First Truly Habitable Planet Discovered, Experts Say

Astronomers studying a nearby star say they've found the first potentially habitable planet—likely a rocky place with an atmosphere, temperate regions, and crucially, liquid water, considered vital for life as we know it.

Other extrasolar planets have been called Earthlike, but, astronomer Paul Butler assured, "this is really the first Goldilocks planet"—not too hot, not too cold.

Orbiting the red dwarf star Gliese 581 about every 37 days, the new planet, named Gliese 581g, is "just the right size and just at the right distance [from its star] to have liquid water on the surface," added Butler, of the Carnegie Institution for Science in Washington, D.C., during an online press briefing today.

(Related: "Earthlike Planet Spied in Distant Solar System.")

Gliese 581g in Goldilocks Zone

Located some 20 light-years from Earth, Gliese 581 is among the hundred closest stars to us. Already scientists have detected six planets orbiting the red dwarf, making Gliese 581 the hub of the largest known planetary system outside our solar system.

The star has also inspired perhaps the largest number of habitable-planet headlines.

For starters, planet Gliese 581c was announced in 2007 as potentially habitable but later found to orbit too close to the star—making the planet too hot for life.

Another planet, Gliese 581d, is thought to orbit on the cold side of the habitable zone. While Gliese 581d could harbor life, the planet would need a thick atmosphere with a strong greenhouse effect to warm up to the point of habitability. (Find out why some think Gliese 581d may hold water.)

"They are very close to habitable, but not quite," Steve Vogt, a professor of astronomy and astrophysics at the University of California, Santa Cruz, said during the briefing.

"This one is right between the two, in the same system."

(Also see "'Sex c' New Planet Discovered.")

Terminator Aliens on Earthlike Planet?

Roughly three times more massive than Earth, the newfound planet is tidally locked to its star, which means that one side is perpetually basked in daylight, the other side constantly dark.

Aliens, if they exist, are most likely to live along the line between shadow and light, a temperate region known as the terminator, the scientists said.

Imagining the view from the terminator, Vogt said, "You basically see this star sitting on the horizon. You see an eternal sunrise or sunset, depending on whether you are an optimist or a pessimist."

Planet Discovery Suggests Billions More

The Gliese 581g discovery is based on 11 years of observations, largely via the W.M. Keck Observatory in Hawaii. The data allowed scientists to detect the wobble in a star's orbit caused by the gravitational pull of an orbiting planet—a technique called radial velocity.

Given the relative ease of finding this planet, 10 to 20 percent of all stars may have potentially habitable planets, Vogt said in a press release. (See an interactive guide to the hundreds of known planets.)

"There could be tens of billions of these systems in our galaxy."

The Gliese 581g habitable-planet findings are to be reported in a future issue of The Astrophysical Journal.