Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Hunting planets with laser rulers

Laser frequency combs can be used for the calibration of astronomical spectrographs.

This will help to find extra-solar planets, i.e. planets that are orbiting a star outside our solar-system. By refining this technology it might become possible to directly measure even very small changes in the expansion velocity of the universe.

Use of Doppler shift measurements in the search for 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 towards the observer (above), and the light waves appear to be squeezed. This means the radiation is shifted towards higher frequencies, which is called a “blue shift”. If, on the other hand, the star is travelling away from the observer (see below), the waves seem to be stretched, resulting in a so-called “red-shift” towards lower frequencies. Graphic: Th. Udem, MPQ

A frequency comb is a light source with a comb-like spectrum. The frequency difference f_r between two neighbouring 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 colours (i.e. its frequency components) by the spectrograph and imaged on the CCD detector. The comb-like 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.

Since their invention some ten years ago, laser frequency combs have proven to be a vital tool in many laser laboratories around the world. Originally developed for the investigation of the quantum world, they are now making their way into astronomy and astrophysics. A team of scientists from the Laser Spectroscopy Division of Professor Theodor W. Hänsch at the Max Planck Institute of Quantum Optics (Garching), in collaboration with the European Southern Observatory (ESO), the Instituto de Astrofísica de Canarias, and the Menlo Systems GmbH (Martinsried), has now modified the frequency comb technique in a way that it can be applied for the calibration of astronomical spectrographs (Nature, 31 May 2012, DOI:10.1038/nature11092).

The new instrument has been tested successfully with the High Accuracy Radial velocity Planet Searcher (HARPS), a spectrograph at the 3.6-metre-telescope at the La Silla Observatory in Chile. A tenfold improvement of precision was obtained as compared with traditional spectral lamp calibrators. This will greatly enhance the chances to find earth-like planets outside our solar system. Ultimately, the search for extra-solar planets shall answer the question whether our solar system is the only place in the universe that provides the conditions for life as we know it.

These planets cannot be imaged directly, even with the largest telescopes. One of the most successful detection methods is the measurement of the tiny Doppler shifts in the spectrum of the parent star which is due to the recoiling motion caused by the planet. The light which reaches us from distant stars is composed of numerous spectral lines that are characteristic of the different chemical elements in the star’s gas atmosphere. When the star is moving towards the observer or away from her, these lines are shifted to slightly higher or lower frequencies. Vice versa, the measurement of the Doppler shifts gives information about the star’s movement. This provides a very promising way to find extra-solar planets which reveal their identity only indirectly: while travelling around their central star they push and pull it a little bit, causing a relatively small change of its velocity. (For comparison: While the sun orbits the galactic centre with a speed of 220 kilometres per second, the recoil on the sun caused by the Earth amounts to 9 centimetres per second.)

Hence, 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.

“Measuring” a physical parameter is equivalent to comparing it with a calibrated standard. So far the precision in frequency measurements was limited by slow drifts like aging of the calibration source, e.g. a thorium spectral lamp. The laser-frequency combs invented in the late nineties – for which Theodor W. Hänsch and John Hall were awarded the Nobel Prize in 2005 – have greatly improved the precision of frequency measurements. In 2005 MPQ and ESO decided to cooperate on the development of frequency combs for the calibration of astronomical spectrographs. As the first test measurements carried out at the VTT in Tenerife in 2008 turned out to be very promising, the scientists began to work on a frequency comb for the HARPS-spectrograph at the La Silla Observatory.

A frequency comb is emitted by a laser delivering light with a spectrum composed of many lines at constant intervals. By electronic feedback circuits each line is defined in reference to an atomic clock. By comparing the spectral lines of a star with the lines of this laser ruler, which does not change over time, it is possible to observe even very small variations of the star light, caused e.g. by an orbiting planet.

The adaption of the laser-frequency comb for its application in astronomical spectroscopy posed a few major technical challenges. Even precision spectrographs like HARPS provide a limited frequency resolution, typically of 105. Hence the lines of the frequency comb to be developed have to be spaced at intervals of more than 10 GHz, otherwise the spectrograph would not be able to resolve them. Furthermore, astronomical spectrographs operate in the visible spectral region.

In order to ensure robust and stable operation fibre-laser systems were chosen as the basis of the frequency comb. Fibre-laser systems, however, emit light in the infrared region, with spectral distances of a few 100 MHz. The scientists were able to change these properties by implementing a cascade of several spectral filters and using advanced fibres (developed by Philip Russell from the Max Planck Institute for the Science of Light, Erlangen). This resulted in a frequency comb with the desired mode spacing and a broad spectrum in the visible region. The calibration of the HARPS spectrograph with this frequency comb resulted in a sensitivity for velocity changes of 2.5 cm/s. This was demonstrated in a series of measurements in November 2010 and January 2011. By observing a star with a well known planet for a couple of nights the team could prove the high stability of the system over time.

