Plane-finding requires fine-tuning

Steven Schultz

Princeton NJ -- Imagine the light of a star streaming toward Earth in the form of 10 billion little packets -- photons, as they are known in science. If it is a star with a planet circling it, like Earth orbiting the sun, then perhaps one of those photons will have been reflected from the planet's surface and carry some information about it.

Detecting that one photon is the core goal of a group of Princeton scientists designing a NASA mission to discover and observe Earth-like planets around other stars.

That's like picking out one person among the entire population of the planet, said Charles Beichman, a NASA scientist who oversees the agency's Terrestrial Planet Finder mission.

"Getting it right to a part in a million is not enough, you need to get it right to one part in 10 billion," said Beichman. "It's a very, very challenging technical problem."

Princeton astrophysicist David Spergel has hit upon an idea that goes a long way toward solving the problem. He designed a new kind of telescope that, simply because of its shape, deletes some of the starlight and allows the dim light of a planet to shine through.

His innovation is to reshape the pupil of the telescope from the conventional circle to a pair of cat-eye-shaped openings. When light passes through such an opening, interference between light waves cancels some of the star's light, so the star appears as a single hourglass-shaped image, with two dark spots carved out of either side of the star. If a planet happens to be in one of these dark spots, its light will no longer be overshadowed by the star's light. The planet will appear as a much smaller, fainter hourglass shape floating next to the star.

"It's really a brilliant solution and it's very simple," said engineering professor Michael Littman, who created a lab demonstration of the technique using a laser, which he shined through a 35 mm slide containing a negative image of Spergel's cat-eye shapes.

So far the device is able to cancel the star's light by a factor of 100,000. That performance would need to be increased at least 10,000-fold to detect the billion-to-one difference between a star and a planet. Nonetheless, the Princeton team believes Spergel's device may be easier to implement than NASA's original proposal for the telescope. That plan calls for a network of five satellites, each with its own telescope, flying in formation in space. The satellites, separated by 100 meters, would beam their images to a central location, which would use interference between the images to cancel the starlight.

Whatever technique they use, scientists will need to employ innovative tactics to gather information once they obtain an image of a distant planet. Astrophysicist Ed Turner, working with Sara Seager of the Institute for Advanced Study and graduate student Eric Ford, has shown that it may be possible to detect the planet's weather and the presence of oceans simply by reading how that one dim spot of light gets dimmer and brighter over a period of time.

To demonstrate their idea, Ford developed a computer model that shows how the total light intensity reflected from Earth changes as continents and oceans pass through the sun's light.

"If you looked at our solar system from afar, you would see basically three terrestrial planets -- Venus, Earth and Mars," said Turner. "And if you looked at their light curves, you would immediately see that the Earth is the most interesting of the three."

But all such observations would be impossible without securing a good vantagepoint. That is one of the projects of Jeremy Kasdin, assistant professor of mechanical and aerospace engineering, who, with post-doctoral associate Pinchas Gurfil, has developed several innovative orbits that could be used to position the space telescope in the optimum location to view nearby stars.

The challenge is to keep the satellite away from the Earth, which would get in the way of observations, and away from the band of "space dust" that orbits the sun between here and Mars. Kasdin showed how rockets could sling the satellite above the orbital plane of the Earth, keeping it close enough for convenient communications, but away from obstacles.

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