earth, very cold at -170 Celsius, and therefore unlikely to have liquid water on its surface, although an atmosphere cannot be ruled out. The planet and star are in the constellation of Ophiuchus, 6 light-years from Earth.