The contribution of secondary eclipses as astrophysical false positives to exoplanet transit surveys

¹ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: alexandre.santerne@astro.up.pt
² Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
³ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁴ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Portugal

Received: 14 March 2013
Accepted: 13 July 2013

Abstract

We investigate the astrophysical false-positive configuration in exoplanet-transit surveys. It involves eclipsing binaries and giant planets that present only a secondary eclipse, as seen from the Earth. To test how an eclipsing binary configuration can mimic a planetary transit, we generated synthetic light curves of three examples of secondary-only eclipsing binary systems that we fit with a circular planetary model. Then, to evaluate its occurrence we modeled a population of binaries in double and triple systems based on binary statistics and occurrence. We find that 0.061% ± 0.017% of main-sequence binary stars are secondary-only eclipsing binaries that mimics a planetary transit candidate with a size down to the size of the Earth. We then evaluate the occurrence that an occulting-only giant planet can mimic an Earth-like planet or even a smaller one. We find that 0.009% ± 0.002% of stars harbor a giant planet that only presents the secondary transit. Occulting-only giant planets mimic planets that are smaller than the Earth, and they are in the scope of space missions like Kepler and PLATO. We estimate that up to 43.1 ± 5.6 Kepler objects of interest can be mimicked by this configuration of false positives, thereby re-evaluating the global false-positive rate of the Kepler mission from 9.4 ± 0.9% to 11.3 ± 1.1%. We note, however, that this new false-positive scenario occurs at relatively long orbital periods compared with the median period of Kepler candidates.