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It has long been thought that spotting Earth-like planets around distant stars will require space-based telescopes with exquisite optics. But now, one astronomer thinks he can do it with an already-planned telescope and what he modestly describes as "a big fuel tank with a hefty bag attached".

In this week's Nature¹, Webster Cash, an astronomer at the University of Colorado, Boulder, lays out his plan to use a mobile flower-shaped 'starshade' to spot Earth-sized planets within a few light years of the Solar System. The shade, a 30-50-metre sheet of black plastic-bag-like material, would block out light from the target star, allowing a space telescope to look directly for planets.

In recent years, astronomers have detected 194 extrasolar planets using a variety of indirect methods, but they have yet to see starlight reflecting off the surface of a small and distant planet. A picture of an Earth-like 'pale blue dot' is the holy grail of extrasolar planet astronomy because it would allow researchers to learn about the planet's atmosphere and look for signs of life, things that cannot be done through indirect observation.

A distant 'Earth' would probably be about one ten-billionth the brightness of its mother star. Most astronomers think it will take a specially designed, multibillion-dollar telescope to spot something that dim. Adding to the expense is the fact that the telescope would have to be placed 1.5 million kilometres from Earth to avoid atmospheric interference. The costs were so high that earlier this year NASA 'deferred indefinitely' its Terrestrial Planet Finder (TPF) mission.

However, Cash thinks he can do the job on the cheap using his contraption with the already-planned successor to the Hubble: the James Web Space Telescope (JWST). His scheme calls for perfectly positioning the shade some 50,000 kilometres away from the JWST and directly between it and the target star. When the telescope turns to the star, the starlight should be perfectly blocked, allowing JWST to spot the dim light of any planets that might be nearby. The 'fuel tank' would carry propellant for driving the black sunshade from one spot to the next.

Flower power

The starshade has a unique petalled shape in order to wipe out as much light as possible from the star behind, including any light that may diffract or bend around the edge of the shade. Determining the ideal shape and distance of such a shade was a tricky, but solvable problem.

Cash says that the scheme is somewhat limited because the JWST is primarily designed to look at distant galaxies, not planets around nearby stars. Still, he thinks it would be able to see Earth-like planets within 32 light years of us: a distance that includes about 1,000 stars. Furthermore, it may be possible to see whether the brightness of the planet changes as it rotates. In this way, Cash says, "we could directly detect if there were oceans and continents."

"It's a wonderful idea," says Geoff Marcy, an astronomer at the University of California, Berkeley. Marcy calls the idea to use the already planned JWST a "brilliant stroke".

Perfectly poised

But not everyone is so effusive. "It solves the optical problem in principle," says Charles Beichman, the TPF mission's project scientist at the Jet Propulsion Laboratory in Pasadena, California. But, he says, the plan requires keeping the starshade and telescope perfectly positioned. Furthermore, the shade will probably have to travel hundreds of thousands of kilometres to get between the telescope and different stars.

In the end, Beichman says the formidable technical task of aligning and moving the shade may be too great for the scheme to be practical: "I think technical issues will be the killer".

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