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The Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations can take light from any or all of the four 8.4-meter telescopes of the Very Large Telescope.

R. Wesson/ESO

Mountaintop planet hunter turns on

A new exoplanet-hunting instrument, attached to one of the world’s largest telescopes, has seen its first glimpse of the sky, the European Southern Observatory (ESO) announced today. The Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) detects exoplanets by measuring shifts in the spectrum of light from stars caused by the gravity of planets tugging on them. For this technique, the signal of the stellar wobble is bigger for more massive planets in closer orbits. ESPRESSO, with improved spectral resolution, a wider wavelength range, and fixed to ESO’s Very Large Telescope (VLT) at Cerro Paranal in Chile, hopes to discern the fainter tugs of planets with Earth-like masses and orbits.

“It’s the most mature facility in the world of this kind,” says astronomer Didier Queloz of Cambridge University in the United Kingdom, co-discoverer of the first exoplanet around a normal star in 1995.

In the early years of exoplanet science, this “radial velocity” method was the technique of choice, because dim planets are too faint to see so close to the glare of their stars. As an orbiting exoplanet pulls its star back and forth from the perspective of an observer on Earth, the periodic change in the star’s velocity is detectable as a Doppler shift in the frequency of its light. Hundreds of exoplanets have been found in this way. But in recent years, the technique was eclipsed by transit detection, when a planet passes in front of its star and temporarily dims it. Since 2009, NASA’s Kepler satellite has detected several thousand exoplanets using the transit method. 

Because of the way they work, the two methods reveal different characteristics of an exoplanet. Both reveal orbits, but radial velocity points to a planet’s mass, while transits reveal its size. Ideally, astronomers want to know both. Researchers came to “understand that radial velocity was essential for masses, and that created an appetite for these measurements,” Queloz says. A few ground-based instruments had been churning away measuring radial velocities, including ESO’s High Accuracy Radial velocity Planet Searcher (HARPS) and the Automated Planet Finder at the University of California’s Lick Observatory in Mt. Hamilton, but astronomers wanted more.

This has led to a new generation of spectrographs designed to look for exoplanets using different techniques and covering different wavelength ranges. The previous generation of spectrographs could reach stellar wobbles of around 1 meter per second—a slow walking pace. Jupiter, for example, shifts the sun by 13 meters per second, but Earth’s much weaker tug only achieves a velocity of 9 centimeters per second. ESPRESSO, at the forefront of the new generation, aims to put Earth-like planets within reach, with a sensitivity of 10 centimeters per second or even slower. “We’re the first to be mad enough to try to achieve that,” says lead scientist Francesco Pepe of the University of Geneva in Switzerland.

An exact twin of Earth is probably out of reach, but ESPRESSO should be able to detect super-Earths three or four times heavier than Earth that orbit sun-like stars. It may also detect Earth-sized planets around smaller stars, where a weaker tug achieves more movement.

ESPRESSO is not revolutionary compared to the previous generation, admits Pepe, but rather expands on the techniques of HARPS and scales them up for a larger telescope. “We’re doing something unique at the limit of current technology,” he says. One of the most challenging technologies is the laser frequency comb, which takes a laser beam and splits it into hundreds of thousands of equally spaced frequencies to provide a sort of reference grid against which to measure the Doppler shifts of stellar light. Pepe says they are still working to get their frequency comb to cover the whole of the instrument’s detectors.

Queloz says ESPRESSO’s strength lies in the broader range of wavelengths it is sensitive to and the huge light-collecting power of the VLT. In today’s radial velocity measurements, researchers are fighting against the natural noise in stellar light caused by a star’s turbulent atmosphere. With more light collected, across more wavelengths, astronomers can better “remove the effect of the stellar atmosphere,” he says. The VLT comprises four identical 8.4-meter telescopes. ESPRESSO is positioned so that it can take light from any one of the four or combine light from all of them. “It’s extremely versatile and is fully integrated into Paranal’s flexibility,” Queloz says.