Star shade will let alien planets shine

EVEN as a cash-strapped NASA shelves plans for a costly mission to hunt for Earth-like planets, a novel scheme has emerged that promises to achieve the same goal for a fraction of the cost.

Called New World Observer, the proposal will be submitted to NASA next month by Webster Cash, director of the University of Colorado's Center for Astrophysics and Space Astronomy. It could begin spotting "exo-Earths" as early as 2013.

Budget cuts forced NASA to indefinitely postpone its planned Terrestrial Planet Finder (TPF) mission. This would have required the agency to build an expensive new telescope capable of spotting habitable planets in other solar systems. The strategy adopted by Cash is to fly a flowershaped disc in front of the James Webb Space Telescope (JWST), Hubble's successor due for launch in 2013. This would allow the telescope to spot the faint light of distant planets by blocking the much brighter light from their parent stars.

"The problem is not the faintness of the planets but their proximity to stars that are 10 billion times brighter," Cash says. The standard solution, adopted for the TPF, is to fit a telescope with a coronagraph, which can block the light of an individual star while exposing the sky immediately around it. The limitation of this approach is that imperfections within the telescope inevitably scatter some of the star's light back into the field of view. To minimise this scattering an exceptionally well-configured mirror with near-uniform reflectivity is needed, and this is expensive.

Cash's plan would eliminate the scattering problem by using a 40-metre disc hovering 30,000 kilometres in front of the JWST to obscure the target stars before their light enters the telescope. Similar concepts have been deemed unworkable in the past because some starlight would diffract around the disc and drown out any light from the exoplanet. Now Cash has shown that a disc with a particular arrangement of spiked petals causes much of that diffracted light to destructively interfere with itself before it enters the telescope, leaving the field of view dark enough for planet spotting. The "star shade" would cover the inner regions of planetary systems, blocking out the star and any planets orbiting very close to it, but allowing the telescope to observe planets in Earth-like orbits. The system could be used for studying solar systems up to 35 light years away from Earth, as those any further away would be obscured by the disc.

Cash originally envisioned the disc as part of a vast "hypertelescope" that might be developed in the future (New Scientist, 25 February, p 40), but after discussions with JWST astronomers he realised it could be applied to an existing space telescope. "It's not part of the telescope, but rather something that prepares the sky to be examined by the telescope," says John Mather, senior project scientist for JWST.

Cash's proposal does not demand extreme precision. The star-shaped disc can be built to millimetre specifications, and the system will work if the alignment of the disc and the JWST is accurate to within a few metres. "The tolerances of the whole thing are quite forgiving," Cash says. This translates into big savings on its construction and deployment, allowing Cash to propose the project as a "Discovery-class" mission, NASA's least expensive category, with a cost ceiling of $425 million. If approved it could be launched in September 2013, just three months after the JWST.

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THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 11 MARCH 2006

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