Anticipating Tathreeine: searching for stable planetary orbits around a compact hierarchical triple

Anticipating Tathreeine: searching for stable planetary orbits around a compact hierarchical triple

Around a third of the stars in our Galaxy are binary or multiple systems, with triples possibly accounting for up to a tenth. While planets have been found in S-type orbits around the single or binary components of triples, only two P-type orbits have been found, both being circumbinary. A planet in a circumtriple orbit has not yet been found. The anticipated discovery of such exoplanet orbits motivated this study.

Hierarchical triple systems that are compact enough to possibly harbour exoplanets in P-type orbits form a small minority of hierarchical triples, with only 7 of the catalogued 724 systems being suitable. From these we selected and analysed the recently-discovered HD 181068 or Trinity system, using N-body simulations to analyse the stability of the stellar system and possible planetary orbits around it.

Mercury6 was selected as the orbital integration package; it was tested on simple systems and used to replicate a previously published study on HW Virginis, where it performed well. We then generated a stability map for the stellar system, which showed a stability zone for the outer body which differed from the published orbital parameters, suggesting an eccentricity much higher than zero.

Using this geometry we then calculated the limits of the habitability zone and extracted the probable parameters for planetary systems from various exoplanet databases. A comprehensive search using 33 810 test particles failed to find any stable orbits. Simplifying the model by consolidating the central binary into a single body resulted in stable but highly eccentric orbits.

We concluded that Mercury6, which was originally designed for configurations comprising one central dominant body, probably became inappropriate when adding more bodies to a compact triple system of stars of almost equal mass, possibly in the coordinate system used for its test particlesparticles, i.e. in the splitting of the Hamiltonian.