Exoplanets in Transits and their Atmospheres (ExTrA) telescopes at night. Visible in the sky is Orion (right of centre) and the Pleiades star cluster (left of centre). The ExTrA project in La Silla, Chile, is a series of three 60-cm telescopes searching for Earth-sized worlds around stars of a kind known as M dwarfs. These stars were selected because of their small size, which makes it easier to detect orbiting planets via the transit method; a transiting planet will block a greater fraction of the light from a smaller star.