Planetary transit timing variations induced by stellar binarity

The light travel time effect

¹ Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstr., 85741 Garching b. Muenchen, Germany
² Universitaetssternwarte Muenchen, Scheiner Str. 1, 81679 Muenchen, Germany e-mail: montalto@usm.uni-muenchen.de

Received: 23 March 2010
Accepted: 15 June 2010

Abstract

Context. Since the discovery of the first transiting extrasolar planet, transit timing has been recognized as a powerful method to discover and characterize additional planets in these systems. However, the gravitational influence of additional planets is not the only expected source of transit timing variations.

Aims. In this work, we derive the expected detection frequency of stellar companions of hot-jupiter transiting planets host-stars, detectable by means of transit timing analysis. Since roughly half of the stars in the solar neighborhood belong to binary or multiple stellar systems, the same fraction of binary systems may be expected to be present among transiting planet-host stars, unless planet formation is significantly influenced by the presence of a stellar companion. Transit searches are less affected by the selection biases against long-period binaries that plague radial velocity surveys.

Methods. We considered the frequency and the period, mass ratio and eccentricity distributions of known binary systems in the solar neighborhood, and estimated the fraction of transiting planet-hosts expected to show detectable transit timing variations due to the light travel time effect in a binary stellar system, in function of the time since the discovery of the planet.

Results. If the frequency of binaries among hot-jupiter planet host stars is the same as determined in the solar neighborhood, after 5 years since the discovery of a sample of transiting planets 1.0% ± 0.2% of them have a probability > 99% to present transit timing variations > 50 s induced by stellar binarity, and 2.8% ± 0.3% after 10 years, if the planetary and binary orbits are coplanar. Considering the case of random inclinations the probabilities are 0.6% ± 0.1% and 1.7% ± 0.2% after 5 and 10 years respectively. Our estimates can be considered conservative lower limits, since we have taken into account only binaries with periods P > 5 × 10³ days (a 6 AU). Our simulations indicate that transit timing variations due to the light travel time effect will allow us to discover stellar companions up to maximum separations equal to a ~ 36 AU after 5 years since the discovery of the planet (a ~ 75 AU after 10 years).

Conclusions. Comparing the results of the observations with the above predictions allows to understand if stellar companions at critical separations (< 100 AU) are favoring or hindering the formation of hot-jupiter planets. Comparing the results of transit timing detections with those obtained by other complementary methods results in a more complete determination of stellar multiplicity around transiting planet-host stars. Moreover, transit timing analysis allows us to probe stellar multiplicity at critical separations (< 100 AU) around stars located in different regions of the Galaxy, and not just in the solar neighborhood.