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Fri Dec 8, 2017, 12:47 AM

Mountaintop planet hunter turns on

By Daniel Clery Dec. 6, 2017 , 6:00 AM

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.


Very Large Telescope at Cerro Paranal in Chile

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