For the near future the scientists pursue a task that is even more demanding than looking for planets. Astronomical observations have clearly shown that the universe is not static but instead 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 annually one centimetre per second. Such precision is to be delivered by the next ESO-project, the European Extremely Large Telescope (E-ELT) 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 millimetre. Olivia Meyer-Streng

Original publication:
Tobias Wilken, Gaspare Lo Curto, Rafael A. Probst, Tilo Steinmetz, Antonio Manescau, Luca Pasquini, Jonay I. González Hernández, Rafael Rebolo, Theodor W. Hänsch, Thomas Udem & Ronald Holzwarth
A spectrograph for exoplanet observations calibrated at the centimetre-per-second level

Nature, 31 May 2012, DOI:10.1038/nature11092

Dr. Ronald Holzwarth
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching bei München
Phone: +49 (0) 89 / 32905 -262
Dr. Tobias Wilken
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching bei München
Phone: +49 (0) 89 / 32905 -285

Dr. Olivia Meyer-Streng | Max-Planck-Institut
Further information:

More articles from Physics and Astronomy:

nachricht Researchers at Fraunhofer monitor re-entry of Chinese space station Tiangong-1
21.03.2018 | Fraunhofer-Institut für Hochfrequenzphysik und Radartechnik FHR

nachricht Taming chaos: Calculating probability in complex systems
21.03.2018 | American Institute of Physics

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Researchers at Fraunhofer monitor re-entry of Chinese space station Tiangong-1

In just a few weeks from now, the Chinese space station Tiangong-1 will re-enter the Earth's atmosphere where it will to a large extent burn up. It is possible that some debris will reach the Earth's surface. Tiangong-1 is orbiting the Earth uncontrolled at a speed of approx. 29,000 km/h.Currently the prognosis relating to the time of impact currently lies within a window of several days. The scientists at Fraunhofer FHR have already been monitoring Tiangong-1 for a number of weeks with their TIRA system, one of the most powerful space observation radars in the world, with a view to supporting the German Space Situational Awareness Center and the ESA with their re-entry forecasts.

Following the loss of radio contact with Tiangong-1 in 2016 and due to the low orbital height, it is now inevitable that the Chinese space station will...

Im Focus: Alliance „OLED Licht Forum“ – Key partner for OLED lighting solutions

Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, provider of research and development services for OLED lighting solutions, announces the founding of the “OLED Licht Forum” and presents latest OLED design and lighting solutions during light+building, from March 18th – 23rd, 2018 in Frankfurt a.M./Germany, at booth no. F91 in Hall 4.0.

They are united in their passion for OLED (organic light emitting diodes) lighting with all of its unique facets and application possibilities. Thus experts in...

Im Focus: Mars' oceans formed early, possibly aided by massive volcanic eruptions

Oceans formed before Tharsis and evolved together, shaping climate history of Mars

A new scenario seeking to explain how Mars' putative oceans came and went over the last 4 billion years implies that the oceans formed several hundred million...

Im Focus: Tiny implants for cells are functional in vivo

For the first time, an interdisciplinary team from the University of Basel has succeeded in integrating artificial organelles into the cells of live zebrafish embryos. This innovative approach using artificial organelles as cellular implants offers new potential in treating a range of diseases, as the authors report in an article published in Nature Communications.

In the cells of higher organisms, organelles such as the nucleus or mitochondria perform a range of complex functions necessary for life. In the networks of...

Im Focus: Locomotion control with photopigments

Researchers from Göttingen University discover additional function of opsins

Animal photoreceptors capture light with photopigments. Researchers from the University of Göttingen have now discovered that these photopigments fulfill an...

All Focus news of the innovation-report >>>



Industry & Economy
Event News

Virtual reality conference comes to Reutlingen

19.03.2018 | Event News

Ultrafast Wireless and Chip Design at the DATE Conference in Dresden

16.03.2018 | Event News

International Tinnitus Conference of the Tinnitus Research Initiative in Regensburg

13.03.2018 | Event News

Latest News

TRAPPIST-1 planets provide clues to the nature of habitable worlds

21.03.2018 | Physics and Astronomy

The search for dark matter widens

21.03.2018 | Materials Sciences

Natural enemies reduce pesticide use

21.03.2018 | Life Sciences

Science & Research
Overview of more VideoLinks >>>