13 - From hot Jupiters to hot Neptunes … and below

Summary

Recent improvements in radial velocity precision

Since the first discovery of an extrasolar planet around a Solar-type star ten years ago (Mayor and Queloz, 1995), research in this field has been very productive and has led to the detection of more than 140 exoplanets. The vast majority of these discoveries has been made with the radial-velocity (RV) technique, i.e. the precise measurement of the RV wobble that a planet induces in its parent star due to its orbital movement. A major effort to improve the accuracy of the RV measurements has been undertaken by several groups, since this is absolutely necessary to detect the RV signatures of giant planets, in the range 1–100 ms⁻¹. Two main techniques were developed: one using a ThAr calibration simultaneously with each observation (Baranne et al., 1996) to track instrumental drifts, and one using an iodine absorption cell, superimposing a reference spectrum on the stellar spectrum (Butler et al., 1996). Both techniques have been able to deliver RV precision at the level of ∼3 ms⁻¹, opening the way to the discovery of many planetary systems.

Over the past decade, the exoplanet group at Geneva Observatory has been operating two high-resolution spectrographs able to achieve high RV precision, namely the ELODIE instrument mounted on the 1.93 m telescope at Observatoire de Haute-Provence (France), and the CORALIE instrument installed on the Swiss 1.2 m telescope at La Silla Observatory (Chile). Both ELODIE and CORALIE are high-resolution (R = 50 000), fiber-fed echelle spectrographs.