Artist's impression of the planetary system around the red dwarf Gliese 581. (Courtesy: ESO)

It's a major discovery: a planet orbiting a red dwarf star some 20.5 light years away with Earth-like qualities, including a radius only 50 per cent larger and a mass about five more than our own planet.

But the potential of the planet to hold liquid water on its surface is what makes the discovery, unveiled Tuesday by the European Southern Observatory in Germany, so intriguing.

Though the planet is 14 times closer to its star than Earth is to our own sun, the star it orbits — the red dwarf Gliese 581 — is smaller and colder than our sun, meaning the planet lies in a potentially ideal temperature zone.

"We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius, and water would thus be liquid," explained Stephane Udry, the lead author of the paper reporting the result, in a statement.

"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," said Udry, from the Geneva Observatory in Switzerland.

Water covers over 70 per cent of the surface of the Earth, but the simple compound H2O has not proven so simple to find outside our own planet, particularly in liquid form, making it the Holy Grail for astronomers searching for planets that might one day support life.

What makes liquid water so important to life as we know it on Earth? A key property is the compound's bipolar nature, said Victoria Hipkin, a scientist with the Canadian Space Agency.

When an oxygen atom binds with two hydrogen atoms to form water, the larger oxygen atom moves to one side of the molecule while the hydrogen atoms move to the opposite side. This makes the oxygen side slightly negative in charge and the hydrogen side slightly positive. The bipolar nature of the molecule means water interacts strongly with other elements and chemical compounds it mixes with, making it an ideal solvent.

It's these interactions that — combined with a stable heat source — help stimulate the biochemical processes needed to create organic material and later life, said Hipkin.

The interactions work best when water is in liquid form, since molecules have freedom of movement but are also contained in a more defined area. The high range between water's freezing point (0 C) and boiling point (100 C) also helps create a stable environment for chemicals interacting in the liquid.

Water also possesses the unique quality of being less dense as a solid than as a liquid, which allows ice to float atop the liquid water and potentially act as a protective shield for life when a planet goes through a freezing cycle.

Rare in our solar system

Liquid water's uniqueness as an agent for life is matched only by its elusiveness in our own solar system, one of the reasons a planet over 20 light years away could spark such interest.

Of all the heavenly bodies orbiting our sun, only three seem likely candidates to support liquid water: the planet Mars, Jupiter's moon Europa and Saturn's moon Enceladus.

Mars, because of its proximity to Earth, has been the focus of an intense search for water. We already know water is on Mars; the planet's northern polar ice cap is made up mostly of frozen water. Finding liquid water, however, would require searching underground, since the planet's colder climate — the planet's surface temperatures can drop to -111 C — and low atmospheric pressure would either freeze liquid water into ice or disperse it in the atmosphere as gas.

The Phoenix Mars lander, slated to launch in August 2007, hopes to answer these questions when it lands on the planet's northern arctic plains. The lander will investigate whether the Martian arctic can support life, look into the history of water at the landing site and, using a meteorological station designed by the Canadian Space Agency, investigate how the Martian climate is affected by the polar conditions.

Farther away from Earth, two moons have attracted the attention of astronomers. One is Europa, an ice-covered moon orbiting Jupiter. Images from the Voyager and Galileo spacecrafts suggest Europa is covered in ice and perhaps even liquid water, though recent estimates based on crater depths suggest the solid ice on the exterior is at least 19 kilometres thick. But again, as with Mars, it's what's underneath the icy exterior that intrigues scientists.

Hipkin said the notion of an ocean underneath the icy surface makes Europa one of many targets for future exploration.

Saturn's tiny moon Enceladus, about one-seventh the size of our own moon, has also attracted attention ever since the Cassini-Hyugens spacecraft first spotted a geyser spewing what might be liquid water and ice crystals in 2005.

Like Mars, however, both moons have extremely cold surface areas that would likely not support liquid water for very long.

New viewing power

It's one of the potential differences between the ESO's newly discovered planet and our own solar system's candidates for liquid water. The planet is also distinct from many of the planets already discovered outside our solar system — mostly gas giants like Jupiter that could not sustain life as we know it.

The newly discovered planet, on the other hand, has a radius 50 per cent larger than Earth and five times our planet's mass, making it the smallest planet discovered outside our solar system.

The European scientists found it using a precise spectograph known as HARPS: for High Accuracy Radial Velocity for Planetary Searcher. HARPS has discovered 11 of the 13 known planets with a mass below 20 Earth masses, said Michel Mayor, the HARPS Principal Investigator from the Geneva laboratory.

It may not be the only tool astronomers will have available, however, as new innovations in telescope technology may also mean the discovery of more Earth-like planets may be around the corner.

NASA's Jet Propulsion Laboratory unveiled a new system in April 2007 called a High Contrast Imaging Testbed that reduces the glare from larger, brighter objects like stars and helps make planets more visible. The system will be placed aboard the Terrestrial Planet Finder, a NASA project currently without a launch date.

Getting to these far-off planets may prove more difficult. NASA is currently planning to build a moon base by 2020, with a mission to Mars to follow, but a trip to the red dwarf Gliese 581 may remain for now the realm of science fiction writers.

But in a statement announcing the dwarf star planet's discovery, a member of the astronomical team from the ESO remains undaunted.

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

Hipkin knows visiting planets outside our solar system is a long way off, but won't rule out the possibility altogether.

"One hundred and fifty years ago we had neither the motorcar or airplane, so it's very difficult to predict where we'll be over such a large time scale," she said. "One day it could be within our reach."

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