Transit timing effects due to an exomoon

Abstract

As the number of known exoplanets continues to grow, the question as to whether such bodies harbour satellite systems has become one of increasing interest. In this paper, we explore the transit timing effects that should be detectable due to an exomoon and predict a new observable. We first consider transit time variation (TTV), where we update the model to include the effects of orbital eccentricity. We draw two key conclusions. In order to maintain Hill stability, the orbital frequency of the exomoon will always be higher than the sampling frequency. Therefore, the period of the exomoon cannot be reliably determined from TTV, only a set of harmonic frequencies. The TTV amplitude is ∝MSaS where MS is the exomoon mass and aS is the semimajor axis of the moon's orbit. Therefore, MS and aS cannot be separately determined. We go on to predict a new observable due to exomoons – transit duration variation (TDV). We derive the TDV amplitude and conclude that its amplitude is not only detectable, but the TDV signal will also provide two robust advantages. The TDV amplitude is ∝MSa−1/2S and therefore the ratio of TDV to TTV allows for MS and aS to be separately determined. TDV has a π/2 phase difference to the TTV signal, making it an excellent complementary technique.