Astronomers say they have for the first time spotted a planet in what is sometimes called "the Goldilocks zone" - an area where conditions to support life are just right.

Not too far from its star, and not too close, so it could contain liquid water. The planet itself is neither too big nor too small for the proper surface, gravity and atmosphere. Just like Earth.

"This really is the first 'Goldilocks' planet," said co-discoverer R Paul Butler of the Carnegie Institution of Washington.

The planet sits in the middle of what astronomers refer to as the habitable zone, unlike any of the nearly 500 other planets astronomers have found outside Earth's solar system.

It also is in Earth's galactic neighbourhood, suggesting that plenty of Earth-like planets circle other stars.

Finding a planet that could potentially support life is a major step toward answering the timeless question: Are we alone?

Scientists have jumped the gun before on proclaiming that planets outside Earth's solar system were habitable only to have them turn out to be not quite so conducive to life.

This one, however, is so clearly in the right zone that five outside astronomers told the Associated Press it seems to be the real thing.

"This is the first one I'm truly excited about," said Penn State University's Jim Kasting. He said this planet is a "pretty prime candidate" for harbouring life.

Life on other planets does not mean ET. Even a simple single-cell bacteria or the equivalent of shower mould would shake perceptions about the uniqueness of life on Earth.

But there remain many unanswered questions about this strange planet. It is about three times the mass of Earth, slightly larger in width and much closer to its star - 14 million miles away instead of 93 million.

It is so close to its version of the sun that it orbits every 37 days. And it does not rotate much, so one side is almost always bright, the other dark.

Temperatures can be as hot as 71C or as cold as minus 4C, but in between - in the land of constant sunrise - it would be "shirt-sleeve weather", said co-discoverer Steven Vogt of the University of California at Santa Cruz.

It is unknown whether water exists on the planet, and what kind of atmosphere it has. Because conditions are ideal for liquid water, however, and because there always seems to be life on Earth where there is water, Mr Vogt believes "that chances for life on this planet are 100%".

The astronomers' findings are being published in Astrophysical Journal and were announced by the National Science Foundation.

The planet circles a star called Gliese 581. It is about 120 trillion miles away, so it would take several generations for a spaceship to get there. It may seem like a long distance, but in the scheme of the vast universe, this planet is "right in our face, right next door to us", Mr Vogt said in an interview.

That proximity and the way it was found so early in astronomers' search for habitable planets hints to scientists that planets like Earth are probably not that rare.

Mr Vogt and Mr Butler ran some calculations, with giant margins for error built in, and figured that as much as one out of five to 10 stars in the universe have planets that are Earth-sized and in the habitable zone.

With an estimated 200 billion stars in the universe, that means maybe 40 billion planets that have the potential for life, Mr Vogt said. However, Ohio State University's Scott Gaudi cautioned that is too speculative about how common these planets are.

Mr Vogt and Mr Butler used ground-based telescopes to track the star's precise movements over 11 years and watch for wobbles that indicate planets are circling it.

The newly discovered planet is actually the sixth found circling Gliese 581. Two looked promising for habitability for a while, another turned out to be too hot and the fifth is likely to be too cold. This sixth one - called Gliese 581g - bracketed right in the sweet spot in between, Mr Vogt said.

"It's not a very interesting name and it's a beautiful planet," Mr Vogt said. Unofficially, he's named it after his wife: "I call it Zarmina's World."

The star Gliese 581 is a dwarf, about one-third the strength of Earth's sun. Because of that, it cannot be seen without a telescope from Earth, although it is in the Libra constellation, Mr Vogt said.

But if you were standing on this new planet, you could easily see Earth's sun, Mr Butler said.

The low-energy dwarf star will live for billions of years, much longer than Earth's sun, he said. And that just increases the likelihood of life developing on the planet, the discoverers said.

"It's pretty hard to stop life once you give it the right conditions," Mr Vogt said.

His colleague, Paul Butler of the Carnegie Institution of Washington, in Washington, D.C., wasn't willing to put a number on the odds of life, though he admitted he's optimistic.

"It's both an incremental and monumental discovery," Sara Seager, an astrophysicist at the Massachusetts Institute of Technology, told Incremental because the method used to find Gliese 581g already has found several planets (all super-Earths, more massive than our own world) outside their stars' habitable zone, along with non-Earth-like planets within the habitable zone.

