Transit Duration and Timing Variations from Binary Planets

TL;DR

Binary planet systems can produce deceptive single-planet transit signals with large transit duration variations and timing variations, which can be identified with analytic models and simulations, aiding in their detection and differentiation from true planets.

Contribution

This work introduces an analytic approximation for transit duration bounds and demonstrates the detection of binary planets through their unique TDV and TTV signatures, supported by simulations and a Python tool.

Findings

Binary planets cause large amplitude TDVs of hours.

Binary planets produce detectable TTVs within the exomoon corridor.

Chimera transits can be distinguished from true planets using their unique signatures.

Abstract

Systems of two gravitationally bound exoplanets orbiting a common barycenter outside their physical radii ("binary planets") may result from tidal capture during planet-planet scattering. These objects are expected to form in tight orbits of just a few times their summed radii due to dynamical tides. As a result of their close proximity, their transits overlap heavily, leading to the deceptive illusion of a single planet of larger effective size, an effect compounded in the presence of noisy data and/or long integration times. We show that these illusory single-component transits, dubbed "chimera transits", exhibit large-amplitude Transit Duration Variation (TDV) effects on the order of hours, as well as smaller Transit Timing Variations (TTVs). We compute an analytic approximation for the transit duration upper bound, assuming binary planets with low impact parameter and orbits coplanar with the stellarcentric orbit. We verify the accuracy of our expressions against dynamical simulations of binary Jupiters using the luna algorithm, and provide a Python code for numerical calculations of the TDV signal in binary planet systems (github.com/joheenc/binary-planet-transits). Additionally, chimera transits from binary planets exhibit TTVs of detectable amplitude and high frequency, falling within the recently identified exomoon corridor. Due to their anomalous shapes, depths, and durations, such objects may be flagged as false positives, but could be clearly surveyed for in existing archives.