The scientists determined that the planet, which they have called 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.
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, 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, eleven years of observation is considered a short time and 20 light years, which is roughly 117.5 trillion miles, 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," said Vogt.
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.
With surface temperatures decreasing the further one goes toward the dark side of the planet and increasing as one goes into the light side, the most habitable part of the new planet would be the line between darkness and light, which is known as the "terminator".
The researchers estimate that the average surface temperature of the planet would be between -24 and 10 degrees Fahrenheit (-31 to -12 degrees Celsius).