In either case, Gliese 581 c will likely become a target for missions in search of extraterrestrial life, they added.
"We still have a long way to go before reaching that point. But for sure it's the best candidate we know of right now," Udry commented.
"The planet is really close to us," he said. Still, it would take 20 years to get there if traveling at the speed of light, and another 20 to return.
Gliese 581 c is better suited to life than larger planets like Jupiter, which tend to be dense masses of gas, Udry explained.
(See an interactive map of the solar system.)
"You need a rocky planet to find life—the big giants are not the best places for that," he said.
More precise instruments have recently enabled astronomers to detect small "exoplanets"—worlds found outside our solar system.
"We started to find them two or three years ago," Udry said. Thirteen exoplanets that have less than 20 times the weight of Earth have been discovered so far, he noted.
"We found them very easily, so it looks like they are much more numerous than the giant planets we were finding before," the astronomer said.
The new planet was detected using an instrument called a spectrograph at the European Southern Observatory at La Silla, Chile.
Known as the High Accuracy Radial Velocity Planetary Searcher (HARPS), the device is described by the team as a "unique planet-hunting machine."
It works by detecting the pull of an unseen exoplanet on the star it orbits. An orbiting planet causes its star to wobble slightly, and this effect can be measured by instruments such as HARPS.
Advanced spectrographs are enabling astronomers to detect ever smaller planets, said Michael Perryman of the European Space Agency's Astrophysics Missions Division in the Netherlands.
"The wobble for these planets that they are detecting now is very, very tiny—about three meters [nine feet] per second, which is about the speed you run at," Perryman said.
"New planets are being discovered every few weeks or so," he added. "The interesting development is when you start getting these lower-mass planets closer to [the weight] of the Earth."
The newfound planet is especially noteworthy, Perryman said.
"As soon as you find a planet at the right distance [from its star] such that liquid water might exist, then you're saying this is the kind of environment in which one might start looking for life," he added.
Udry, of the Geneva Observatory, said the goal of future programs is to find a planet and star pairing to match that of Earth and the sun.
"We are now developing instruments which will allow us to find those," Udry said
"We hope, and even expect, to have these habitable planets all over the place."
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