Some 360 such planets have now been detected, but none is exactly equivalent to the Earth.The closest so far is Gliese 581c, which was discovered in 2007 by Dr Mayor's colleague, Stéphane Udry.
Unfortunately, astronomers do not yet know if its orbit transits Gliese 581's disc, as viewed from Earth.
Gliese 581e is a rocky planet like Earth, but it probably could not sustain life; it is too close to its star for liquid water to survive on its surface.
Gliese 581d is the third planet discovered around the red dwarf star Gliese 581, which has a mass 0.31 times that of the Sun and is 20.4 light-years from Earth.
Gliese 581d orbits its star every 66.64 days at a distance of 32.9 million km (20.5 million miles), which is within the habitable zone.
Gliese 581, extrasolar planetary system containing four planets.
Another, Gliese 581e, is the smallest planet seen in orbit around an ordinary main sequence star other than the Sun.
Hence, Gliese 229 B was the first object widely accepted as a brown dwarf.
Gliese 581d was discovered in 2007 and has a mass at least 5.6 times that of Earth.
Gliese 581e, discovered in 2009, has a mass 1.94 times that of Earth and is the smallest planet in the Gliese 581 system.
In the two years since Gliese 581g (pronounced GLEE-za) was announced, NASA's Kepler satellite has identified more than 2,300 candidate planets in a small portion of the Milky Way in the Cygnus constellation, which scientists are sifting in hopes of soon discovering what some call Earth 2.0.
