The planet furthest out in the Gliese 581 system, Gliese 581 d, orbits its host star in 66.8 days. Gliese 581 d is probably too massive to be made only of rocky material, but astronomers speculate that it is an icy planet that has migrated closer to the star. The new observations were made with the HARPS exoplanet hunter and have revealed that the planet is in the habitable zone, where liquid water could exist. It could thus be covered by a large and deep ocean and is so the first serious “water world” candidate, according to astronomers.