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  • Star comb aids search for exoplanets

Photonics Spectra
May 2012

BOULDER, Colo. – A new laser frequency comb soon may be able to answer one of the most intriguing scientific questions: Are there other earthlike planets in our galaxy capable of supporting life as we know it?

A collaboration of the National Institute of Standards and Technology (NIST), the University of Colorado at Boulder and Penn State University brings together the best of laser science, frequency combs and precision astronomical instrumentation. The researchers’ goal is to determine whether life might exist on other planets, said Scott Diddams, a NIST physicist and co-creator of the frequency comb.

The device has for the first time calibrated measurements of infrared starlight from stars other than the sun by precisely measuring the frequencies of their emitted light. The results suggest that combs eventually will fulfill their potential to boost the search for Earth-like planets to the next level.

“The laser frequency comb we used is unique because its mode spacing of 25 GHz is significantly larger than typical combs, which might have a mode spacing of 100 MHz to 1 GHz,” Diddams told Photonics Spectra. “Another unique feature is that it was designed to be completely transportable, which enabled us to take it out of our lab in Boulder, move it to the McDonald Observatory and operate it there with the 9.2-m Hobby-Eberly Telescope and the Penn State University Pathfinder spectrograph.”

Another distinctive feature is that it operates in the near-infrared. All other combs currently used for astronomical observations operate in the visible spectrum.

“Despite its challenges, the near-infrared has the particular advantage that a certain class of stars (called M stars) emit most of their light in this spectral region,” said Steve Osterman, an astronomer at the University of Colorado. “Moreover, there are many M stars in our galaxy nearby the Earth (60 percent of the stars within about 30 light-years of the Earth are M stars), making them excellent candidates in a search for exoplanets.”


NIST researchers and collaborators measured the frequencies of infrared starlight (three solid bands with faint tick marks indicating where light is absorbed by the atmosphere) by comparing the missing light to a laser frequency comb reference “ruler” (sets of bright vertical bars indicating precise wavelengths, which increase from left to right). The three sets of starlight and comb light are shown in false color, from deeper orange (the most light) to orange-white (slightly less light) to black (very little light). Courtesy of CU Boulder/NIST/Penn State.


To search for planets orbiting distant stars, astronomers look for periodic variations in the apparent colors of starlight over time. A star’s nuclear furnace emits white light, which is modified by elements in the atmosphere that absorb certain narrow bands of color. Periodic changes within the characteristic “fingerprint” can be caused by the star’s wobbling from the gravitational pull of an orbiting planet.

“It is such periodic changes in the emitted wavelengths that are used to infer the presence of a planet orbiting the star,” said Suvrath Mahadevan, an assistant professor of astronomy and astrophysics at Penn State. “In our experiments, we typically took a series of approximately 10 exposures of five minutes in duration while the telescope was pointed at a particular stellar object. Repeating this over a few nights, we were able to determine that our system could measure changes in the near-infrared emitted wavelengths with a resolution of about 50 femtometers.”

The scientists note that many factors play into how precisely they can make such measurements. Although observation time played a significant role, they discovered that the biggest limitation was modal noise present in the beam that illuminates the spectrometer.

“Such noise arises in the type of optical fibers we used to transport the light between the comb, the telescope and the spectrograph,” Diddams said.

Astronomers have discovered more than 600 planets using star wobble analysis, but a planet analogous to Earth, with low mass and orbiting at just the right distance from a star – in the so-called “Goldilocks zone” – is hard to detect with conventional technology.

“The challenge is that a planet like our Earth orbiting ‘not too close or not too far’ from a star like our sun would make the star ‘wobble’ with a velocity of only 10 centimeters per second,” said Gabe Ycas, a University of Colorado graduate student who helped build the comb. “That is about the speed of a fast spider crawling across the floor. This small velocity change would result in a Doppler shift of the wavelength of the starlight of only 0.5 femtometers, which is very small indeed and difficult to detect – particularly if you consider that such measurements are made across enormous distances and with faint stellar sources.”

Combing the galaxy for exoplanets

The NIST comb, which spans wavelengths from 1450 to 1700 nm, provides strong signals at narrowly defined target frequencies and is traceable to international measurement standards. When combined with Penn State’s Pathfinder spectrograph, the frequency comb acts as a precise ruler to calibrate and track the exact colors in the star’s fingerprint and to detect any periodic variations.

The comb calibrated the spectrograph at the Hobby-Eberly Telescope in the Texas mountains, where it measured star wobble with a precision of about 10 m/s. This accuracy is comparable to the best achieved in the infrared region of the electromagnetic spectrum.

“Observation time was limited in our first field test because ours was a very new and high-risk experiment that had to fit into the schedule of a significant research telescope,” Diddams said. “The biggest technical difficulties arose in part from the fact that such an experiment had never been done before.”

However, the scientists say that the device has the inherent capability of measuring star wobble of just a few centimeters per second – 100 times better – although limitations in the spectrograph and in the stability of the star itself may constrain the ultimate precision.

Next, the NIST and University of Colorado team plans to create a comb with 30-GHz mode spacing that covers the 1000- to 1200-nm spectral range.

“Our partners at Penn State have been funded by the NSF [National Science Foundation] to turn their Pathfinder spectrograph into a facility instrument which will be called the Habitable Zone Planet Finder (HPF),” Diddams said.

“Demonstrating that the laser comb enabled precision radial velocity measurements on the prototype Pathfinder is an important component of the successful HPF proposal,” added Mahadevan, who is the HPF’s principal investigator. “We are continuing to work together in the development and building stages, and we hope to return to the Hobby-Eberly telescope in two to three years to join the comb with the HPF, which would then begin a dedicated campaign of exoplanet searches.”

The study was published in the open-access journal Optics Express (http://dx.doi.org/10.1364/OE.20.006631).


GLOSSARY
infrared astronomy
The study and the interpretation of the infrared emittances of celestial bodies and phenomena.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
spectrograph
An optical instrument for forming the spectrum of a light source and recording it on a film. The dispersing medium may be a prism or a diffraction grating. A concave grating requires no other means to form a sharp image of the slit on the film, but a plane grating or a prism requires auxiliary lenses or concave mirrors to act as image-forming means in addition to the dispersing element. Refracting prisms can be used only in parallel light, so a collimating lens is required before the prism and...
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