Photospheric stellar activity ( i.e .
dark spots or bright plages ) might be an important source of noise and confusion in stellar radial-velocity ( RV ) measurements .
Radial-velocimetry planet search surveys as well as follow-up of photometric transit surveys require a deeper understanding and characterization of the effects of stellar activities to differentiate them from planetary signals .
We simulate dark spots on a rotating stellar photosphere .
The variations in the photometry , RV , and spectral line shapes are characterized and analyzed according to the stellar inclination , the latitude , and the number of spots .
We show that the anti-correlation between RV and bisector span , known to be a signature of activity , requires a good sampling to be resolved when there are several spots on the photosphere .
The Lomb-Scargle periodograms of the RV variations induced by activity present power at the rotational period P _ { rot } of the star and its two first harmonics P _ { rot } / 2 and P _ { rot } / 3 .
Three adjusted sinusoids fixed at the fundamental period and its two-first harmonics allow us to remove about 90 % of the RV jitter amplitude .
We apply and validate our approach on four known active planet-host stars : HD 189733 , GJ 674 , CoRoT-7 , and \iota Hor .
We succeed in fitting simultaneously activity and planetary signals on GJ674 and CoRoT-7 .
This simultaneous modeling of the activity and planetary parameters leads to slightly higher masses of CoRoT-7b and c of respectively , 5.7 \pm 2.5 M _ { Earth } and 13.1 \pm 4.1 M _ { Earth } .
The larger uncertainties properly take into account the stellar active jitter .
We exclude short-period low-mass exoplanets around \iota Hor .
For data with realistic time-sampling and white Gaussian noise , we use simulations to show that our approach is effective in distinguishing reflex-motion due to a planetary companion and stellar-activity-induced RV variations provided that 1 ) the planetary orbital period is not close to that of the stellar rotation or one of its two first harmonics , 2 ) the semi-amplitude of the planet exceeds \sim 30 % of the semi-amplitude of the active signal , 3 ) the rotational period of the star is accurately known , and 4 ) the data cover more than one stellar rotational period .