After more than four years of observations using the most successful low-mass-exoplanet hunter in the world, the HARPS spectrograph attached to the 3.6m ESO telescope at La Silla, Chile, astronomers have discovered in this system the lightest exoplanet found so far: Gliese 581-e (foreground) is only about twice the mass of our Earth.

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 color). The planet furthest out, Gliese 581 d, orbits its host star in 66.8 days, while Gliese 581 e completes its orbit in 3.15 days. Credit: ESO/L. Calçada

(PhysOrg.com) -- Exoplanet researcher Michel Mayor announces the discovery of the lightest exoplanet found so far. The planet, "e," in the system Gliese 581, is only about twice the mass of our Earth. The team also refined the orbit of the planet Gliese 581-d, first discovered in 2007, placing it well within the habitable zone, where liquid water oceans could exist. These amazing discoveries are the outcome of observations using the HARPS spectrograph attached to the 3.6m ESO telescope at La Silla, Chile.