The hunt for a second Earth began in earnest last Thursday with the launch of a space probe that will peer beyond the solar system to distant planets warmed by the faintest of stars.

At 2.23pm UK time a modernised Soyuz rocket tore into the sky over Baikonur Cosmodrome in Kazakhstan, carrying Corot, the first space telescope designed to find habitable planets orbiting stars in remote solar systems.

The mission, which will take place over a two-and-a-half-year period, will look for rocky worlds about twice the size of Earth that lie in what space scientists call habitable zones, the "Goldilocks" regions of space in every solar system where heat from the nearest star is neither too hot nor too cold to sustain liquid water - believed to be the essential ingredient for life.

Warm rocky planets similar in size to Earth are astronomers' best hope of finding extraterrestrial life, and any spotted by Corot will be studied intensely by future missions scheduled for the next decade. The mission will also prove invaluable for scientists hoping to understand how planets form and how common other "Earths" may be.

More than 200 planets have been discovered around stars beyond our own solar system, using ground-based telescopes that detect the way light is bent by a planet's gravitational field. But the technique only works well for huge planets, and nearly all so far discovered are gas giants, planets at least as big as Jupiter.

To discover smaller extra-solar planets, scientists need much larger, ground-based telescopes, or orbiting planet-finders, which do not have to "see" through our own atmosphere.

Corot, a four-metre-long probe built by the French space agency, CNES, with help from British and other European scientists, will use its 27 centimetre-diameter telescope to gaze at 120,000 stars from an orbit more than 500 miles above Earth. It will look for minuscule changes in the brightness of the stars caused by orbiting planets passing in front of them. By detecting changes in brightness as slight as 300 parts per million, scientists expect the telescope to spot between 10 and 40 small rocky planets. Many are expected to be geologically active and capable of supporting an atmosphere. Ian Roxburgh, a cosmologist at Queen Mary University, London, and a Corot mission scientist, said the telescope would monitor stars as distant as 10,000 million million miles away.

"Until we have a way of travelling much, much faster in space, the idea of being able to reach even the nearest of these planets is firmly in the realm of science fiction. What this will tell us is the frequency with which these planets form around other stars."

Corot, which stands for "Convection Rotation and Planetary Transits", will also track the acoustic waves that ripple across the surface of distant stars and plunge into their interiors in the same way as monitoring seismic waves from earthquakes sheds light on the structure of Earth.

The probe is the first of a flotilla of planet-hunting satellites due to launch in the near future. In 2015 Corot is set to be followed by the European Space Agency's "Darwin" mission, a fleet of four or five space telescopes that will analyze the atmospheres of distant rocky planets. At around the same time the US space agency Nasa will launch Terrestrial Planet Finder, another space telescope designed to locate Earth-like worlds.

Planets likely to harbour life are expected to be found in rare "Goldilocks" regions of space. Only solar systems close to the centre of a galaxy have enough heavy elements to form rocky planets and many of the complex molecules believed necessary for life. But too near the centre, other hazards, including radiation, comets and asteroids make it difficult for life to gain a foothold.

Within a solar system the most habitable planets are found in a zone where water can exist as a liquid. Too far out and water is permanently frozen, but too near the star and it is turned into steam. In our solar system the habitable zone is roughly between Venus, the second rock from the sun, and Mars, the fourth.

This year, scientists at the Carnegie Institute of Washington identified a star, Epsilon Indi A, as the best candidate to have Earth-like planets. The star is 11.8 light-years away, one of the closest to Earth, in the constellation Indus. The smallest planet found beyond the solar system is more than 20,000 light years away in the constellation Sagittarius. With surface temperatures of -220C it is unlikely to harbour life.

Ian Sample

The Guardian