YOU can鈥檛 put a square peg in a round hole. But putting a square aperture on
a round telescope might be the best way to see the light of distant, Earth-like
planets. At least, that鈥檚 what computer simulations of a new extraterrestrial
telescope predict.
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鈥檚 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.
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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鈥檛 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鈥檚 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鈥檚 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. 鈥淲e had thought of looking
in the infrared, where the planet is only a million rather than a billion times
less bright than the star,鈥� 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. 鈥淧otentially they are simpler systems,
less complex, with lower risk,鈥� he says.
The planet hunters themselves are also keen. 鈥淭his could be a big
breakthrough,鈥� says Frank Shu at the University of California, Berkeley.