The planet, found by astronomers at the University of California, Santa Cruz, and the Carnegie Institution of Washington, is orbiting in the middle of the "habitable zone" of the red dwarf star Gliese 581, which means it could have water on its surface.
The scientists determined that the planet, named Gliese 581g, has a mass three to four times that of Earth and an orbital period of just under 37 days.
Its mass indicates that it is probably a rocky planet and has enough gravity to hold on to an atmosphere, according to Steven Vogt, professor of astronomy and astrophysics at the University of California, Santa Cruz, and one of the leaders of the team that discovered the planet.
"Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 per cent," Mr Vogt said.
"I have almost no doubt about it."
If Gliese 581g has a rocky composition similar to Earth's, its diameter would be about 1.2 to 1.4 times that of the Earth, the researchers said.
The surface gravity would be about the same or slightly higher than Earth's, so that a person could easily walk upright on the planet, Mr Vogt said.
Gliese 581g was discovered by scientists working on the Lick-Carnegie Exoplanet Survey, during 11 years of observing the red dwarf star Gliese 581, which is only 20 light years from Earth.
For astronomers, 11 years of observation is considered a short time and 20 light years, which is roughly 190 trillion kilometres, rather close. The sun is around eight and a half light minutes from Earth.
"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," Mr Vogt said.
The planet is tidally locked to its star, meaning that one side is always facing the star and basking in perpetual daylight, and the other is in perpetual darkness because it faces away from the star.
This would make the line between darkness and light the most habitable part of the new planet, which is known as the "terminator".
The researchers estimate that the average surface temperature of the planet would be between -31 to -12 degrees Celsius.
But actual temperatures would range from "blazing hot on the side facing the star, to freezing cold on the dark side", they said.
That means the probability of life existing in a band of perpetual twilight or "Goldilocks zone" (not too hot, not too cold) around the planet is high.
The findings, which will be published in the Astrophysical Journal and posted online at arXiv.org, "offer a very compelling case for a potentially habitable planet", Mr Vogt said.
In their report, the scientists in fact announce the discovery of two new planets around Gliese 581, bringing the total number of known planets around this star to six.
Two previously detected planets around Gliese lie at the edges of the habitable zone, one on the hot side and one on the cold side of the star, and are probably not habitable.
The newly discovered planet g, however, lies right in the middle of the habitable zone.
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