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.
Changing features of the laser light likewise has a dramatic effect on the renderings.
"Light from a pulsed laser produces a very fine-tooth line, with each line corresponding to a different wavelength," Gordon Walker, a retired University of British Columbia astrophysicist, told Discovery News. "With current technology, that pitch is too fine, all the lines are blended together."
The new technique filters out some of the wavelengths, creating spaces that are closely calibrated to wavelengths in the electromagnetic spectrograph, the Rosetta stone for analyzing starlight.
"Li and colleagues' brainchild is an optically filtered comb of evenly spaced frequency references, all derived from a single frequency source -- a pulsed laser," writes Walker in a related Nature article.
"The idea is not new … nor is its application to astronomy. But [they] are the first to realize the concept in a way suitable for astronomical practice, in what could be a breakthrough in the precision of astronomical spectroscopy."
The new system, which is expected to get is first test later this year, will use a laser comb to take a spectrum of a target star to establish a baseline. At various points in the future, additional spectra will be taken of the star to look for any shifts.
Like the sound of a train approaching or receding into the distance, wavelengths of light shift as a star moves to and fro. Scientists are hopeful the new technique will reveal if there are any Earth-like planets doing the tugging.
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