Planet transit: The dimming of stellar flux by an orbiting planet passing through the line-of-sight to Earth allows us to measure the size of the planet with respect to the star, to get the size of the orbit, and the inclination of the orbital plane with respect to line-of-sight.
PLATO will achieve its science objectives by performing:
- Planet detection and radius determination from photometric transits;
- Determination of planet masses from ground-based radial velocity follow-up;
- Determination of accurate stellar masses, radii, and ages from asteroseismology;
- Identification of bright exoplanet hosts that will be targets for atmospheric spectroscopy.
Stellar radial velocity: periodic Doppler shifts of spectral features allows us measuring the wobbling of the star around the center of mass of the planetary systems. In this way the planet mass relative to the stellar mass times sin i, with i the inclination of the orbital plane with respect to the line-of-sight, are measured
Planets will be detected by the PLATO satellite, whose data will enable the measurements of the planets’ size, the characterisation of the orbits, and – through asterosismology – the characterisation of stellar mass and ages. Radial velocity data, collected from the ground for the discovered planets, will render it possible the measurement of planets’ mass and the assessment of the rocky vs. gases nature of the detected planets.
All these information will be used to select the most interesting planets for atmospheric spectroscopy.
References
PLATO – Revealing habitable worlds around solar-like stars
Definition Study Report, ESA-SCI(2017)1, April 2017
platomission.com 
