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Mountain planet hunter turns on | Science



The Echelle spectrograph for Rocky Exoplanet and stable spectroscopic observations can take light from one or all of the 8.4-meter telescopes of the Very Large Telescope.

R. Wesson / ESO

A new exoplanet search instrument, connected to one of the largest telescopes in the world, has seen the sky for the first time, announced today the European Southern Observatory (ESO). The Echelle spectrograph for Rocky Exoplanet and stable spectroscopic observations (ESPRESSO) detects exoplanets by measuring changes in the spectrum of starlight caused by the gravity of the planets dragging them. For this technique, the stellar wobble signal is larger 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, expects to discern the weakest tugboats of planets with Earth-like orbits and masses.

"It is the most mature installation in the world of this type," says astronomer Didier Queloz of the University of Cambridge in the United Kingdom, co-discoverer of the first exoplanet around a normal star in 1

995.

the early years of exoplanet science, this "the radial velocity method was the technique of choice, because the dark planets are too faint to see themselves so close to the brightness of their stars." As an exoplanet in orbit pulls its star forward and toward back from the perspective of an observer on Earth, the periodic change in the velocity of the star 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 The technique was eclipsed by the detection of traffic, when a planet passes in front of its star and temporarily darkens it.Since 2009, NASA's Kepler satellite has detected several thousand exopl anetas using the transit method.

Due to the way they work, the two methods reveal different characteristics of an exoplanet. Both reveal orbits, but the radial velocity points to the mass of a planet, while the transits reveal its size. Ideally, astronomers want to know both. The researchers came to "understand that radial velocity was essential for the masses, and that created an appetite for these measurements," says Queloz. A few ground instruments had been beating radial velocity measurements, including the ESO High Precision Radial Velocity Planer (HARPS) and the Automated Plane Finder at the Lick Observatory at the University of California at Mt. Hamilton, but the astronomers wanted more.

This has led to a new generation of spectrographs designed to search for exoplanets using different techniques and covering different ranges of wavelengths. The previous generation of spectrographs could achieve stellar oscillations of around 1 meter per second, a slow pace to walk. Jupiter, for example, changes the sun by 13 meters per second, but the much weaker pull of the Earth only reaches a speed of 9 centimeters per second. ESPRESSO, at the forefront of the new generation, aims to put planets similar to Earth within reach, with a sensitivity of 10 centimeters per second or even slower. "We are the first to be crazy enough to try to achieve it," says lead scientist Francesco Pepe of the University of Geneva in Switzerland.

It is likely that an exact twin of Earth is 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 can also detect planets the size of the Earth. Earth around smaller stars, where a weaker pull achieves more movement.

ESPRESSO is not revolutionary compared to the previous generation, admits Pepe, but it extends HARPS techniques and expands them for a larger telescope. "We are doing something unique to the limit of current technology," he says. One of the most challenging technologies is the laser frequency comb, which takes a laser beam and divides it into hundreds of thousands of equally spaced frequencies to provide a kind of reference grid against which to measure the Doppler changes of starlight. Pepe says they are still working to have their frequency comb cover all the instrument's detectors.

Queloz says that the strength of ESPRESSO lies in the wider range of wavelengths at which it is sensitive and the enormous light-gathering power of the VLT. In current measurements of radial velocity, researchers are fighting against the natural noise in starlight caused by the turbulent atmosphere of a star. With more light collected, at more wavelengths, astronomers can "better eliminate 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 of the four or combine the light of all of them. "It is extremely versatile and is fully integrated into the flexibility of Paranal," says Queloz.


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