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The detection of small mass planets with the radial-velocity technique is nowconfronted with the interference of stellar noise. HARPS can now reach aprecision below the meter-per-second, which corresponds to the amplitudes ofdifferent stellar perturbations, such as oscillation, granulation, andactivity. Solar spot groups induced by activity produce a radial-velocity noiseof a few meter-per-second. The aim of this paper is to simulate this activityand calculate detection limits according to different observational strategies.Based on Sun observations, we reproduce the evolution of spot groups on thesurface of a rotating star. We then calculate the radial-velocity effectinduced by these spot groups as a function of time. Taking into accountoscillation, granulation, activity, and a HARPS instrumental error of 80 cm/s,we simulate the effect of different observational strategies in order toefficiently reduce all sources of noise. Applying three measurements per nightof 10 minutes every three days, 10 nights a month seems the best testedstrategy. 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 Earth massesin the habitable zone of a K1V dwarf. Using Bern's model of planetaryformation, we estimate that for the same range of activity level, 15 to 35 % ofthe planets between 1 and 5 Earth masses and with a period between 100 and 200days should be found with HARPS. A comparison between the performance of HARPSand ESPRESSO is also emphasized by our simulations. Using the same optimizedstrategy, ESPRESSO could find 1.3 Earth mass planets in the habitable zone ofearly-K dwarfs. In addition, 80 % of planets with mass between 1 and 5 Earthmasses and with a period between 100 and 200 days could be detected.

Planetary detection limits taking into account stellar noise. II. Effect of stellar spot groups on radial-velocities