An artist's impression of 'Planet e' , forground left, released by the European Organisation for Astronomical Research in the Southern Hemisphere Tuesday April 21, 2009. Exoplanet researcher Michel Mayor announced Tuesday the discovery of the lightest exoplanet found so far. The planet, 'e', in the famous system Gliese 581, in the constellation of Libra and 20.5 light years (192 trillion km or 119 trillion miles) away, is only about twice the mass of Earth.[China Daily/Agencies]