May 07, 2009

Improved Spectroscopy Will Help Detect Earth Sized Planets

Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. have created an "astro-comb" to help astronomers detect lighter planets, more like Earth, around distant stars.

Presentation CMII1, "Femtosecond Laser Frequency Comb for Precision Astrophysical Spectroscopy," Chih-Hao Li et al, 3:45 p.m., Monday, June 1.

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

Smithsonian researcher David Phillips says that he and his colleagues expect to reach a velocity resolution of 60 cm/sec, and maybe even 1 cm/sec, 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.

Astro-comb: revolutionizing precision spectroscopy in astrophysics

Searches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision better than 60cm/s. To find a 1-Earth mass planet in an Earth-like orbit, a precision of 5cm/s is necessary. The combination of a laser frequency comb with a Fabry-Perot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration. Here we report the fabrication of such a filtered laser comb with up to 40 GHz (~1 Angstrom) line spacing, generated from a 1 GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-line-spacing comb (astro-comb) is well matched to the resolving power of high-resolution astrophysical spectrographs. The astrocomb should allow a precision as high as 1cm/s in astronomical readial velocity measurements.

Improved precision in the detection of the motion of stars and galaxies will improve all kinds of astronomy. It will reveal the structure of the universe with several hundred times more accuracy.

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.

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