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Laser Help for Terrestrial Planet Search

Boosting the sensitivity of our exoplanet search tools by a hundredfold is no small matter, yet that’s just what optical frequency combs, when implemented with an ultrafast laser, may be able to do. A frequency comb is created by a laser that generates short, equally spaced pulses of light. ‘Locking’ the individual frequencies — keeping them in phase with each other — is essential, as is producing pulses that are no more than a few million billionths of a second long. The image below explains the name, the graph giving the impression of nothing more than a fine-toothed comb (and see this National Institute of Standards and Technology backgrounder for further details on how these combs work).

Laser frequency comb

We’ve looked at laser combs before, in particular in the work being performed at the Harvard Smithsonian Center for Astrophysics, which is involved in the deployment of such a comb at the William Herschel Observatory in the Canary Islands. The resultant instrument, called the HARPS-NEF (High-Accuracy Radial-velocity Planet Searcher of the New Earths Facility) spectrograph, should be useful in studying Earth-sized planets detected by the Kepler mission. The spikes on the comb can be used to measure the frequency of other light sources to a high degree of precision, useful to everything from the exoplanet hunt to making more accurate global positioning measurements.

Image: Experimental data from a NIST “gap-toothed” frequency comb that are false colored to indicate the range from low power (red) to high power (blue). The comb is specially designed for astronomy. Each “tooth” is a precisely known frequency, and the teeth are widely separated (by 20 gigahertz) in comparison to a standard comb. Credit: M. Kirchner & S. Diddams/NIST.

Now scientists at the University of Konstanz in Germany and the National Institute of Standards and Technology (NIST) have pushed a laser into record territory with a combination of high speed, short pulses and high average power. The new laser produces pulses ten times more often than a standard NIST frequency comb, creating shorter pulses than other lasers operating at comparable speeds. This is significant because the shorter the laser pulses, the wider the spacing between the ‘teeth’ of the comb. While standard combs use teeth too closely spaced for precise exoplanetary work, an ultrafast laser in this range offers the potential for more precise measurements of distant light.

The interest in laser comb technologies stems from the need to improve our Doppler methods of planet detection. Tiny shifts in the frequency of a star’s light as measured on a spectrograph tell astronomers about the presence of an otherwise unseen planet around a star. The trick is to measure those shifts with ever increasing detail, an area in which frequency combs hold out rich promise. The shifts induced by an Earth-like planet — equivalent to a wobble of only a few centimeters per second — are far beyond the capabilities of today’s instruments, which are limited to about one meter per second.

No wonder, then, that laser combs are under study at a widening number of institutions, a list that also includes the Max-Planck Institute for Quantum Optics. The new ultrafast laser is one way to push the envelope. Another is to spread the ‘teeth’ of the comb using other methods. The above mentioned CfA work pursues these, and such techniques are also under investigation by the NIST group and Steve Osterman (University of Colorado, Boulder), who are working with sets of mirrors to eliminate periodic blocks of teeth to create a ruler that should be more than adequate for such minute planetary detections.

Comments on this entry are closed.

  • Daniel May 10, 2008, 8:09

    this new method “astro-comb” are excellent news that could detect the rocky planets around centauri B that is a quiet photosphere https://centauri-dreams.org/?p=1737 ever now around centauri A and Proxima centauri that very active with this level 1cm s−1 could detect the planet of low mass easily. this its very exciting moment in exoplanetary science

  • dad2059 May 10, 2008, 9:00

    Promising, but it involves alot of tweaking it seems.

    What would be the range on this be Paul? Probably depends how powerful the laser is wouldn’t it?

  • Administrator May 10, 2008, 13:41

    dad2059, radial-velocity detections at this level would presumably be much easier with nearby stars, but I don’t have any idea whether the use of a laser frequency comb could extend the range appreciably. Nonetheless, there’s plenty of work to do nearby, including Daniel’s interesting point about stars like Centauri B, where the odds on Earth-mass planets seem good.

  • ljk May 15, 2008, 11:47

    Exoplanet Explosion from Cosmos Online

    In late 2008 astronomers will start a massive search for new planets by observing about 11,000 nearby star systems over a period of six years. This number dwarfs the roughly 3,000 stars that astronomers have searched to date for the presence of planets.

    Scientists hope that the NASA-funded project, called MARVELS (Multi-object Apache Point Observatory Radial Velocity Exoplanet Large-area Survey), will find at least 150 new planets, but potentially many, many more.

    “We’re looking in particular for giant planets like Jupiter,” says Jian Ge, principal investigator for MARVELS and an astronomer at the University of Florida in Gainesville, USA. He likens big planets to beacons of lighthouses: signalling the presence of entire solar systems. “Once we find a big planet around a star, we know that smaller planets could be there, too.”

    MARVELS will do much more than just catalogue a few hundred more planets. By surveying the Jupiter-like planets around such a large number of stars, the project aims to give astronomers the data they need to test competing theories for how planetary systems form and evolve.

    Full article here:


  • ljk May 27, 2008, 11:43

    ExoFit: Orbital Parameters of Extra-solar Planets from Radial Velocities

    Authors: Sreekumar T. Balan (UCL), Ofer Lahav (UCL)

    (Submitted on 23 May 2008)

    Abstract: Retrieval of orbital parameters of extra-solar planets poses considerable statistical challenges. Due to sparse sampling, measurement errors, parameters degeneracy and modelling limitations there are no unique values of basic parameters such as period and eccentricity.

