The planet orbits a red dwarf star called Gliese 581, 20 light years from Earth, in the middle of the star's habitable zone, meaning that temperatures on its surface are just right for life to develop.
US-based scientists found the planet using precise measurements from the Keck telescope in Hawaii, which has been scrutinising Gliese 581 for more than a decade.
The new planet, called Gliese 581g, is one of several known to be orbiting the star, but is the first to be discovered in the so-called Goldilocks Zone, where the distance from the star means that temperatures are neither too hot or too cold for life to exist.
"We had planets on both sides of the habitable zone - one too hot and one too cold - and now we have one in the middle that's just right," Steven Vogt of the University of California, who worked on the team that discovered the planet, said.
He said that less than 500 planets have been discovered outside of the solar system, and the fact that this one lies in the middle of the Goldilocks Zone suggests that habitable planets could be extremely common.
"The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common," he said.
"If the local stellar neighbourhood is a representative sample of the galaxy as a whole, our Milky Way could be teeming with potentially habitable planets."
Relative term
"Nearby" is a relative term in astronomy, where the distances between stars and galaxies are enormous. A light-year is the distance light can travel in one year at a speed of 300,000km a second, or about 10 trillion km. Lying 20 light years from Earth, Gliese 581 is at least 200 trillion km away.
At that distance, it is impossible to make direct observations of planets. Instead, astronomers observe tiny "wobbles" in the star they orbit caused by the gravitational pull of orbiting planets.
This allows them to calculate the distance and size of the planets circling the star, and whether or not they could support life. The technique has allowed scientists to discover at least five planets orbiting Gliese 581, although none lay the correct distance from the star for life to exist.
The latest discovery has average temperatures averaging from -31 to -12 degrees Celsius. Because the planet does not rotate, one side of it is locked in perpetual darkness and is extremely cold, while the other is always facing its star and is much hotter. The habitable parts of planet are thought to be in the "dusk" areas, where the star would appear low in the sky.
Scientists say if the planet was rocky, like Earth, its gravity would be similar. The temperature would also theoretically allow liquid water to exist on its surface, but the team say they have not detected any evidence of water so far.
