Astronomers searching for extrasolar planets may be a step closer to finding other Earthlike places in the universe around sunlike stars, thanks to a new tool that promises to increase the accuracy of planet-hunting devices by tenfold. Scientists from the Max Planck Institute of Quantum Optics, in collaboration with the European Southern Observatory (ESO), the Instituto de Astrofisica de Canarias and the Menlo Systems GmbH, have modified the laser frequency comb technique in such a way that it can be applied for the calibration of astronomical spectrographs. Laser frequency combs are calibration tools designed to precisely measure “wobble” in stars. (See: Star Comb Joins Quest for Other Earths) The researchers successfully tested the instrument with the High Accuracy Radial velocity Planet Searcher (HARPS), a spectrograph at the 3.6-m telescope at the La Silla Observatory in Chile. They achieved a tenfold improvement in precision over traditional spectral lamp calibrators, which will significantly enhance the chances of discovering Earthlike planets outside our solar system. This modification could help astronomers determine whether our solar system is the only place in the universe that provides the conditions needed for life as we know it. Use of Doppler shift measurements in the search for extrasolar planets. When a planet (red ball) orbits a star (yellow ball), the recoil it exerts gives rise to a periodic movement: At one time the star is moving toward the observer (above), and the light waves appear to be squeezed. This means the radiation is shifted toward higher frequencies, which is called a “blue shift.” If, on the other hand, the star is traveling away from the observer (see below), the waves seem to be stretched, resulting in a so-called “red-shift” toward lower frequencies. (Image: Th. Udem, MPQ) Even with the largest telescopes, these types of planets cannot be imaged directly. Measuring the tiny Doppler shifts, or wobble, in the spectrum of a parent star — due to the recoiling motion caused by the planet — is the most successful detection method to date. The light that reaches us from distant stars is composed of multiple spectral lines that are characteristic of the different chemical elements in the star’s gas atmosphere. When the star is moving toward or away from the observer, these lines shift slightly to higher or lower frequencies. By measuring the Doppler shifts, astronomers can obtain information about the star’s movement. This provides a promising way of locating extrasolar planets that reveal their identity only indirectly: While traveling around their central star, they push and pull it a little bit, causing a relatively small change in its velocity. For this reason, the amount of Doppler shift in the star’s spectrum is very small and can be detected only with the help of high-precision measurement tools. Unfortunately, adapting laser frequency combs for astronomical spectroscopy applications has posed a few major technical challenges. Even precision spectrographs like HARPS provide limited frequency resolution — typically around 105. This means that the lines of the frequency comb would need to be spaced at intervals of more than 10 GHz, or it would not be able to resolve them. Another challenge is that astronomical spectrographs operate in the visible spectral region. A frequency comb is a light source with a comblike spectrum. The frequency difference between two neighboring lines is always exactly the same. It is kept stable by comparing it with an atomic clock. The comb light is guided to the spectrograph in an optical fiber. The light is separated into its colors (i.e., its frequency components) by the spectrograph and imaged on the CCD detector. The comblike spectrum appears as a row of dots of which each dot corresponds exactly to one line of the frequency comb. This “laser ruler” can now be used to calibrate the spectrograph. (Image: T. Wilken) To overcome these challenges, the researchers chose a fiber laser system as the basis of the frequency comb. These systems emit light in the infrared region and have spectral distances of a few hundred megahertz. The scientists changed these properties, however, by implementing a cascade of several spectral filters and by using advanced fibers developed by Philip Russell from the Max Planck Institute for the Science of Light in Erlangen. A frequency comb with the desired mode spacing and a broad spectrum in the visible range was the result. When calibrated with the HARPS spectrograph, the modified frequency comb delivered 2.5-cm/s sensitivity for velocity changes. This was demonstrated in a series of measurements taken in November 2010 and January 2011. Next, the researchers plan to pursue a task even more challenging than looking for planets. Astronomical observations have shown that the universe is not static but rather expanding continuously. New results on the microwave background radiation and the observation of supernovae suggest that this expansion is accelerating over time. However, the change of the velocity is expected to be very small, of the order of 1 cm/s annually. This precision is expected to be delivered by the next ESO project, the European Extremely Large Telescope, which is planned to be constructed in Chile in the next decade. High-precision frequency combs will be at the heart of its Codex spectrograph, providing a calibration precision of one part per 300 billion — a feat equivalent to measuring the circumference of the Earth to half a millimeter. The findings appeared in Nature. For more information, visit: www.mpq.mpg.de