    Here we estimate the orbital parameters from radial velocity data in a Bayesian framework by utilising Markov Chain Monte Carlo (MCMC) simulations with the Metropolis-Hastings algorithm. We follow a methodology recently proposed by Gregory and Ford, but our implementation is different, based on the object oriented approach outlined by Graves.

    We make our resulting code, ExoFit, publicly available with this paper. As an illustration we re-analysed the orbital solution of companions to HD 187085 and HD 159868 from the published radial velocity data. We confirm the degeneracy reported for orbital parameters of the companion to HD 187085 and show that a low eccentricity orbit is more probable for this planet. For HD 159868 we obtained slightly different orbital solution and a relatively high ‘noise’ factor indicating the presence of an unaccounted signal in the radial velocity data.

    ExoFit is designed in such a way that it can be easily extended for a variety of probability models, including different Bayesian priors. It can be modified to solve the multi-planet problem, or to be extended for analysing transit and micro-lensing data.

    Comments: Submitted to MNRAS, 11 pages, 6 Tables and 10 Figures. ExoFit may be downloaded from this http URL

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.3532v1 [astro-ph]

    Submission history

    From: Sreekumar Thaithara Balan [view email]

    [v1] Fri, 23 May 2008 08:12:13 GMT (227kb)


  • ljk June 20, 2008, 17:22

    Near Infrared Monitoring of Ultracool Dwarfs: Prospects for Searching for Transiting Companions

    Authors: Cullen H. Blake, Joshua S. Bloom, David W. Latham, Andrew H. Szentgyorgyi, Michael F. Skrutskie, Emilio E. Falco, Dan S. Starr

    (Submitted on 17 Jun 2008)

    Abstract: Stars of late-M and L spectral types, collectively known as Ultracool Dwarfs (UCDs), may be excellent targets for searches for extrasolar planets. Owing to their small radii, the signal from an Earth-size planet transiting a UCD is, in principle, readily detectable. We present results from a study designed to evaluate the feasibility of using precise near infrared (NIR) photometry to detect terrestrial extrasolar planets orbiting UCDs.

    We used the Peters Automated InfRared Imaging TELescope (PAIRITEL) to observe a sample of 13 UCDs over a period of 10 months. We consider several important systematic effects in NIR differential photometry and develop techniques for generating photometry with a precision of 0.01 mag and long-term stability. We simulate the planet detection efficiency of an extended campaign to monitor a large sample of UCDs with PAIRITEL.

    We find that both a targeted campaign with a single telescope lasting several years and a campaign making use of a network of telescopes distributed in longitude could provide significant sensitivity to terrestrial planets orbiting UCDs, potentially in the habitable zone.

    Comments: 22 pages, 5 figures, 3 tables. Accepted for publication in PASP

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0806.2883v1 [astro-ph]

    Submission history

    From: Cullen Blake [view email]

    [v1] Tue, 17 Jun 2008 23:13:27 GMT (170kb)


  • ljk July 1, 2008, 12:37

    Comparing the performance of stellar variability filters for the detection of planetary transits

    Authors: A. S. Bonomo, A. F. Lanza

    (Submitted on 27 Jun 2008)

    Abstract: We have developed a new method to improve the transit detection of Earth-sized planets in front of solar-like stars by fitting stellar microvariability by means of a spot model. A large Monte Carlo numerical experiment has been designed to test the performance of our approach in comparison with other variability filters and fitting techniques for stars of different magnitudes and planets of different radius and orbital period, as observed by the space missions CoRoT and Kepler. Here we report on the results of this experiment.

    Comments: 4 pages, 3 postscript figures, Transiting Planets Proceeding IAU Symposium No.253, 2008

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0806.4463v1 [astro-ph]

    Submission history

    From: Aldo Stefano Bonomo Mr. [view email]

    [v1] Fri, 27 Jun 2008 08:56:51 GMT (308kb)


  • ljk July 1, 2008, 12:39

    Optimal strategies of radial velocity observations in planet search surveys

    Authors: Roman V. Baluev

    (Submitted on 27 Jun 2008)

    Abstract: Applications of the theory of optimal design of experiments to radial velocity planet search surveys are considered. Different optimality criteria are discussed, basing on the Fisher, Shannon, and Kullback-Leibler informations.

    Algorithms of optimal scheduling of RV observations for two important practical problems are considered. The first problem is finding the time for future observations to yield the maximum improvement of the precision of exoplanetary orbital parameters and masses. The second problem is finding the most favourable time for distinguishing alternative orbital fits (the scheduling of discriminating observations).

    These methods of optimal planning are demonstrated to be potentially efficient for multi-planet extrasolar systems, in particular for resonant ones. In these cases, the optimal dates of observations are often concentrated in quite narrow time segments.

    Comments: 8 pages, 2 figures, no tables, Accepted to MNRAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0806.4544v1 [astro-ph]

    Submission history

    From: Roman Baluev [view email]

    [v1] Fri, 27 Jun 2008 15:49:52 GMT (119kb)