Radial velocities: ELODIE, SOPHIE, HARPS, ESPRESSO, SPIRou
The instruments used by the team of Michel Mayor and Didier Queloz, Elodie, Sophie, Coralie and Harps, installed int the Observatoire de Haute Provence (OHP) in France and the European Southern Observatory (ESO), in Chile, are among the greatest discoverers of exoplanets using the radial velocity ? method.
These instruments measure thousands of spectral lines ? , which enable them to measure the radial velocities ? of stars of about three orders of magnitude ? smaller than $\frac{c}{R}$, where $c$ is the speed of light and $R$ the instrument’s resolving power.
The spectrographs at the OHP
The ELODIE instrument was installed in 1993 on the 1.93 m telescope in the Observatoire de Haute Provence. It was a high-resolution ? spectrograph which observed in the visible spectrum (between 385 and 680 nm) and was capable of obtaining precision of $7 \space m/s$ on the radial velocity ? of the star observed. It led to the discovery of the first exoplanet, 51 Pegasi b.
Light from the star is heavily dispersed to be able to measure tens of thousands of spectral lines ? and compare them to the same immobile lines.
Spectrographs are extremely large instruments which cannot be sent into space.
It has been replaced by SOPHIE, a spectrograph with a higher resolution ? mode ($R=75,000$), which enabled precision of $2 \text{ or } 3 \space m/s$ to be achieved.
Sophie helped discover numerous exoplanets, but this instrument had another role. The American Kepler satellite ? discovered a large number of exoplanet candidates using the transit method. To be certain that these objects are planets, their mass has to be measured with the radial velocity ? method. These observations with SOPHIE were therefore essential in confirming the numerous exoplanet candidates detected by Kepler.
HARPS and HARPS-Nord
The HARPS (High Accuracy Radial velocity ? Planet Searcher) spectrograph is optimised for detecting planets and was installed in 2003 on a 3.6 m diameter telescope on the La Silla site of the European Southern Observatory (ESO).
It is designed to achieve precision better than one $m/s$ thanks to very high resolution ? ($R=98,000$) in the visible spectrum (between 378-691 nm), in particular owing to a very high precision reference spectrum (optical frequency comb).
In 2012, a new version of the instrument was installed in the Northern hemisphere on the Galileo National Telescope in La Palma to complete the coverage of the sky and observe all the stars in it.
ESPRESSO
The ESPRESSO spectrograph, installed on ESO’s Very Large Telescope (VLT), aims for a precision of several tens of cm/s. Its goal is to hunt for Earth’s “twins” in the habitable zone of Sun-type stars, which are still inaccessible in 2025.
SPIRou
SPIrou was installed in 2019 on the Canada-France-Hawaii Telescope (CFHT). It is a spectropolarimeter which takes high-precision radial velocity ? measurements in the near infrared and discovers sub-Neptune class planets around cold stars with temperate atmospheres.