Paris - Astronomers reported on Wednesday they had discovered a "super-Earth" more than 20 light years away that is the most intriguing world found so far in the search for extraterrestrial life.
About five times the mass of Earth, the planet orbits a cool, dim "red dwarf" star located in the constellation of Libra, the team from the European Southern Observatory (ESO) said in a press release.
The star, Gliese 581, has already been identified as hosting a planet similar in size to Neptune, the frigid gas giant on the edge of our own Solar System.
The new planet is 14 times closer to Gliese 581 than the Earth is to the Sun. But because Gliese 581 is so cool, the planet is not scorched by solar radiation. It zips around the star at express speed, making just 13 days to complete an orbit.
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| 'Its radius should be only 1.5 times the Earth's radius' |
"We have estimated that the mean temperature of this super-Earth lies between 0 and 40°C, and water would thus be liquid," said lead researcher Stephane Udry of Switzerland's Geneva University.
"Moreover, its radius should be only 1.5 times the Earth's radius, and models predict that the planet should be either rocky - like our Earth - or covered with oceans."
"Liquid water is critical to life as we know it," said Xavier Delfosse, a team member from France's Grenoble University.
"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.
He added: "On the treasure map of the Universe, one would be tempted to mark this planet with an X."
In 1995, two astronomers, also at Geneva, spotted the first extrasolar planet, a term for a planet orbiting a star other than our own. Since then, 227 such planets have been spotted, according to the Extrasolar Planets Encyclopaedia.
Gliese 581 is among the 100 closest stars to us. Its "red dwarf" monicker comes from the fact that it is only one-third of the mass of the Sun.
Because such stars emit far less heat, they offer a far greater chance of having planets in the so-called Goldilocks zone where liquid water - and thus the potential for life - can exist.
The search for extrasolar planets has thrown up very few potential home-from-homes.
The finds are typically uninhabitable, gassy giant planets, rather than small rocky ones. Or they orbit so close to their star that their atmosphere is stripped away by solar radiation or, alternatively, at such a distance that they are ice balls.
Two years ago, the same ESO team found a Neptune-sized planet around Gliese 581, and their observations that time gave them a hint that a smaller planet, closer to Earth in size, might also encircle the star.
Not only did they find the "super-Earth", they also found indications of another planet, eight times the size of Earth, that completes an orbit in 84 days -- a discovery that makes Gliese 581 "quite a remarkable system," ESO said.
The discovery is being reported in the journal Astronomy and Astrophysics.
It came thanks to a highly sensitive instrument at ESO's facility at Silla, Chile.
The detection technique works by measuring the so-called radial velocity of the star.
A planet orbiting around a star produces changes in the star's position relative to Earth. This causes a measurable "wobble" in the starlight, from which the orbiting object and its size can be inferred.
Even though Gliese 581 offers such promise, it would be impossible for mankind to reach it - or even send an unmanned scout probe - using current technology. Chemical rockets generate only a fraction of the light speed needed to get there within a human timescale.
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