By watching for a special kind of flare, astronomers have identified the fingerprints of an Earth-size planet orbiting a distant star.The planet orbits its host star, a dim “M dwarf,” just at the edge of the habitable zone — the region where liquid water could exist. Jupiter’s moon Io — the solar system’s most volcanic world — has inspired a new way to find distant exoplanets. As the moon orbits Jupiter, it tugs on the planet’s magnetic field, generating bright auroras in Jupiter’s atmosphere. Even if we couldn’t see Io itself, the enormous auroras, pulsing to the beat of...