THE search for life on other planets could be transformed by launching a huge “star shade” into space, allowing a telescope to see Earth-like worlds orbiting distant stars.

The plastic, daisy-shaped shield, 50 metres (165ft) in diameter, would be used to block the light from stars that may have solar systems capable of supporting life. Astronomers would then be able to see dim lights from small planets that are normally hidden by the glow of their parent stars.

If the star shade were used with a sufficiently powerful telescope, it could reveal a wealth of detail about worlds many light years away that cannot even be seen at present, according to Professor Webster Cash, of the University of Colorado at Boulder.

Scientists would be able to observe planetary features such as oceans, continents, polar ice-caps and clouds, and even detect the signatures of water, oxygen and methane, which would indicate the likely presence of life, he said.

The star shade, or “occulter”, could be built with existing technology and launched within a decade, most likely to work in tandem with the James Webb Space Telescope, the successor to the Hubble Space Telescope, which is scheduled for launch in 2013.

“For over a century, science-fiction writers have speculated on the existence of Earth-like planets around nearby stars,” Professor Cash said. “If they actually exist, use of an occulter could find them within the next decade.

“We will be able to study Earth-like planets tens of trillions of miles away and chemically analyse their atmospheres for signs of life.”

Details of Professor Cash’s concept, known as the New Worlds Observer, are described today in the journal Nature.

Nasa is so interested in the idea that it has backed the research with an initial study grant of $400,000 (£218,000), and is considering a $400 million proposal to build a star shade to fly with the James Webb Space Telescope.

Working with a star shade 50m in diameter, the more powerful successor to Hubble would be capable of detecting planets as small as the Moon over distances of up to 30 light years.

Although more than 175 “exoplanets” beyond our solar system have been discovered, the search is difficult because the small size even of gas giants such as Jupiter means that they glow very dimly compared with their parent stars.

The presence of large planets can be inferred from their influence on light from the central star, but this technique is not suitable for finding small, rocky worlds such as Earth, which are the best candidates for supporting extraterrestrial life.

Professor Cash’s paper shows that a star shade could make this search possible by blotting out the light from a central star so the faint glow of its surrounding planets, even small ones, could become visible to a telescope orbiting Earth. The shade is daisy-shaped to allow very small amounts of light reflected by planets to filter around its “petals”.

“Think of a fielder holding up one hand to block out the sunlight as he tracks a ball,” he said. “We would use the star shade as a giant hand to suppress the light emanating from a central star by a factor of about 10 billion.” The star shade would be launched separately from the telescope with which it works, and would orbit about 15,000 miles away from it.

An external star shade of this sort would be simpler and cheaper than the alternative — fitting an orbiting telescope with a coronagraph, which does the same thing as the star shade, but inside the telescope.

An even more advanced version of the project — the New Worlds Imager — would involve a ring of telescopes placed on the Moon, beneath a fleet of orbiting star shades. This would allow scientists to photograph distant, Earth-like planets, Professor Cash said.

“There is a bit of Buck Rogers in the New Worlds Imager concept, but seeking and mapping new lands is something that seems to ring deep in the human psyche,” he added.

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