Astronomers say they've found the first planet beyond our solar system that could have the right size and setting to sustain life as we know it, only 20 light-years from Earth.

"My own personal feeling is that the chances of life on this planet are 100 percent," Steven Vogt, an astrophysicist at the University of California at Santa Cruz, told reporters today. "I have almost no doubt about it."

The discovery, published online in The Astrophysical Journal, is the result of 11 years of observations at the Keck Observatory in Hawaii. Astronomers participating in the Lick-Carnegie Exoplanet Survey detected the planet by tracking the faint gravitational wobbles it produced in its parent star. Now they say there may well be many more planets out there like this one.

"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," Vogt said in a news release.

One of Vogt's co-authors, Paul Butler of the Carnegie Institution, reminded reporters during a teleconference today that the first exoplanet orbiting a normal star was detected 15 years ago. Since then, almost 500 other alien planets have been found. "We're at exactly that threshold now with finding habitable planets," Butler said.

The newfound planet, known as Gliese 581g, is estimated to be 3.1 to 4.3 times as massive as Earth, and makes a complete circuit around its sun in just under 37 days. If the planet has a rocky composition like Earth's, it would be 1.2 to 1.4 times as wide as our own planet, qualifying it as a "super-Earth."

Even more intriguingly, the red dwarf star's dimness and the planet's orbital distance (0.146 AU, less than half the distance between Mercury and our sun) suggest that the planet's average surface temperature is not that far below water's freezing point (somewhere between 10 and -24 degrees Fahrenheit, or -12 and -31 degrees Celsius).

Although that average may sound chilly, the astronomers say Gliese 581g appears to be tidally locked to its star, with one side perpetually in the sun and the other side perpetually dark. That means the highs on the day side would be hellishly hot. The lows on the night side would be unendurably cold. But there would be a livable zone along the line between shadow and light.

"Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude," Vogt said.

Based on this analysis, Vogt and his colleagues say Gliese 581g is in a planetary zone that is, in the words of the Goldilocks tale, "not too hot and not too cold, but just right" for water to exist somewhere in liquid form. Astrobiologists say that life seems to exist anyplace on Earth that has liquid water, and that such a Goldilocks zone should be conducive to alien life as well. Some astronomers have even proposed that super-Earths could be friendlier to life than our own home world.

The Gliese 581 system is already well-known to planet hunters. Gliese 581g is the sixth planet to be detected around the parent star. Two other planets in the system are on the edges of the Goldilocks zone: Gliese 581c (potentially "too hot") and Gliese 581d (potentially "too cold"). "Now we have one in the middle that's just right," Vogt said.

The method that was used to detect the latest member of Gliese 581's planetary family, known as radial velocity, requires painstaking observations over a number of years. As the method is currently practiced, it's not capable of finding Earthlike planets around sunlike stars. The Lick-Carnegie Exoplanet Survey was able to spot this super-Earth because it could exert a relatively large pull on a relatively small star. But the observations weren't easy: It took 238 measurements, conducted over 11 years with the aid of the European-led HARPS team, to confirm Gliese 581g's existence.

Astronomers believe it will be easier in the future to find habitable planets — not only because they're building up a larger database of radial velocity measurements, but also because new space probes such as NASA's Kepler and Europe's CoRoT satellite are detecting hundreds of exoplanets using a different technique known as the transit method.

"The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that's a large number," Vogt said. "There could be tens of billions of these systems in our galaxy."

But how accessible would they be? Relatively speaking, Gliese 581g is in our celestial neighborhood, but it would take tens of thousands of years to get there using conventional rocket technologies. Vogt said it might be possible to send a robotic probe to the planet using an experimental nuclear propulsion system, such as the Project Orion system that was proposed a half-century ago but never built.

"If you're traveling at a tenth of the speed of light, you could reach this thing in 200 years," Vogt told reporters, "Now, you probably wouldn't send humans there, because that would be multiple generations and you'd need a big crew cabin and there wouldn't be much to do for 200 years. But you could send sophisticated robot cameras. Basically, the equivalent of a Droid cell phone would do pretty well. ... In 220 years, if we started now, you would be able to get close-up pictures and a sense of what kind of atmosphere was there, and radio communications, that sort of thing. And it would be a great thing to do with the world's stockpile of nuclear weapons. Just put 'em up on a rocket and send 'em up there."

More on alien planets:

• Too big? 55 Cancri appears to have a planet in the Goldilocks zone, but it's 45 times as massive as Earth. That's probably too massive for life as we know it, but life could conceivably exist on a moon (such as Pandora in the movie "Avatar").
• Too hot? As mentioned above, Gliese 581c might be on the edge of the Goldilocks zone, but astronomers are coming around to the view that it's too hot to sustain life. The planet is at least five times as massive as Earth.
• Too cold? When Gliese 581c fizzled, astronomers suggested that Gliese 581d might be on the other side of the star system's habitable zone. Astronomers say life could be a possibility if the planet had a thick atmosphere to hold onto heat. Otherwise, it's probably too cold. It's also seven times as massive as Earth.
• Planet theory: Four types of habitable planets proposed
• Countdown: Watch Keith Olbermann's chat with astronomer Derrick Pitts
• The world next door: Discovery suggests Mars was once habitable
• Millions of Earths? Astronomer's talk causes a stir
• More on the way: Hunt for new worlds goes into overdrive
• Looking for alien Earths? Probes could turn up hundreds

In addition to Vogt and Butler, authors of "The Lick-Carnegie Exoplanet Survey: A 3.1 Me Planet in the Habitable Zone of the Nearrby M3V Star Gliese 581" include Eugenio Rivera, Nader Haghighipour, Gregory W. Henry and Michael H. Williamson. The research was funded by the National Science Foundation, NASA and the Carnegie Institution. Watch NSF's webcast of today's news briefing on the discovery.

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