Star Comb Joins Quest for Other Earths

A new laser frequency comb soon may be able to determine whether life on other planets really exists.

Developed by scientists from the University of Colorado at Boulder and the National Institute of Standards and Technology (NIST), the device has for the first time calibrated measurements of starlight from stars other than the sun by precisely measuring the frequencies, or colors, of their emitted light. The results suggest that combs eventually will fulfill their potential to boost the search for Earth-like planets to a new level.

“The comb worked great,” said Scott Diddams, a NIST physicist and co-creator of the frequency comb. “In a few days, it enabled measurement precision comparable to the very best achieved in the same wavelength range with much more established techniques, and we hope the comb will do much better as the new technique is perfected.”

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). (Image: CU/NIST/Penn State)

The NIST comb calibrated measurements of infrared starlight, a type of light emitted predominantly by M dwarf stars, which are plentiful in Earth’s part of the galaxy and might have orbiting planets suitable to life.

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 this characteristic “fingerprint” can be caused by the star wobbling from the gravitational pull of an orbiting planet.

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

Enter the NIST comb, which spans an infrared wavelength range from 1450 to 1700 nm. It provides strong signals at narrowly defined target frequencies and is traceable to international measurement standards. When combined with a spectrograph, the frequency comb — which has widely spaced “teeth” tailored to the reading capability of spectrographs — acts as a precise ruler to calibrate and track the exact colors in the star’s fingerprint and detect any periodic variations.

The comb calibrated a spectrograph at the Hobby-Eberly Telescope in the Texas mountains, where it measured star wobble with a precision of about 10 m/s, comparable to the best ever achieved in the infrared region of the electromagnetic spectrum. The first field results are limited by the short observation time and technical issues associated with the newly developed experimental approach. However, the scientists say the device has the inherent capability of measuring star wobble of just a few cm/s — 100 times better — although limitations in the spectrograph and in the stability of the star itself may constrain the ultimate precision.

The team hopes to soon improve precision to 1 m/s, or roughly the limit to date for measuring visible light from the sun, which would put the technique at the cutting edge of infrared astronomy.

The work was supported by NIST and the National Science Foundation. Pennsylvania State University students and astronomers collaborated on the project.

The study was published in Optics Express.

For more information, visit: www.nist.gov