  We consider an alternative to the cross-correlation function (CCF), that uses
the autocor- relation function (ACF), to measure in spectra the reflected light
of the stars by their planetary companion. The objective of this work is to
assess and analyse the efficiency of the ACF in detecting planetary signals by
a detection of reflected light. To do so, we first compare the ACF and the CCF
using artificial spectra containing a planetary signal. We then use the ACF to
analyse spectra of the 51 Peg + 51 Peg b system and compare our results with
those obtained by Martins et al. (2015) who previously analysed the same system
using the CCF. The functionalities of the ACF and the way it is implemented are
similar to that of the CCF. One of the main advantages of the ACF is the fact
that, unlike the CCF, no weighted binary mask is required. This makes the ACF
easier to use. The results related to simulated spectra showed that the ACF
allowed us to decrease the boundary condition threshold for the use of spectra
compared to the CCF so that more data could be used to recover the planetary
signal. For the 51 Peg b planet, we achieved a detection significance of 5.52
{\sigma}noise with the ACF compared to 3.70 {\sigma}noise with the CCF (Martins
et al. 2015). We conclude that the ACF has the potential to become a prominent
technique in detecting exoplanets considering its efficiency, ease of use and
rapid execution time.
