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Nobel Prize-Winning Laser Technology to Help Find Earth-like Planets

30 Maggio 2012

The new technology of laser frequency combs (eso0826) [1] has now been tested with the HARPS [2] planet-finder on the ESO 3.6-metre telescope at the La Silla Observatory in Chile. Frequency combs provide reference light sources of extraordinary stability and have the potential to allow HARPS and similar instruments to make much more precise measurements than is currently possible [3]. This novel technique is expected to become a revolutionary tool for the astronomical community and help astronomers to find Earth-like planets in the habitable zones around nearby stars. The results are being presented in a paper to appear in the 31 May 2012 edition of the journal Nature.

A team of scientists from ESO, the Max Planck Institute of Quantum Optics (MPQ, Garching, Germany) and the Instituto de Astrofisica de Canarias (IAC, Tenerife, Spain) — led by Tobias Wilken, a researcher at MPQ — has used a laser frequency comb to perform a test observing run with HARPS. When using the new laser comb, the attainable precision of the measurements was found to have improved by a factor of at least four when compared to the limit achieved using older hollow-cathode lamp technology.

By applying this technique for the first time with HARPS, they mapped the orbit of the known planet orbiting the star HD75289. These measurements were consistent with previous results, showing the robustness of this tool for use with the next generation of spectrographs.

The frequency comb that was tested is a prototype of a system that is being developed by a collaboration between ESO, MPQ, Menlo Systems GmbH (Germany), the IAC and the Universidade Federal do Rio Grande do Norte (Brazil). It will be installed for routine operations in HARPS in the near future.

Several current hot astronomical research areas will benefit from this innovative technique, most significantly the detection of Earth-like planets. One of the most successful methods for finding planets around other stars is measuring the planet’s effect on the star’s motion by looking for tiny shifts in the parent star's spectral lines due to the Doppler effect [4]. These shifts are measured relative to a reference light source that must be extremely stable. The laser frequency comb offers a source that is significantly more stable than any available before. This means that measurements of velocities down to the level of only centimetres per second should be attainable.

If an observer elsewhere in the Galaxy wanted to detect the presence of the Earth as it orbits the Sun, they would have to measure the wobble of the Sun backwards and forwards over one year with equipment that was sensitive enough to pick up velocity changes with an amplitude of just 9 centimetres per second. This means that the use of frequency combs will make possible the detection of Earth-mass planets in the habitable zones around nearby stars by the radial velocity technique. Such planets are among the best candidates for bearing life outside the Solar System.

Looking further ahead, when the next generation of ground-based telescopes becomes available, such as the European Extremely Large Telescope (E-ELT), Laser frequency combs will become a vital tool for providing direct measurements of the acceleration of the expansion of the Universe.

Notes

[1] A Laser Frequency Comb is a coherent light source, emitting a narrow line spectrum, whose frequency differences lie in the radio regime and are exactly equal across its entire, comb-like spectrum. They are as accurate and stable as the atomic clock to which they are stabilised. The development of the first frequency comb was achieved separately by both the groups of T.W. Hänsch at the Max Planck Institute of Quantum Optics and J. L. Hall at the United States National Institute of Standards and Technology. It enabled measurement of transitions in atomic and molecular systems with unprecedented accuracy. In recognition of this achievement, T. W. Hänsch and J. L. Hall were awarded the 2005 Nobel Prize in Physics, the other half of which was awarded to R.J. Glauber.

[2] High Accuracy Radial velocity Planet Searcher.

[3] The spectrum of a laser frequency comb injected into a spectrograph like HARPS appears as a set of equal intensity, equally spaced emission lines, as opposed to the commonly used hollow cathode lamps, where the lines are defined by atomic transitions and their spacing and intensities are therefore not adjustable.

[4] The Doppler effect is the change in frequency of a wave for an observer moving relative to the source of the wave. Astronomical spectra are composed of numerous spectral lines of different chemical elements at well-defined frequencies. The Doppler effect is recognisable because these lines are not always at the frequencies that are obtained from the spectrum of a stationary light source. By using this technique, it is possible to reconstruct the orbits of exoplanets around distant stars.