Transit timing effects due to an exomoon

TL;DR

This paper investigates transit timing effects caused by exomoons, introduces a new observable called transit duration variation (TDV), and shows how combining TTV and TDV can determine exomoon properties.

Contribution

It extends transit timing variation models to include eccentric orbits and predicts TDV as a new observable for exomoon detection.

Findings

TTV amplitude is proportional to exomoon mass and semi-major axis.

TDV amplitude depends on exomoon mass and inverse square root of semi-major axis.

Combining TTV and TDV allows separate estimation of exomoon mass and orbital distance.

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: 1) 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. 2) The TTV amplitude is proportional to M_S a_S where M_S is the exomoon mass and a_S is the semi-major axis of the moon's orbit. Therefore, M_S and a_S 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 provide two robust advantages: 1) The TDV amplitude is proportional to M_S a_S^{-1/2} and therefore the ratio of TTV and TDV allows for a separate determination of M_S and a_S. 2) TDV has a 90 degrees phase difference to the TTV signal, making it an excellent complementary technique.