"Astro Comb" May Boost Hunt for Earthlike Planets

Sara Goudarzi
for National Geographic News
April 2, 2008

The scientific equivalent of a fine-toothed comb may soon sweep across the skies looking for Earthlike planets outside our solar system.

Dubbed the astro-comb, the technology improves on a highly successful planet-hunting technique called the radial velocity—or wobble—method, which looks for small shifts in the wavelengths of a star's light caused by the gravitational tug of an orbiting planet.

So far astronomers have found more than 270 exoplanets, many using the wobble method, but none of them are Earth-size.

That's because the technology has not been able to measure the very small spectral shifts caused by smaller worlds orbiting in a star's habitable zone. (Related: "Earth 'Twin' Might Circle Star Next Door" [March 10, 2008].)

The performance of the instruments astronomers use to observe starlight often "drifts" due to Earthly factors such as temperature and pressure variations.

Scientists therefore need to continuously recalibrate the instruments by comparing starlight to a standard light.

The astro-comb boosts sensitivity to smaller wobbles via a "comb" of laser lines covering a wide range of the optical spectrum, creating a perfect ruler for calibration.

"This improved sensitivity is required to find lighter planets like Earth analogs, because such lighter planets pull less strongly on stars," said lead study author Chih-Hao Li, a researcher at the Harvard-Smithsonian Center for Astrophysics.

The study appears in this week's issue of the journal Nature.

Stable and Precise

For now, the wobble method is limited in part by the stability and precision of spectrographs—instruments that split light from astronomical objects into component wavelengths to show the object's spectrum.

Theoretically, the astro-comb could provide calculations with a hundred times better precision than existing measurements.

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