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Ramping Up Doppler, Finding New Earths

Keep your eye on a project in the Canary Islands called the New Earths Facility. Using a laser measuring device now being tuned up for the job, scientists intend to continue the hunt for terrestrial worlds with a greater than ever chance of success. Called an astro-comb, the device brings far greater precision to our existing Doppler techniques for finding exoplanets. In fact, early reports suggest it may increase the resolution of these methods by as much as one hundred times, making the detection of an Earth-like world in an orbit similar to ours feasible.

Now we’re getting into interesting territory indeed, not only in terms of planetary detections themselves but synergies with the ambitious Kepler mission, to be launched in 2009. Read on.

New Earths in the universe

Studying the Doppler shift of distant starlight has already achieved a remarkable precision, capable of finding planets down to about five Earth masses in orbits as far from the star as Mercury. But the farther we get from the star, the trickier these observations become. The same is true for planetary size. Larger worlds are much easier to find, making finding planets as small as the Earth in an orbit similar to our own a challenge. The astro-comb ought to make a difference.

Image: CfA astronomers are developing a new device that may be the first to spot Earth-like planets, like the hypothetical world with two moons shown in this artist’s concept. The “astro-comb” uses a laser to provide an ultrasensitive way of measuring a distant star’s wobbling motion, which is induced by an orbiting planet. Credit: David A. Aguilar (CfA).

Pulses of laser light linked to an atomic clock create a standard against which the astro-comb can measure the incoming starlight. The technology is based on so-called ‘laser combs’ that have been in use for creating precision clocks. Such a comb creates spikes of laser light evenly spaced in wavelength — hence the comb metaphor — that can be projected into a spectrograph. Ronald Walsworth (Smithsonian Astrophysical Observatory) added a filtering device to the laser comb that spreads the ‘teeth’ of the comb to make the technology workable for astrophysics.

If the device proves out, numerous applications should be possible beyond the planet hunt. The paper on this work, for example, talks about measuring the decelerating expansion of the early universe (as opposed to what is now believed to be the renewed acceleration of same in a much later era). But planets, Earth-like ones at that, are what inevitably come to mind, taking pride of place in the paper on this work:

Beyond our first demonstration, astro-combs should enable many observations that have previously been considered technically unachievable. One example is the search for a 1-Earth-mass planet in an Earth-like orbit around a Sun-like star, which requires a sensitivity of 5 cm s−1 and stability on at least a 1-year timescale.

Deploying an astro-comb at the William Herschel Observatory in the Canaries should boost the powers of the spectrograph to be located there under the auspices of the Harvard Origins of Life Initiative and the Geneva Observatory to the point where that instrument can find Earth-like planets. Known as the HARPS-NEF (High-Accuracy Radial-velocity Planet Searcher of the New Earths Facility) spectrograph, the instrument is similar to the HARPS spectrograph in Chile.

Interestingly, the Canaries facility, its instrumentation augmented by an astro-comb, offers useful support to the upcoming Kepler mission. Kepler will go after planetary transits, looking at 100,000 solar-type dwarf stars in a single field of view for four years. Kepler’s problem: The space-based observatory will be able to detect Earth-sized planets but unable to measure their mass because of the tiny Doppler values they generate. Current models for terrestrial planets suggest that the combination of mass and radius information will help us tell the difference between rocky planets and water worlds.

HARPS-NEF, especially as boosted by the astro-comb, should help make this possible. A prototype astro-comb will be tested this summer at the Mount Hopkins Observatory in Arizona, with improvements feeding into the Canaries project. The paper is Chih-Hao Li et al., “A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s-1,” Nature 452 (3 April 2008), pp. 610-612 (available online).

Comments on this entry are closed.

  • Adam April 9, 2008, 5:49

    The precision claimed (hoped for?) is impressive, but will stars be quiet enough to extract such a tiny signal from? Stellar photospheres are pretty noisy places apparently.

  • David Nataf April 9, 2008, 20:46

    Signal to noise ratio increases as the square root of time, so it’ll probably just take longer.

  • Adam April 10, 2008, 6:14


    That is good news! Excellent. I wasn’t sure what the relationship was for extracting a periodic radial velocity signal out of photospheric noise was, so thanks for the info.

    Seems like good cause for optimism :-)

  • Daniel April 11, 2008, 13:05

    this new method “astro-comb” are excellent news that could detect the rocky planets around centauri B that is a quiet photosphere https://www.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

  • ljk April 15, 2008, 15:13

    Space artists play an increasingly important role in imagining
    distant worlds

    Boston Globe


    By Colin Nickerson, Globe Staff

    April 14, 2008

    Cambridge – When an exoplanet called 2M1207B was caught in a
    colossal smash-up earlier this year, the Harvard-Smithsonian Center
    for Astrophysics summoned David A. Aguilar to recreate the scene.

    No way scientists could just snap pictures. The surmised collision
    with another planet was unfurling 173 light-years from Earth – too
    distant and too tiny, relatively, for the Hubble Space Telescope or
    other mighty lenses to catch meaningful images.

    The astronomers needed a space artist, and fast: Without a
    “visual,” the spectacular finding, detected through complex
    gauging of luminosity, temperature, and densities, would be
    hard to explain to nonscientists….

