Planetary detection limits taking into account stellar noise

Abstract

Context. The detection of small mass planets with the radial-velocity technique is now confronted with the interference of stellar noise. HARPS can now reach a precision below the meter-per-second, which corresponds to the amplitudes of different stellar perturbations, such as oscillation, granulation, and activity. Aims: Solar spot groups induced by activity produce a radial-velocity noise of a few meter-per-second. The aim of this paper is to simulate this activity and calculate detection limits according to different observational strategies. Methods: Based on Sun observations, we reproduce the evolution of spot groups on the surface of a rotating star. We then calculate the radial-velocity effect induced by these spot groups as a function of time. Taking into account oscillation, granulation, activity, and a HARPS instrumental error of 80 cm s-1, we simulate the effect of different observational strategies in order to efficiently reduce all sources of noise. Results: Applying three measurements per night of 10 min every three days, 10 nights a month seems the best tested strategy. Depending on the level of activity considered, from log R'HK = - 5 to - 4.75, this strategy would allow us to find planets of 2.5 to 3.5 M⊕ in the habitable zone of a K1V dwarf. Using Bern's model of planetary formation, we estimate that for the same range of activity level, 15 to 35% of the planets between 1 and 5 M⊕ and with a period between 100 and 200 days should be found with HARPS. A comparison between the performance of HARPS and ESPRESSO is also emphasized by our simulations. Using the same optimized strategy, ESPRESSO could find 1.3 M⊕ planets in the habitable zone of K dwarfs. In addition, 80% of planets with mass between 1 and 5 M⊕ and with a period between 100 and 200 days could be detected.