The exomoon corridor: Half of all exomoons exhibit TTV frequencies within a narrow window due to aliasing

Abstract

ABSTRACT Exomoons are expected to produce potentially detectable transit timing variations (TTVs) upon their parent planet. Unfortunately, distinguishing moon-induced TTVs from other sources, in particular planet–planet interactions, has severely impeded its usefulness as a tool for identifying exomoon candidates. A key feature of exomoon TTVs is that they will always be undersampled, due to the simple fact that we can only observe the TTVs once per transit/planetary period. We show that it is possible to analytically express the aliased TTV periodicity as a function of planet and moon period. Further, we show that inverting an aliased TTV period back to a true moon period is fraught with hundreds of harmonic modes. However, a unique aspect of these TTV aliases is that they are predicted to occur at consistently short periods, irrespective of what model one assumes for the underlying moon population. Specifically, 50 per cent of all exomoons are expected to induce TTVs with a period between 2 to 4 cycles, a range that planet–planet interactions rarely manifest at. This provides an exciting and simple tool for quickly identifying exomoons candidates and brings the TTV method back to the fore as an exomoon hunting strategy. Applying this method to the candidate, Kepler-1625b i, reveals that its TTV periodicity centres around the median period expected for exomoons.