  We explore the feasibility of detecting Earth analogs around Sun-like stars
using the radial velocity method by investigating one of the largest radial
velocities datasets for the one of the most stable radial-velocity stars
HD20794. We proceed by disentangling the Keplerian signals from correlated
noise and activity-induced variability. We diagnose the noise using the
differences between radial velocities measured at different wavelength ranges,
so-called "differential radial velocities". We apply this method to the radial
velocities measured by HARPS, and identify four signals at 18, 89, 147 and 330
d. The two signals at periods of 18 and 89 d are previously reported and are
better quantified in this work. The signal at a period of about 147 d is
reported for the first time, and corresponds to a super-Earth with a minimum
mass of 4.59 Earth mass located 0.51 AU from HD20794. We also find a
significant signal at a period of about 330 d corresponding to a super-Earth or
Neptune in the habitable zone. Since this signal is close to the annual
sampling period and significant periodogram power in some noise proxies are
found close to this signal, further observations and analyses are required to
confirm it. The analyses of the eccentricity and consistency of signals provide
weak evidence for the existence of the previously reported 43 d signal and a
new signal at a period of about 11.9 d with a semi amplitude of 0.4 m/s. We
find that the detection of a number of signals with radial velocity variations
around 0.5\,m/s likely caused by low mass planet candidates demonstrates the
important role of noise modeling in searching for Earth analogs.
