Astro Comb Scours for Earth-Like Worlds

April 3, 2008 -- Compared to the sun, Earth is a fleck. You would need 109 of them to cover the sun's face and another 1.3 million to fill its interior. Yet puny Earth and all its sibling planets make their presence known with gravitational tugs that cause the mother star to wobble just a bit.

The shifts are revealed by analyzing changes in the light coming from the sun.

Earth gets a 10-centimeter-per-second nod by the sun over the course of a year. Jupiter, which is 300 times larger, commands a more perceptible 13 meter-per-second shift throughout its 12-year orbit.

Planets' gravitational tugs become profoundly more difficult to detect if you are far away -- like in another solar system. This is especially true, considering that stars like the sun regularly explode and convulse, causing shifts in their radiance that have nothing to do with any planets in tow.

Nevertheless, scientists have charted 277 extrasolar worlds, though none as small as Earth. A new technique, described in this week's Nature, adds a formidable arrow to the planet-hunters' quivers.

Harvard University's Chih-Hao Li and colleagues have developed a method to filter laser pulses so that infinitesimally minute changes in starlight spectras can stand out. They call their technique "astro-combing."

It's a little like looking at the reflection of trees, clouds and sky in a pool of standing water. Just as water is used to bear the images, laser light serves as the reflecting pond for analyzing starlight.

Color the water red, and the trees, clouds and sky will look different, as some features disappear and others become more pronounced.