Eight new 'Earths' which may hold liquid water and could sustain life have been spotted orbiting a distant sun in our galaxy.
The new worlds double the number of small exo-planets believed to be circling their stars in the 'Goldilocks zone' - neither too hot, nor too cold - where water would not evaporate or freeze.
Since liquid water is critical to life on Earth, many scientists believe the search for extraterrestrial life should focus on planets where it occurs. ''Most of these planets have a good chance of being rocky, like Earth,'' said lead scientist Dr Guillermo Torres, from the Harvard-Smithsonian Centre for Astrophysics in Cambridge, US.
The two most Earth-like planets out of the eight are known as Kepler-438b and Kepler-442b. They orbit red dwarf stars that are smaller and cooler than the Sun.
With a diameter just 12pc bigger than Earth, Kepler-438b has a 70pc chance of being rocky, the scientists have calculated. Kepler-442b is about one-third larger than Earth, and the likelihood of it being rocky is around 60pc. The planets are in the zone where they receive roughly as much sunlight as Earth. Too much heat from its star, and any water would boil away as steam. Too little, and the water would freeze solid.
''For our calculations we chose to adopt the broadest possible limits that can plausibly lead to suitable conditions for life,'' Dr Torres added. Kepler-438b receives about 40pc more light than the Earth, giving it a 70pc probability of having a habitable zone orbit. In comparison, baking-hot Venus has twice as much.
The other star gets about two-thirds as much light as Earth and is 97pc likely to be in the habitable zone.
Co-author Dr David Kipping, also from the Centre for Astrophysics, said: ''We don't know for sure whether any of the planets in our sample are truly habitable. All we can say is that they're promising candidates.''
The new planet was discovered using Nasa's Kepler telescope, which was launched in March 2009 to search for habitable zone, Earth-sized planets in the Milky Way. The telescope detects planets as their orbits cross in front of their star and cause a very tiny but periodic dimming of the star's brightness. However, visiting the planets to check for life is virtually impossible. Kepler-438b is located 470 light-years from Earth while Kepler-442b is 1,100 light-years away. Even the nearer planet is 2,726 trillion miles away and the light reaching Hubble is hundreds of years old.
The team, whose findings were presented at the American Astronomical Society's annual meeting in Washington DC, studied planetary candidates first identified by the US space agency. (© Daily Telegraph, London)