A ''super-Earth'' that could have a life-supporting climate has been discovered in a multi-world solar system 42 light years from the Sun.
The planet, which is several times more massive than the Earth, lies just the right distance from its star to allow the existence of liquid surface water.
It orbits well within the star's ''habitable'' or ''Goldilocks'' zone - the region where temperatures are neither too hot nor too cold to sustain life.
The new world is one of six, all with masses a few times that of the Earth, believed to circle the dwarf star HD 40307 in the constellation Pictor.
All the others are located outside the habitable zone, too close to their parent star to support liquid water.
Three of these were originally identified in 2009 by European Southern Observatory astronomers. They used an instrument called Harps (High Accuracy Radial Velocity Planet Searcher) to look for changes in the colour of starlight that give away the presence of planets.
A new analysis of the Harps data, using special computer software, has now revealed three more super-Earths.
One of them, HD 40307g, excited astronomers because it was much further from the star than its companions - between half and three quarters of the Earth's distance from the Sun - and comfortably within the habitable zone.
Professor Hugh Jones, from the University of Hertfordshire, a member of the international team, said: ''The longer orbit of the new planet means that its climate and atmosphere may be just right to support life.
''Just as Goldilocks liked her porridge to be neither too hot nor too cold but just right, this planet or indeed any moons that is has lie in an orbit comparable to Earth, increasing the probability of it being habitable.''
Scientists believe the planet turns on its axis, giving it a regular day and night cycle and making its environment even more Earth-like.
Planets orbiting too close to their stars risk being ''tidally locked'' so they always have the same face pointing towards the star. In the same way, the Moon always has its nearside facing the Earth. One side of such a planet is likely to be scorching hot while the other is dark and cold. Scientists doubt that tidally locked planets could be habitable.
The discovery, published in the journal Astronomy & Astrophysics, arose from a European planet-hunting initiative called RoPACS (Rocky Planets Around Cool Stars),
Like many other extra-solar planets, HD 40307g was found by observing wobbles in the motion of its parent star caused by the tug of its gravity.
The wobbles stretch or compress the wavelength of light from the star as seen from the Earth, making it redder as it moves away and bluer as it comes closer. Analysing these patterns can provide information about the planet's mass.