  Different stellar phenomena affect radial velocities (RVs), causing
variations large enough to make it difficult to identify planet signals from
the stellar variability. RV variations caused by stellar oscillations and
granulation can be reduced through some methods, but the impact of rotationally
modulated magnetic activity on RV, due to stellar active regions is harder to
correct. New instrumentation promises an improvement in precision of one order
of magnitude, from about 1 m/s, to about 10 cm/s. In this context, we report
our first results from 24 spectroscopic ESPRESSO/VLT observations of the solar
twin star HIP 11915, spread over 60 nights. We used a Gaussian Process approach
and found for HIP 11915 a RV residual RMS scatter of about 20 cm s$^{-1}$,
representing an upper limit for the performance of ESPRESSO.
