An “Astrocomb” for Detecting Earth-Like Worlds

Astronomers on the quest for Earth-like exoplanets sometimes talk of the “Goldilocks zone”—the distance from a planet’s parent star that’s “just right” for liquid water to exist, and thus for life as we know it on Earth to arise.

But for so-called M-dwarf stars, the most abundant star type in our galaxy, finding potential habitable-zone rocky planets is a tough proposition. That’s because the proximity of such planets to their host stars means that one common method for detecting and assessing them—the gravitationally induced Doppler shift in the star’s light due to the orbiting planet—requires detection of a faint infrared signal that’s been hard to pick up using current technology.

Now, a multi-institution team of U.S. researchers has paired up a near-infrared optical frequency comb and a precision infrared spectrograph to create an instrument specifically tuned to catch signals from habitable-zone planets circling M-dwarf stars (Optica, doi: 10.1364/OPTICA.6.000233). The team believes that the instrument, which has already been tested on the 10-m Hobby–Eberly Telescope at McDonald Observatory in Texas, could significantly expand the catalog of potential Earth-like planets surrounding some of the most abundant and closest stars. And the researchers hope that the use of the souped-up spectrograph, dubbed the Habitable Zone Planet Finder (HPF), will offer up a nice menu of planets for future study using the James Webb Space Telescope (JWST).

Elusive infrared

One of the most common ways of detecting distant extrasolar planets is the so-called radial-velocity (RV) method. As the planet revolves around its host star, the planet’s small gravitational tug on the star causes the star to move in a tiny periodic ellipse, which can be detected by Doppler shifts in the star’s light—a slight blue shift when the star is moving toward the observer; a slight red shift when the star is moving away.

This method has enabled the detection of hundreds of exoplanets, particularly large, Jupiter-like planets, for which the RV shift is measured in 10 or more meters per second. For-M dwarf stars—which account for some 70 percent of the stars in our galaxy, and which many astronomers view as the best candidates for hosting rocky planets similar to our own—the RV shift for an orbiting habitable-zone planet is a much smaller 1 m/s.

That makes the search for planets surrounding these abundant dwarf stars much more difficult than for the Jupiter-like worlds. The “hot Jupiters” discovered thus far have tended to surround stars that emit in the visible wavelength band, where the Doppler shift is easily measured using low-noise silicon detectors. M dwarfs, in contrast, emit mainly in the near-infrared. In that wavelength region, even spectrographs kitted out with state-of-the-art cryogenic infrared detectors have been able to detect Doppler shifts equivalent to RVs only as small as 5–10 m/s. That’s an order of magnitude greater than the shifts relevant for habitable-zone planets around M-dwarf stars.

Tailored light from the astrocomb

The team behind the new work, which includes scientists from the U.S. National Institute of Standards and Technology (NIST), nine U.S. universities and one Australian university, has now put together a package that neatly extends the observational reach for these tough-to-access exoplanets.

The package begins with a NIST-developed electro-optic frequency comb spanning the NIR wavelength range from 700 nm to 1600 nm, with a spacing of 30 GHz between the discrete “teeth” of the comb. To create the comb, light from a 1064-nm continuous-wave semiconductor laser source feeds into a set of waveguide electro-optic modulators driven by a 30-GHz microwave source. The result: a comb of around 100 single-frequency “teeth,” each separated from the next by the driving microwave frequency. To keep things ultra-stable, the comb is locked to the signal from an atomic clock.

This “astrocomb”—a relatively compact setup installed on a 60×152-cm breadboard—provides, after several additional filtering and amplification steps, a source of tailored light for the HPF spectrograph. The HPF itself has already been designed from the ground up to detect exoplanets around M-dwarf stars, with a wavelength range of 800–1300 nm, a custom-built, 125-pound echelle grating and cooling to 180 K to tamp down infrared noise from the instrument itself. The frequency comb provides the spectrograph with a standard ruler, at a bandwidth and tooth spacing precisely calibrated to amplify signals at the target frequencies. That extra calibration tool hammers down the resolvable RVs to levels relevant for detecting rocky planets around M-dwarf stars.

Another window for habitable-zone exoplanets?

The astrocomb addition to the HPF saw first light a year ago, and has been running continuously at the Hobby-Eberly Telescope in Texas since May. Thus far, in observations of Barnard’s star, a nearby M dwarf, the team has demonstrated that the astrocomb-HPF combination can detect RV changes of 1.53 m/s in the near-infrared—with an instrument RV precision of low as 6 cm/s. In a press release accompanying the research, coauthor and OSA Fellow Scott Diddams of NIST said that the addition of the astrocomb to HPF “should allow us to find habitable planets around the most ubiquitous stars in our galaxy.”

That combination could gain extra relevance with the launch of JWST, currently slated for spring 2021. A specific target of the space telescope’s next-gen spectrometers will be analysis of the atmospheric chemistry of transiting terrestrial planets for possible biomarkers—potential signs of life. New rocky-planet candidates around M dwarf stars, revealed by the HPF and other instruments, could give JWST a lot to look at. Stay tuned …