New Constraints on Gliese 876 -- Exemplar of Mean-Motion Resonance

TL;DR

This study reanalyzed the Gliese 876 exoplanetary system using six years of new radial velocity data, revealing it is deeply in resonance and likely in a low energy, quasi-regular state, with implications for its formation history.

Contribution

It provides updated system parameters using advanced modeling techniques, highlighting the importance of correlated noise models in dynamical characterization.

Findings

System is deeper in resonance than previously thought

No evidence found for an additional planet in the resonant chain

System may be in a low energy, quasi-regular double apsidal resonance

Abstract

Gliese 876 harbors one of the most dynamically rich and well-studied exoplanetary systems. The nearby M4V dwarf hosts four known planets, the outer three of which are trapped in a Laplace mean-motion resonance. A thorough characterization of the complex resonant perturbations exhibited by the orbiting planets, and the chaotic dynamics therein, is key to a complete picture of the system's formation and evolutionary history. Here we present a reanalysis of the system using six years of new radial velocity (RV) data from four instruments. This new data augments and more than doubles the size of the decades-long collection of existing velocity measurements. We provide updated estimates of the system parameters by employing a computationally efficient Wisdom-Holman N-body symplectic integrator, coupled with a Gaussian Process (GP) regression model to account for correlated stellar noise. Experiments with synthetic RV data show that the dynamical characterization of the system can differ depending on whether a white noise or correlated noise model is adopted. Despite there being a region of stability for an additional planet in the resonant chain, we find no evidence for one. Our new parameter estimates place the system even deeper into resonance than previously thought and suggest that the system might be in a low energy, quasi-regular double apsidal corotation resonance. This result and others will be used in a subsequent study on the primordial migration processes responsible for the formation of the resonant chain.