Astronomers claim to have discovered, for the first time, an Earth-like planet outside our solar system that could support water -- and potentially life.

Researchers have not directly seen the planet and it will be years before more sensitive instruments will be developed to look for signs of life on the planet that is about 190 trillion kilometres away from Earth.

But according to measurements, the planet is about 50 per cent larger and five times heavier than Earth. It orbits Gliese 581 -- a diminutive "red dwarf" star located in the constellation Libra.

Red dwarfs are low-energy stars that give off dim red light and last longer than stars like the Earth's sun. Until a few years ago, astronomers didn't consider these stars as possible hosts of planets that might sustain life.

The new planet is being named Gliese 581 C after its star that is about one-third the size of the Sun and is dimmer and cooler.

Researchers are touting the discovery as a milestone in planet detection and the search for extraterrestrial life

"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 extra-terrestrial life," said Xavier Delfosse, a member of the research team from Grenoble University in France, in a statement.

"On the treasure map of the universe, one would be tempted to mark this planet with an X."

Gliese 581 C is so far the smallest planet found beyond our solar system and the first that meets scientists' standards of being potentially hospitable to life.

But scientists say there is still a lot that is unknown. And under current standards scientists use to declare a planet habitable, Mars should be able to host life.

The results of the astronomers' discovery have not been published but have been submitted to the journal Astronomy and Astrophysics.

Gliese 581 C 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. The wobbles can reveal the existence of other worlds.

It's uncertain whether its surface is rocky like Earth or if it's a frozen ice ball with liquid water on the surface.

Based on theory, 581 c should have an atmosphere, said University of Geneva astronomer Michel Mayor, one of 11 European scientists on the team that found the planet.

However, what is in that atmosphere is still a mystery; and if it's too thick that could make the planet's surface temperature too hot to host life, Mayor said.

But researchers believe the average temperature to be somewhere between 0 and 40 degrees Celsius (32 and 104 degrees Fahrenheit).

Until now, all 220 planets astronomers have found outside our solar system have suffered from the "Goldilocks problem" -- that is, they have been either too hot, too cold or too monstrous and gaseous like Jupiter.

Scientists believe this new planet, however, could be just right.

"This could be very important," NASA astrobiology expert Chris McKay, who was not part of the discovery team, told the Associated Press. "It doesn't mean there is life, but it means it's an Earth-like planet in terms of potential habitability."

Besides having the right temperature, the discovery team says the new planet is probably full of liquid water which is critical to life. But that is based on theory about how planets form and not on any evidence, said Stephane Udry, the discovery team's lead author and another Geneva astronomer.

Two teams of astronomers, one in Europe and one in the U.S., have been racing to be the first to find a planet like 581 C outside the solar system. The European researchers looked at 100 different stars using a tool called HARPS (High Accuracy Radial Velocity for Planetary Searcher) to find this one planet.

The discovery of 581 C will likely fuel studies of planets circling red dwarf stars similar to Gliese 581. Red dwarf stars are the most common type of stars in our galaxy. About 80 per cent of the stars near Earth are red dwarfs.

Other details of the new planet:

• It orbits its star every 13 days.
• It circles its star 14 times closer than the Earth orbits the sun.
• Gravity is 1.6 times as strong as the Earth's -- so a 150-pound person would feel like 240 pounds on 581 C.
• Astronomers previously found a Neptune-sized planet circling the star Gliese 581, as well as evidence of a third planet about eight times the mass of Earth.

With a report from The Associated Press