Activity time series of old stars from late F to early K VI. Exoplanet mass characterisation and detectability in radial velocity

Abstract

Stellar variability impacts radial velocities at various timescales and therefore the detectability of exoplanets and the mass determination based on this technique. It is necessary to implement systematic studies, to delineate the current limitations of RV techniques to detect Earth-like planets. This paper aims are to investigate whether the targeted 10% mass uncertainty from RV follow-up of transits detected by PLATO can be reached, and to analyse and quantify Earth-like planet detectability for various spectral types. We implemented blind tests based on a large data set of realistic synthetic time series reproducing different phenomena leading to stellar variability such as complex magnetic activity patterns as well as flows, covering F6-K4 stars and a wide range of activity levels. The 10% mass uncertainty for a 1 MEarth in the habitable zone of a G2 star cannot be reached, even with an improved version of a usual correction of stellar activity and even for long-duration (ten years) well-sampled observations. This level can be reached for masses above 3 MEarth or for K4 stars alone. We quantify the maximum dispersion of the RV residuals needed to reach this 10% level, assuming the correction method and models do not affect the planetary signal. Several other methods were tested and do not allow a significantly improvement of this limited performance. Similarly, such low-mass planets in the habitable zone cannot be detected with a similar correction: blind tests lead to very low detection rates for 1 MEarth and a very high level of false positives. Very significant and new improvements with respect to methods based on activity indicators to correct for stellar activity must be devised at all timescales to reach the objective of 10% uncertainty on the mass or to detect such planets in RV. Methods based on the correlation with activity indicators are unlikely to be sufficient.