Planetary detection limits taking into account stellar noise. I. Observational strategies to reduce stellar oscillation and granulation effects

The radial velocity signature of stellar noise is small, around the meter-per-second, but already too much for the detection of Earth mass planets in habitable zones. In this paper, we address the important role played by observational strategies in averaging out the radial velocity signature of stellar noise. We also derive the planetary mass detection limits expected in presence of stellar noise. We start with HARPS asteroseismology measurements for 4 stars (beta Hyi, alpha Cen A, mu Ara and tau Ceti) available in the ESO archive plus very precise measurements of alpha Cen B. This sample covers different spectral types, from G2 to K1 and different evolutionary stage, from subgiant to dwarf stars. Since the span of our data ranges between 5 to 8 days, we will have access to oscillation modes and granulation phenomena, without important contribution of activity noise which is present at larger time scales. For those 5 stars, we generate synthetic radial velocity measurements after fitting corresponding models of stellar noise in Fourier space. These measurements allows us to study the radial velocity variation due to stellar noise for different observational strategies as well as the corresponding planetary mass detection limits. Applying 3 measurements per night of 10 minutes exposure each, 2 hours apart, seems to average out most efficiently the stellar noise considered. For quiet K1V stars as alpha Cen B, such a strategy allows us to detect planets of ~3 times the mass of Earth with an orbital period of 200 days, corresponding to the habitable zone of the star. Since activity is not yet included in our simulation, these detection limits correspond to a case, which exist, where the host star has few magnetic features. In this case stellar noise is dominated by oscillation modes and granulation phenomena.