Astrocombs for Planet-Hunting Telescopes

The hunt for Earth-like planets, and perhaps extra­terrestrial life, just got more precise, thanks to record-setting starlight measure­ments made possible by a National Institute of Standards and Tech­nology NIST “astrocomb.” NIST’s custom-made frequency comb – which precisely measures frequencies of light – ensures the precision of starlight analysis by a spectro­graph at the Hobby-Eberly Tele­scope in Texas. The project is a colla­boration involving NIST, the University of Colorado Boulder (CU) and Pennsyl­vania State University, the primary partner in the telescope and spectrograph.

Scott Diddams views the NIST frequency comb designed to ensure the precision of starlight analysis at the Hobby-Eberly Telescope in Texas. (Source: NIST)

The new comb apparatus for the first time provides the precision needed for discovering and characterizing planets orbiting M dwarf stars, which comprise 70 percent of the stars in the galaxy and are plentiful near Earth. “The comb imme­diately allowed our Penn State colleagues to make measure­ments they could not otherwise make,” NIST Fellow Scott Diddams said. “These improved tools should allow us to find habitable planets around the most ubiquitous stars in our galaxy.”

A star’s nuclear furnace emits white light, which is modified by elements in the atmo­sphere that absorb certain narrow bands of color. To search for planets orbiting distant stars, astro­nomers look for periodic changes in this characteristic fingerprint, that is, very small variations in the apparent colors of starlight over time. These oscil­lations in color are caused by the star being tugged to and from by the gravi­tational pull of an unseen orbiting planet. This apparent wobble is subtle, and measurements are limited by the frequency standards used to calibrate spectro­graphs.

Hundreds of exoplanets have been discovered using star wobble analysis, but a planet with a mass similar to that of Earth and orbiting at just the right distance from a star is hard to detect with conven­tional techno­logy. Data collected by the NIST-CU-Penn State research team show the astrocomb will make it possible to detect Earth-mass planets that cause color shifts equivalent to a star wobble of about 1 meter per second, at least 10 times better than previously achieved in the infrared region of the electro­magnetic spectrum. Infrared light is the main type emitted by M dwarf stars.

Over the past 20 years, NIST researchers in Boulder, Colorado, first invented and then pioneered further advances in optical frequency combs. The comb delivered to Texas is unique in having about 5,000 widely spaced teeth as specific color cali­bration points. It’s tailored to the reading capability of Penn State’s Habitable Zone Planet Finder sprectro­graph and spans the target infrared wave­length band of 800-1300 nm. Just 60 by 152 square centimeters in size and made of rela­tively simple commercial components, the comb is also robust enough to withstand continuous use at a remote site.

In providing tailored light to the spectro­graph, the new comb acts like a very precise ruler to calibrate and track exact colors in a star’s finger­print and detect any periodic variations. The comb, made with new electro-optic laser tech­nology, provides strong signals at accurately defined target frequencies that can be traced to inter­national measurement standards. The project has been in the works for years. The NIST-CU-Penn State research team did a test run in 2012 that showed the promise of the new approach. The new comb was delivered and saw “first light,” as they say in astronomy, in February 2018, and has been running nightly since May of 2018. The new comb has a broader light range and is more stable than the earlier demo version.

While the idea of using frequency combs to aid planet discovery has generated a lot of interest around the world, the new astrocomb is the first in operation at near-infrared wave­lengths. Other combs currently operating on a tele­scope, such as the High Accuracy Radial Velocity Planet Searcher (HARPS) in Chile, are dedicated to visible light measure­ments. (Source: NIST)

Reference: A. J. Metcalf et al.: Stellar spectroscopy in the near-infrared with a laser frequency comb, Optica 6, 233 (2019); DOI: 10.1364/OPTICA.6.000233

Link: Time and Frequency Division, National Institute of Standards and Technology NIST, Boulder, USA

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