  Context. Detecting regular dips in the light curve of a star is an easy way
to detect the presence of an orbiting planet. COROT is a Franco-European
mission launched at the end of 2006, and one of its main objectives is to
detect planetary systems using the transit method. Aims. In this paper, we
present a new method for transit detection and determine the smallest detected
planetary radius, assuming a parent star like the Sun. Methods. We simulated
light curves with Poisson noise and stellar variability, for which data from
the VIRGO/PMO6 instrument on board SoHO were used. Transits were simulated
using the UTM software. Light curves were denoised by the mean of a low-pass
and a high-pass filter. The detection of periodic transits works on light
curves folded at several trial periods with the particularity that no rebinning
is performed after the folding. The best fit was obtain when all transits are
overlayed, i.e when the data are folded at the right period. Results. Assuming
a single data set lasting 150d, transits from a planet with a radius down to 2
Rearth can be detected. The efficiency depends neither on the transit duration
nor on the number of transits observed. Furthermore we simulated transits with
periods close to 150d in data sets containing three observations of 150d,
separated by regular gaps with the same length. Again, planets with a radius
down to 2 Rearth can be detected. Conclusions. Within the given range of
parameters, the detection efficiency depends slightly on the apparent magnitude
of the star but neither on the transit duration nor the number of transits.
Furthermore, multiple observations might represent a solution for the COROT
mission for detecting small planets when the orbital period is much longer than
the duration of a single observation.
