Ground-based radial velocity measurements of candidate planets detected by PLATO will be carried out to confirm or reject the planet detection and to determine the planet mass and the complete orbital parameters.
As a planet orbits its host, the star moves back and forth along the line-of-sight due to the gravitational influence of the planet, and the Doppler effect will thus alternately blue- and red-shift the spectral signatures of the star. By measuring a large number of spectral absorption lines (typically several thousand per star), it is possible to determine the position of the line centre to a precision of a few parts in a thousand, consequently providing a velocity value that nowadays can be measured with a precision better than 1 m/s. Assuming enough observations have been made, this results in a usable radial velocity curve with the same precision. The presence of an influencing body can be inferred and its Mpsin idetermined (whereiis the inclination of the orbital plane). Today, the Radial Velocity method enables the discovery of small planets (a few Earth masses) close to low-mass stars. The detection of an Earth orbiting a true Sun-analogue requires an increase of an order-of-magnitude in precision to be achieved by future instruments. With the ESPRESSO instrument to be installed on the VLT in 2017, we will reach 10 cm/s in 15 min for a V= 8 star, or 20 cm/s in 1 hour for a V= 11 star. These estimates demonstrate the need for bright stars and large collecting areas when considering the radial velocity follow- up of very low-mass planet candidates detected by PLATO.