    The images are important to scientists not only to educate the
    public about their work but to keep enthusiasm stoked among
    the politicians and big institutions that pay for nearly all such

    “We walk a pretty fine line,” said Lynette Cook, a California-based
    illustrator who possesses degrees in both biology and fine arts, and
    is widely regarded as the doyenne of 21st-century space artists….

    …Since the mid-1990s, she has collaborated with prominent
    astronomers, including GEOFFREY W. MARCY OF THE UNIVERSITY
    OF CALIFORNIA AT BERKELEY, a leading finder of exoplanets.

    Said Marcy: “I can tell you that we’ve discovered a planet with
    6.48 Saturn masses and 2.64 times the orbit of the Earth, and
    provide equations to make your head spin. Lynette can make
    you feel like you are setting foot on that planet.”…

  • ljk May 20, 2008, 12:10

    Microlensing Detections of Moons of Exoplanets

    Authors: Cheongho Han

    (Submitted on 17 May 2008)

    Abstract: We investigate the characteristic of microlensing signals of Earth-like moons orbiting ice-giant planets. From this, we find that non-negligible satellite signals occur when the planet-moon separation is similar to or greater than the Einstein radius of the planet.

    We find that the satellite signal does not diminish with the increase of the planet-moon separation beyond the Einstein radius of the planet unlike the planetary signal which vanishes when the planet is located well beyond the Einstein radius of the star. We also find that the satellite signal tends to have the same sign as that of the planetary signal. These tendencies are caused by the lensing effect of the star on the moon in addition to the effect of the planet. We determine the range of satellite separations where the microlensing technique is optimized for the detections of moons.

    By setting an upper limit as the angle-average of the projected Hill radius and a lower limit as the half of the Einstein radius of the planet, we find that the microlensing method would be sensitive to moons with projected separations from the planet of $0.05 {\rm AU} \lesssim d_{\rm p} \lesssim 0.24 {\rm AU}$ for a Jupiter-mass planet, $0.03 {\rm AU}\lesssim d_{\rm p} \lesssim 0.17 {\rm AU}$ for a Saturn-mass planet, and $0.01 {\rm AU} \lesssim d_{\rm p} \lesssim 0.08 {\rm AU}$ for a Uranus-mass planet. We compare the characteristics of the moons to be detected by the microlensing and transit techniques.

    Comments: 6pages, 6 figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.2642v1 [astro-ph]

    Submission history

    From: Cheongho Han [view email]

    [v1] Sat, 17 May 2008 01:42:20 GMT (2345kb)


  • ljk May 8, 2009, 15:09


    May 8, 2009

    ‘Astro-comb’ Will Aid Search for Extra-terrestrial Planets

    Written by Anne Minard

    Artist’s rendering of a hypothetical world with two moons that could be detected using new laser technology called the “astro-comb.” Credit: David A. Aguilar (CfA)

    As the race ramps up to find Earth-like planets around other stars, lasers are a viable option.

    That according to researchers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who have created an “astro-comb,” a sort of calibration tool based on wavelengths of light, to pick up minute variations in a star’s motion caused by orbiting planets.

    In most cases, extrasolar planets can’t be seen directly—the glare of the nearby star is too great—but their influence can be discerned through spectroscopy, which analyzes the energy spectrum of the light coming from the star. Not only does spectroscopy reveal the identity of the atoms in the star (each element emits light at a certain characteristic frequency), it can also tell researchers how fast the star is moving away or toward Earth, courtesy of the Doppler effect, which occurs whenever a source of waves is itself in motion. By recording the change in the frequency of the waves coming from or bouncing off of an object, scientists can deduce the velocity of the object.

    Though the planet might weigh millions of times less than the star, the star will be jerked around a tiny amount owing to the gravity interaction between star and planet. This jerking motion causes the star to move toward or away from Earth slightly in a way that depends on the planet’s mass and its nearness to the star. The better the spectroscopy used in this whole process, the better will be the identification of the planet in the first place and the better will be the determination of planetary properties.

    Right now standard spectroscopy techniques can determine star movements to within a few meters per second. In tests, the Harvard researchers are now able to calculate star velocity shifts of less than 1 m (3.28 feet) per second, allowing them to more accurately pinpoint the planet’s location.

    Smithsonian researcher David Phillips says that he and his colleagues expect to achieve even higher velocity resolution, which when applied to the activities of large telescopes presently under construction, would open new possibilities in astronomy and astrophysics, including simpler detection of more Earth-like planets.

    With this new approach, Harvard astronomers achieve their great improvement using a frequency comb as the basis for the astro-comb. A special laser system is used to emit light not at a single energy but a series of energies (or frequencies), evenly spaced across a wide range of values. A plot of these narrowly-confined energy components would look like the teeth of a comb, hence the name frequency comb. The energy of these comb-like laser pulses is known so well that they can be used to calibrate the energy of light coming in from the distant star. In effect, the frequency comb approach sharpens the spectroscopy process. The resultant astro-comb should enable a further expansion of extrasolar planetary detection.

    The astro-comb method has been tried out on a medium-sized telescope in Arizona and will soon be installed on the much larger William Herschel Telescope, which resides on a mountaintop in the Canary Islands.

    Preliminary results from the new technique were published in the April 3, 2008 issue of Nature. The Harvard group will present the most recent findings at the 2009 Conference on Lasers and Electro Optics/International Quantum Electronics Conference, May 31 to June 5 in Baltimore.