The star shade would orbit a million miles from Earth

The daisy-shaped "occulter", as it is known, would use a powerful telescope trailing thousands of miles behind.

The shade, described in the journal Nature, would stop light from the planet's star swamping the telescope.

The concept by Professor Webster Cash of the University of Colorado has already received funding from Nasa.

He believes an occulter could be in space within seven years "stalking" Hubble's successor, the James Webb Space Telescope, scheduled for launch in 2013.

"We have proposed to build a star shade to launch a couple of months later and follow it out to its orbit," he said. "We believe this the fastest way to get operational."

Planetary search

Scientists are already searching for planets orbiting stars other than the Sun. Researchers hunt for these extra-solar planets, or exoplanets, using a number of techniques.

More than 170 have so far been discovered.

Other concepts to look for small planets are being considered

For example, astronomers look for the dimming of light as planets pass in front of their parent stars. Indirect techniques like this mean that only relatively large planets tend to get identified.

Astrobiologists, though, are really interested in finding smaller, Earth-like planets which could, in theory, have the right conditions for supporting life.

To do this, astronomers need a method of directly imaging the dim planets.

Numerous proposals have been put forward, including massive optical telescopes on Earth, or flotillas of space-based telescopes such as the Europe's Darwin mission or Nasa's Terrestrial Planet Finder.

All these schemes are still in development.

Sun block

Dubbed the New World's Observer, Professor Cash's design would use a giant 45m (148ft) daisy-shaped, plastic shield in tandem with a powerful telescope, trailing 15,000km (9,300 miles) behind.

The pair would orbit about a million miles (1.6 million km) from the Earth at a position known as a Lagrange point.

It's like a cricketer holding up his hand to block out the sunlight

Professor Webster Cash

To search for a planet, astronomers would pick a target star and move the shield in front of the telescope, using thrusters.

When the two align, the position of the shade ensures that excess light from the star is blocked, giving astronomers the maximum chance of spotting any small orbiting planet.

"It's like a cricketer holding up his hand to block out the sunlight as he tracks a ball in the air," said Professor Cash.

The pair would be held in position to give scientists time to image the planet and analyse its atmosphere for the chemical signatures of life.

It could also be used to map entire planetary systems trillions of miles away.

Future mission

The idea has already been given a huge boost by Nasa. The US space agency's Institute for Advanced Concepts gave the proposal $400,000 (£220,000).

Professor Cash and his team have also submitted a proposal to build a shade for the infra-red and visible James Webb Space Telescope.

However, some researchers believe that the Professor Cash and his team may still have some way to go before a star shade blasts into space.

"It's an interesting alternative idea but I suspect that there are enormous technical challenges," said Professor Timothy Naylor, an astrophysicist at Exeter University, UK.

Potential obstacles include carrying enough fuel for the thrusters and developing a method for keeping the shade and telescope in alignment.

"If you are trying to collect the light from a planet then you are going to have to stare at it for a relatively long period of time to do anything really useful," he said.