An artist's impression of the newly discovered planetary system around Gliese 581, including the lightest exoplanet yet discovered, Gliese 581 e. The Gliese 581 planetary system now has four known planets, with masses of about 1.9 (planet e, left in the foreground), 16 (planet b, nearest to the star), 5 (planet c, center), and 7 Earth-masses (planet d, with the bluish colour). Credit: ESO/L. Calcada
