Transit Origami: A Method to Coherently Fold Exomoon Transits in Time Series Photometry

TL;DR

The paper introduces a novel method called Transit Origami for coherently folding exomoon transits in photometric time series, enabling more effective detection of exomoons in systems with TTVs.

Contribution

A new double-folding technique for exomoon detection that uses TTVs and a simple parameter search, demonstrated on simulated and real Kepler data.

Findings

Method successfully applied to simulated light curves.

Upper limits on exomoon size set for Kepler-973b.

Technique can identify or exclude exomoons with specific mass ratios.

Abstract

One of the simplest ways to identify an exoplanetary transit is to phase fold a photometric time series upon a trial period - leading to a coherent stack when using the correct value. Such phase-folded transits have become a standard data visualisation in modern transit discovery papers. There is no analogous folding mechanism for exomoons, which would have to represent some kind of double-fold; once for the planet and then another for the moon. Folding with the planet term only, a moon imparts a small decrease in the surrounding out-of-transit averaged intensity, but its incoherent nature makes it far less convincing than the crisp stacks familiar to exoplanet hunters. Here, a new approach is introduced that can be used to achieve the transit origami needed to double fold an exomoon, in the case where a planet exhibits TTVs. This double fold has just one unknown parameter, the satellite-to-planet mass ratio, and thus a simple one-dimensional grid search can be used to rapidly identify power associated with candidate exomoons. The technique is demonstrated on simulated light curves, exploring the breakdown limits of close-in and/or inclined satellites. As an example, the method is deployed on Kepler-973b, a warm mini-Neptune exhibiting an 8 minute TTV, where the possibility that the TTVs are caused by a single exomoon is broadly excluded, with upper limits probing down to a Ganymede-sized moon.