  Searches for extrasolar planets using the periodic Doppler shift of stellar
spectral lines have recently achieved a precision of 60 cm/s (ref 1), which is
sufficient to find a 5-Earth-mass planet in a Mercury-like orbit around a
Sun-like star. To find a 1-Earth-mass planet in an Earthlike orbit, a precision
of 5 cm/s is necessary. The combination of a laser frequency comb with a
Fabry-Perot filtering cavity has been suggested as a promising approach to
achieve such Doppler shift resolution via improved spectrograph wavelength
calibration, with recent encouraging results. Here we report the fabrication of
such a filtered laser comb with up to 40- GHz (1-A) line spacing, generated
from a 1- GHz repetition-rate source, without compromising long-term stability,
reproducibility or spectral resolution. This wide-line-spacing comb, or
`astro-comb', is well matched to the resolving power of high-resolution
astrophysical spectrographs. The astro-comb should allow a precision as high as
1 cm/s in astronomical radial velocity measurements.
