Looking for exoplanets is a painstakingly precise job. Scientists look for indirect features in their parent stars and then keep an eye on the most promising objects in the hope of confirming a detection. Given the difficulty of this task, astronomers are always looking for ways of improving the observations.

The latest innovation is a calibration tool called a laser frequency comb, an instrument that acts like a ruler against which astronomers can measure the light of the stars with incredible precision. The tool was developed by a team from Caltech, and they are confident that it will allow for a more detailed characterization of exoplanets. They describe their work in the journal Nature Communications.

Don’t think this is a physical comb like you use in your hair, though. Instead, evenly spaced lines of light act like the teeth of a comb, like tick marks on a ruler, against which the light from a distant star can be examined, all at a microscopic scale.

Traditionally, laser combs use pulses of light, but the Caltech team used a microwave-modulated continuous laser source, producing tick marks 10 to 100 times wider than other combs. Any “wobbles” in the light from the star, such as from an orbiting planet, would be detectable by noticing the fluctuations against the ticks. The team said this could be useful for finding planets of all shapes and sizes, even ones similar to Earth. 

The instrument isn’t limited to one telescope, either. It could be used in a variety of observatories, allowing light from a distant star to be analysed and detect the presence of a planet, when other methods might struggle.

^{The spectrum of a cool star and the spectrum of the comb. Small shifts of the spectrum relative to the stable wavelength standard provided by the laser comb would yield a precision measurement, and help with the detection of habitable exoplanets. Yi et al./Caltech}

"We believe members of the astronomical community could greatly benefit in their exoplanet hunting and characterization studies with this new laser frequency comb instrument," said Xu Yi, a graduate student in Vahala's lab and the lead author of the paper, in a statement.

Precision rulers help astronomers to characterize periodic changes in a star’s motion. If there’s an orbiting planet, a star “wobbles”, and by measuring the size of said wobble, scientists can infer the mass and orbital distance of the planet. Without these details, it’s not possible to assess if a planet is habitable or not.

The instrument has been tested both at NASA's Infrared Telescope Facility and with the Near Infrared Spectrometer on the W. M. Keck Observatory's Keck II telescope. The comb worked without problems in both tests, and the team believe that the technology is ready to go.

"Our goal is to make these laser frequency combs simple and sturdy enough that you can slap them onto every telescope, and you don't have to think about them anymore," added paper coauthor Charles Beichman, executive director of the NASA ExoPlanet Science Institute at Caltech.

"Having these combs routinely available as a modest add-on to current and future instrumentation really will expand our ability to find potentially habitable planets, particularly around very cool red dwarf stars.”