Scientists have discovered a warm and rocky "second Earth" circling a star, a find they believe dramatically boosts the prospects that we are not alone.
The planet is the most Earth-like ever spotted and is thought to have perfect conditions for water, an essential ingredient for life.
European researchers detected the planet orbiting one of Earth's nearest stars, a cool "red dwarf" called Gliese 581, 20 light years away in the constellation of Libra.
They say it is potentially habitable, with Earth-like temperatures, and have described the find as a big step in the search for life in the universe.
The planet is just the right size, might have water in liquid form, and in galactic terms is relatively nearby at 120 trillion miles away.
Yet the red dwarf star it closely orbits is much smaller, dimmer and cooler than our sun.
There's still a lot that is unknown about the new planet, and it could be deemed inhospitable to life once more is known about it - it's worth noting that scientists' requirements for habitability count Mars in that category: a size relatively similar to Earth's with temperatures that would permit liquid water.
However, this is the first planet spotted outside our solar system that meets the standards.
"It's a significant step on the way to finding possible life in the universe," said University of Geneva astronomer Michel Mayor, one of 11 scientists on the team that found the planet, on Tuesday.
"It's a nice discovery. We still have a lot of questions," he added.
Alan Boss, who works at the Carnegie Institution of Washington where a US team of astronomers competes in the hunt for an Earth-like planet, called it "a major milestone."
The planet was discovered by the European Southern Observatory's telescope in La Silla, Chile, which has a special instrument that splits light to find wobbles in different wave lengths. Those wobbles can reveal the existence of other worlds.
What they revealed is a planet circling red dwarf Gliese 581 - something of a surprise as until a few years ago astronomers didn't consider these stars as possible hosts of planets that might sustain life.
The new planet is about five times heavier than Earth. Its discoverers aren't certain if it is rocky like Earth or if its a frozen ice ball with liquid water on the surface.
If it is rocky like Earth, which is what the prevailing theory proposes, it has a diameter about 1 1/2 times bigger than our planet. If it is an iceball, as Mayor suggests, it would be even bigger.
Based on the rocky theory the planet, christened 581 c, should have an atmosphere, but what's in that atmosphere is still a mystery and if it's too thick that could make the planet's surface too hot, Mayor said.
However, the research team believes the average temperature to be somewhere between 32 and 104 degrees and that set off celebrations among astronomers.
Until now, all 220 planets astronomers have found outside our solar system have had the "Goldilocks problem." They've been too hot, too cold or too big and gaseous, like uninhabitable Jupiter.
The new planet seems just right or at least that's what scientists think.
Eventually astronomers will rack up discoveries of dozens, maybe even hundreds of planets considered habitable, the astronomers said. But this one simply called "c" by its discoverers when they talk among themselves will go down in cosmic history as the first.
Besides having the right temperature, the new planet is probably full of liquid water, hypothesizes Stephane Udry, the discovery team's lead author.
"Liquid water is critical to life as we know it," co-author Xavier Delfosse of Grenoble University in France, said in a statement.
"Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extraterrestrial life," said Delfosse. "On the treasure map of the Universe, one would be tempted to mark this planet with an X."
Source: China Daily/agencies