First author

Roughly 300 exoplanets — planets outside our Solar System — have been found during the past decade. Most were discovered using a technique that detects a planet's pull on its star through small, periodic changes in the spectrum of the star's light, known as Doppler shifts. So far, this method has been used mainly to detect large planets in close orbit to stars, because current tools for calibrating stars' spectra lack the stability and sensitivity required to find smaller planets. On page 610, Harvard postdoc Chih-Hao Li and his colleagues detail a new laser-based technique — the 'astro-comb' — that provides greater acuity and will revolutionize astrophysics.

Was the astro-comb the initial focus of your postdoctoral work?

No. I came to Harvard to work on tests of fundamental physics with Ron Walsworth. In 2007, Ron and two of our co-authors began brainstorming about how to build a better tool to calibrate astrophysical spectrographs. These instruments separate light from celestial sources according to wavelength. We thought that an optical-frequency comb — a laser-generated spectrum of known visible and infrared light wavelengths — would work best. But we soon realized that the spectral lines of existing comb lasers were too dense to distinguish individual lines in an astrophysical spectrograph.

How did you ultimately improve spectrograph calibration?

We matched the resolution of the comb laser to typical astrophysical spectrographs by filtering the light to suppress enough spectral lines to eliminate overlap.

How important is this discovery?

The astro-comb is revolutionary. It not only increases astrophysical spectrographs' sensitivity to Doppler shifts, but, because it is tied to an atomic clock, also allows measurements to be made over long time periods and permits precise comparisons of measurements from different telescopes. Once we have calibrated our astro-comb at the multi-mirror telescope (MMT) near Tucson, Arizona, we plan to focus on detecting dark matter in globular clusters. After that, we plan to build another astro-comb for the Canary Islands' William Herschel Telescope to look for exoplanets.

What would be your dream findings?

One is to find other 'Earths', or rocky planets, around a range of stars. We'll be able to find planets even smaller than Earth. And the astro-comb may yield other breakthroughs for cosmology — for example, the first direct measurement of the change in the expansion of the Universe.

See also pages 514 and 538