Whenever light passes through an aperture, some of it spreads. This phenomenon is known as diffraction. The effect on an image is to create a blur around it, which on fine astronomical images of stars can swamp tiny signals from nearby planets.

Until now, planet hunters have had to look instead for tiny wobbles in a star's motion caused by the gravity of nearby planets. But Earth-like planets are too small to be spotted this way.

Now researchers at the Harvard-Smithsonian Center for Astrophysics say they have found an ingenious way round the diffraction problem. Rather than using a telescope with a round aperture, why not try a square one, they thought. The idea promises a new lease of life for optical telescopes.

A square aperture causes all the diffracted light from a source to fall along two perpendicular axes, which cross at the point of the image. By rotating the aperture, Costas Papaliolios realised it should be possible to get the image of the planet to fall in dark areas, where it wouldn't be swamped by diffracted light.

People had thought of using square apertures before but were always put off by the fuzzy, grid-like effect they produce. The sharper you make the edges of the square, the fuzzier the image becomes.

But Papaliolios's colleague Peter Nisenson reasoned that this should work in reverse, too: making the edges of the aperture fuzzy might clean up the image. In simulations where a planet was only one-billionth the brightness of a star, he found that fuzzy edges brought the planet into view when it had previously been swamped by blurry starlight.

A fuzzy-square mask should make it possible for telescopes to see Earth-like planets about five times closer to their star than with an ordinary telescope, the group will report in The Astrophysical Journal.

The idea may be a boon to NASA's Terrestrial Planet Finder Project, which aims to launch telescopes like Hubble to look for Earth-like planets. Until recently, NASA was taking a different approach.

"We had thought of looking in the infrared, where the planet is only a million rather than a billion times less bright than the star," says Dan Coulter, project manager for Planet Finder at the Jet Propulsion Lab in Pasadena, California. But optical telescopes using the new idea could have big advantages.

"Potentially they are simpler systems, less complex, with lower risk," he says. The planet hunters themselves are also keen. "This could be a big breakthrough," says Frank Shu at the University of California, Berkeley.

This article appeared in the Feb 10, 2001 issue of New Scientist.

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