Detecting transits from Earth-sized planets around Sun-like stars

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 Universal Transit Modeller 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 150 d, transits from a planet with a radius down to 2 R(circle plus) 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 150 d in data sets containing three observations of 150 d, separated by regular gaps with the same length. Again, planets with a radius down to 2 R(circle plus) 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.