  One of the essential prerequisites for detection of Earth-like extra-solar
planets or direct measurements of the cosmological expansion is the accurate
and precise wavelength calibration of astronomical spectrometers. It has
already been realized that the large number of exactly known optical
frequencies provided by laser frequency combs ('astrocombs') can significantly
surpass conventionally used hollow-cathode lamps as calibration light sources.
A remaining challenge, however, is generation of frequency combs with lines
resolvable by astronomical spectrometers. Here we demonstrate an astrocomb
generated via soliton formation in an on-chip microphotonic resonator
('microresonator') with a resolvable line spacing of 23.7 GHz. This comb is
providing wavelength calibration on the 10 cm/s radial velocity level on the
GIANO-B high-resolution near-infrared spectrometer. As such, microresonator
frequency combs have the potential of providing broadband wavelength
calibration for the next-generation of astronomical instruments in
planet-hunting and cosmological research.
