The first announcement of a planet around the star was Gliese 581b discovered by astronomers at the Observatory of Geneva in Switzerland and Grenoble University in France. Detected in August 2005 and using extensive data from the ESO/HARPS spectrometer it was the fifth planet to be discovered around a red dwarf. Further observations by the same group resulted in the detection of two more planets, Gliese 581c and Gliese 581d. The orbital period of Gliese 581d was originally thought to be 83 days but was later revised to a lower value of 67 days. The revised orbital distance would place it at the outer limits of the habitable zone, the distance at which it is believed possible for liquid water to exist on the surface of a planetary body, given favourable atmospheric conditions. Gliese 581d was estimated to receive about 30% of the intensity of light the Earth receives from the Sun. By comparison, sunlight on Mars has about 40% of the intensity of that on Earth, though if high levels of carbon dioxide are present in the planetary atmosphere, the greenhouse effect could keep temperatures above freezing.