The European Southern Observatory already has one of the world’s best planet-hunting tools in the HARPS spectrograph. Installed at the 3.6-meter La Silla telescope in Chile, HARPS is an instrument that can detect the extremely subtle wobbles in a star’s motion that may be induced by the gravitational pull of an orbiting planet.

But the wobble, or radial-velocity, method most readily turns up large exoplanets, some of them many times more massive than Jupiter, that orbit quite close to their host star. Those are the bodies that exert the most noticeable tug on their parent stars. Astronomers would like to find worlds more like Earth: small, rocky worlds on moderately long orbits that keep the planet at a temperate distance from its star.

HARPS can already detect planets that cause their host stars to move a mere meter per second—about the walking speed of a human. But even that remarkable precision has limited usefulness. Earth, for comparison, imparts a velocity of just nine centimeters per second on the sun. Detecting a change so subtle in the spectral breakdown of a star’s light requires an extremely stable light spectrum for calibration and reference. It’s a bit like using a good tuning fork to check if the note from a piano key has gone ever so slightly flat.

Now a group of researchers has developed a calibration technique to make radial-velocity searches more sensitive to smaller, potentially Earth-like planets. In the May 31 issue of Nature, Tobias Wilken of the Max Planck Institute for Quantum Optics in Garching, Germany, and his colleagues report using a device called a laser frequency comb to give HARPS a boost. (Scientific American is part of Nature Publishing Group.)

A laser frequency comb, the researchers explain, emits a series of evenly spaced spectral lines—like the teeth of a comb—“that are as accurate and stable as the atomic clock relative to which the comb is stabilized.” (Instead of a pendulum, atomic clocks use the frequency of an atom’s oscialltion between quantum states as the basis for timekeeping.) Used as an extremely reliable reference light source, the device allows exoplanet-hunting spectrographs to measure mere centimeters per second of motion, the study’s authors report, which “should make it possible to detect Earth-like planets in the habitable zone.”