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AP – An artist's impression of 'Planet e' , forground left, released by the European Organization for Astronomical …

Impression of 'Planet e' , forground left, released by the European Organization 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. The team also refined the orbit of the planet Gliese 581 d, (coloured blue in image) first discovered in 2007, placing it well within the habitable zone, where liquid water oceans could exist. These discoveries are the outcome of more than four years of observations using the most successful low-mass-exoplanet hunter in the world, the HARPS spectrograph attached to the 3.6-metre ESO telescope at La Silla, Chile.

Organization for Astronomical Research in the Southern Hemisphere.

An American expert called the discovery of the tiny planet "extraordinary."

Ø "The Holy Grail of current exoplanet research is the detection of a rocky, Earth-like planet in the 'habitable zone,'" said Michel Mayor, an astrophysicist at Geneva University in Switzerland.

Ø Gliese 581 e is only 1.9 times the size of Earth — while previous planets found outside our solar system are closer to the size of massive Jupiter, which NASA says could swallow more than 1,000 Earths.

Ø Gliese 581 e sits close to the nearest star, making it too hot to support life. Still, Mayor said its discovery in a solar system 20 1/2 light years away from Earth is a "good example that we are progressing in the detection of Earth-like planets."

Ø Gliese 581 d is probably too large to be made only of rocky material, fellow astronomer and team member Stephane Udry said, adding it was possible the planet had a "large and deep" ocean.

Ø "It is the first serious 'water-world' candidate," Udry said.

Ø Like other planets circling that star — scientists have discovered four so far — Gliese 581 e was found using the European Southern Observatory's telescope in La Silla, Chile.