Launch of race to find Earth-style planets beyond our solar system

The search for an Earth-like planet orbiting a distant star begins in earnest today with the launch of the first scientific satellite designed to find rocky "exoplanets" beyond our solar system.

Instruments on board the French-made Corot satellite are designed to monitor the tiny perturbations to the light coming from a star caused by the orbit of a relatively small planet similar in size to the Earth.

Corot, which stands for convention, rotation and planetary transits, is also designed to monitor seismic "starquakes" - the acoustic waves that cross the surface of a star - which can be used to investigate stellar evolution.

A Russian rocket is scheduled to launch the Corot satellite at 2.43pm British time from the former Soviet Union's launch site at Baikonur in Kazakhstan.

The French National Space Agency is leading the mission with secondary involvement from the European Space Agency, which supplied the telescope's optics, and Britain, which paid for a scientific consultant to advise on the project.

Scientists have identified about 200 exoplanets so far using telescopes on the ground but nearly all of these are thought to be very large, Jupiter-sized objects composed of hot gas, which means they would be very unlikely to harbour life.

To find the smaller, Earth-sized planets composed of rock, and possibly water, it is necessary to refine the techniques still further to pick up the faint perturbations these objects would cause as they pass between their own stars and the Earth.

"A planet passing in front of a star can be detected by the fall in light from that star," said Professor Ian Roxburgh of Queen Mary, University of London, and a member of the European Space Agency's scientific committee.

"Small oscillations of the star also produce changes in the light emitted which reveal what the star is made of and how they are structured internally. This data will provide a major boost to our understanding of how stars form and evolve," Professor Roxburgh said.

Scientists will be particularly interested in exoplanets that are orbiting in the so-called "Goldilocks" zones of their parent stars - not too hot and not too cold for water to remain as a liquid. These rocky planets would therefore be the best contenders for harbouring life.

Once it has reached its 500-mile-high circumpolar orbit, the Corot satellite will monitor about 120,000 stars with its 30cm telescope. It will be looking for the tell-tale signs of planets passing in front of a star and so altering the star's perceived brightness.

The optical telescope on board the Corot satellite will benefit from being positioned well above the Earth's atmosphere, which interferes with the faint light coming from stars, so making it difficult to detect distant exoplanets from the ground.

While it is looking at a star, the Corot satellite will be able to monitor the stellar surface for acoustic waves generated deep inside the star. These seismic events send ripples across the hot surface of the star.

"Corot will lead a bold, new search for planets around other stars. It promises to find many more during its two-and-a-half-year mission, expanding the frontiers of our knowledge toward ever-smaller planets," said a spokesman for the European Space Agency in Paris. "An unknown percentage of those detected are expected to be rocky planets, maybe just a few times larger than the Earth - or smaller, even," he said.

"If Corot finds such planets, they will constitute a new class of planet altogether."

A more ambitious attempt to find Earth-like planets is being planned for launch within the next 10 years. The Darwin mission will be composed of a flotilla of four or five spacecraft that will be positioned well beyond the Moon on the side away from the Sun. This will make it still easier to search for the feeble light changes caused by small, rocky exoplanets.