Predictable patterns in planetary transit timing variations and transit duration variations due to exomoons

TL;DR

This paper introduces analytical and numerical methods to identify and characterize exomoons through transit timing and duration variations, revealing predictable patterns for single and multiple moons, and assesses their detectability with current and future telescopes.

Contribution

It provides new analytical patterns for detecting exomoons in TTV-TDV diagrams and demonstrates their potential detectability using existing and upcoming observational data.

Findings

Single exomoons produce predictable TTV-TDV patterns that approach an ellipse with more data.

Multiple moons in MMR create complex loops in TTV-TDV diagrams, with the number of loops indicating the MMR order.

Detectability of large exomoons is feasible with Kepler data; smaller moons require larger telescopes or brighter stars.

Abstract

We present new ways to identify single and multiple moons around extrasolar planets using planetary transit timing variations (TTVs) and transit duration variations (TDVs). For planets with one moon, measurements from successive transits exhibit a hitherto undescribed pattern in the TTV-TDV diagram, originating from the stroboscopic sampling of the planet's orbit around the planet-moon barycenter. This pattern is fully determined and analytically predictable after three consecutive transits. The more measurements become available, the more the TTV-TDV diagram approaches an ellipse. For planets with multiple moons in orbital mean motion resonance (MMR), like the Galilean moons, the pattern is much more complex and addressed numerically in this report. Exomoons in MMR can also form closed, predictable TTV-TDV figures if the drift of the moons' pericenters is sufficiently slow. We find that MMR exomoons produce loops in the TTV-TDV diagram and that the number of these loops is equal to the order of the MMR, or the largest integer in the MMR ratio. We use a Bayesian model and Monte Carlo simulations to test the discoverability of exomoons using TTV-TDV diagrams with current and near-future technology. In a blind test, two of us (BP, DA) successfully retrieved a large moon from simulated TTV-TDV by co-authors MH and RH, which resembled data from a known Kepler planet candidate. Single exomoons with a 10% moon-to-planet mass ratio, like to Pluto-Charon binary, can be detectable in the archival data of the Kepler primary mission. Multi-exomoon systems, however, require either larger telescopes or brighter target stars. Complementary detection methods invoking a moon's own photometric transit or its orbital sampling effect can be used for validation or falsification. A combination of TESS, CHEOPS, and PLATO data would offer a compelling opportunity for exomoon discoveries around bright stars.