Global dynamics and architecture of the Kepler-444 system

Abstract

Context. S-type planets, which orbit one component of multiple-star systems, place strong constraints on planet formation and evolution models. A notable case study is Kepler-444, a triple-star system whose primary is orbited by five planets smaller than Venus in a compact configuration, and for which the stellar binary companion revolves around the primary on a highly eccentric orbit. Aims. Several open questions remain about the formation and evolution of Kepler-444. Having access to the most precise up-to-date masses and orbital parameters is highly valuable when tackling those questions. We provide the first full dynamical exploration of this system, with the goal being to refine those parameters. Methods. We apply orbital stability arguments to refine the system parameters on models with and without the stellar binary companion in order to understand the origin of the dynamical constraints. This approach makes use of the numerical analysis of fundamental frequencies fast chaos indicator. We also explore potential two- and three-planet mean-motion resonances (MMRs) in the system. Prior to investigating the dynamics of a model that includes the binary companion, we update its orbital parameters and mass using new observational constraints from both HIRES radial velocity and Gaia astrometric data, as well as archival imaging of the system. Results. The planetary system does not appear in any of the low-order two- or three-planet MMRs. We provide the most precise up-to-date dynamical parameters for the planets and the stellar binary companion. The orbit of the latter is constrained by the new observations, and also by the stability analysis. This update further challenges the planets formation processes. We also test the dynamical plausibility of a sixth planet in the system, following hints found in HST data. We find that this putative planet could exist over a broad range of masses, and with an orbital period of between roughly 12 and 20 days. Conclusions. We note the overall good agreement of the system with short-term orbital stability. This suggests that a diverse range of planetary system architectures could be found in multiple-star systems, potentially challenging the planet formation models further.