In order to determine the wavelengths of light produced by a star, its spectrograph is compared with a standard light source that provides precisely known wavelengths that are stable over time. Exoplanet measurements take place over periods of months or even decades–astronomers light years away would need to watch our Sun for a year to see the effects of Earth. The current standards are stable but limited in the wavelengths they provide.
Since the late 1980s, astronomers have been interested in using optical-frequency combs, spectra of light made by rapidly pulsing lasers, to provide better standards for high-resolution spectroscopy, says John Hall, a fellow at JILA, a joint research institute of the National Institute of Standards and Technology and the University of Colorado, in Boulder, CO. (Hall won the Nobel Prize in physics in 2005, in part for his work on optical-frequency combs.) Such a spectrum is made by a single laser that produces pulses of light of precisely calibrated wavelengths, spaced billionths of a second apart.
But until now, no one has been able to figure out how to adapt this technology for astronomy: the fringes of light in an unmodified optical-frequency comb are too closely spaced together. To solve this problem, Walsworth and Li coupled an optical-frequency comb with a filtering chamber called a Fabry-Pérot cavity that uses a series of mirrors to cancel out most of the fringes. The resulting fringes act like a ruler for precisely identifying the wavelengths of light in a spectrograph.
“There are a bunch of people working on this, and they beat them out,” Seager says of the Harvard researchers. Other groups working on the problem are starting from the top down, designing techniques that need to be combined with better telescopes and better spectrographs also currently under development. Walsworth took a simple approach to the problem. His calibration technique can be used on existing telescopes, with existing spectrographs.
Walsworth says that it will still be a few years before new Earth-like planets are detected using this technique. His group is currently adapting the frequency-comb system for transport from Massachusetts to Arizona, where it will be used to calibrate the spectrograph at the Multiple Mirror Telescope. In 2010, the system will be installed on a state-of-the-art spectrograph in the Canary Islands. “The ultimate goal is to find planets of Earth mass with a one-year period around a Sun-like star,” says Walsworth. But he notes that the first discoveries using the new system will be rocky planets orbiting more closely to dimmer, cooler stars.