"It really is monumental if you accept this as the first Earth-like planet ever found in the star's habitable zone," said Seager, who was not directly involved in the discovery.

Vogt, Butler and their colleagues will detail the planet finding in the Astrophysical Journal.

The newfound planet joins more than 400 other alien worlds known to date. Most are huge gas giants, though several are just a few times the mass of Earth.

Stellar tugs

Gliese 581g is one of two new worlds the team discovered orbiting the red dwarf star Gliese 581, bumping that nearby star's family of planets to six. The other newfound planet, Gliese 581f, is outside the habitable zone, researchers said.

The star is located 20 light-years from Earth in the constellation Libra. One light-year is about 6 trillion miles (10 trillion km).

Red dwarf stars are about 50 times dimmer than our sun. Since these stars are so much cooler, their planets can orbit much closer to them and still remain in the habitable zone.

Estimates suggest Gliese 581g is 0.15 astronomical units from its star, close enough to its star to be able to complete an orbit in just under 37 days. One astronomical unit is the average distance between the Earth and sun, which is approximately 93 million miles (150 million km).

The Gliese 581 planet system now vaguely resembles our own, with six worlds orbiting their star in nearly circular paths.

With support from the National Science Foundation and NASA, the scientists -- members of the Lick-Carnegie Exoplanet Survey -- collected 11 years of radial velocity data on the star. This method looks at a star's tiny movements due to the gravitational tug from orbiting bodies.

The subtle tugs let researchers estimate the planet's mass and orbital period, how long it takes to circle its star.

Gliese 581g has a mass three to four times Earth's, the researchers estimated. From the mass and size, they said the world is probably a rocky planet with enough gravity to hold onto an atmosphere.

Just as Mercury is locked facing the sun, the planet is tidally locked to its star, so that one side basks in perpetual daylight, while the other side remains in darkness. This locked configuration helps to stabilize the planet's surface climate, Vogt said.

"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, suggesting that life forms that like it hot would just scoot toward the light side of that line while forms with polar-bear-like preferences would move toward the dark side.

Between blazing heat on the star-facing side and freezing cold on the dark side, the average surface temperature may range from 24 degrees below zero to 10 degrees Fahrenheit (minus 31 to minus 12 degrees Celsius), the researchers said.

Are you sure?

Supposedly habitable worlds have been found and later discredited, so what makes this one such a breakthrough?

There's still a chance that further observations will dismiss this planet, also. But over the years, the radial velocity method has become more precise, the researchers point out in their journal article.

In addition, the researchers didn't make some of the unrealistic assumptions made in the past, Seager said.

For instance, another planet orbiting Gliese 581 (the planet Gliese 581c) also had been considered to have temperatures suitable for life, but in making those calculations, the researchers had come up with an "unrealistic" estimate for the amount of energy the planet reflected, Seager pointed out. That type of estimate wasn't made for this discovery.

"We're looking at this one as basically the tip of the iceberg, and we're expecting more to be found," Seager said.

One way to make this a reality, according to study researchers, would be "to build dedicated 6- to 8-meter-class Automated Planet Finder telescopes, one in each hemisphere," they wrote.

The telescopes -- or "light buckets" as Seager referred to them -- would be dedicated to spying on the nearby stars thought to potentially host Earth-like planets in their habitable zones. The result would be inexpensive and probably would reveal many other nearby potentially habitable planets, the researchers wrote.

Beyond the roughly 100 nearest stars to Earth, there are billions upon billions of stars in the Milky Way, and with that in mind, the researchers suggest tens of billions of potentially habitable planets may exist, waiting to be found.

Planets like Gliese 581g that are tidally locked and orbit the habitable zone of red dwarfs have a high probability of harboring life, the researchers suggest.

Earth once supported harsh conditions, the researchers point out. And since red dwarfs are relatively "immortal" living hundreds of billions of years (many times the current age of the universe), combined with the fact that conditions stay so stable on a tidally locked planet, there's a good chance that if life were to get a toe-hold it would be able to adapt to those conditions and possibly take off, Butler said.