New Constraints on Gliese 876—Exemplar of Mean-motion 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 meanmotion 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.

Chaotic Excitation and Tidal Damping in the GJ 876 System

“…Additional ambiguity in our calculations arises from the fact that the eccentricity of planet d itself is relatively poorly constrained by the radial velocity data. To this end, the recent analyses of Trifonov et al (2018) and Millholland et al (2018) favor a somewhat lower eccentricity than that derived by Nelson et al (2016) i.e., e d ≈0.06-0.08. This revision implies a considerably shorter circularization timescale of τ e ∼20 Myr, which in turn translates to a lower estimate of the tidal quality factor (Q∼10 4 for our adopted parameters).…”

“…Additionally, the short-period super-Earth GJ 876 d exhibits an intriguingly moderate eccentricity of e d =0.05-0.15 (Correia et al 2010;Nelson et al 2016;Millholland et al 2018;Trifonov et al 2018). As we show below, any finite eccentricity is at odds with the standard model of tidal dissipation.…”

The CARMENES search for exoplanets around M dwarfs

“…This is expected since constraining the mutual inclinations from RV data requires strong interactions between the planets over the temporal baseline of the observations. The best example is the other massive pair around an M dwarf; for the GJ 876 system, Rivera et al (2010), Nelson et al (2016), and Millholland et al (2018) successfully measured the orbital inclinations from RV data. For GJ 876, the timescales of the perturbations are shorter than the observational data time span, and thus it is possible to constrain the mutual inclination.…”

Dynamics and Formation of the Near-resonant K2-24 System: Insights from Transit-timing Variations and Radial Velocities

“…Typically, RVs constrain M i sin p , the planet mass modulo (an unknown inclination angle). For high signal-to-noise data sets, deviations from sinusoidal RV curves can reveal orbital eccentricities, and for a few exceptional systems, nonKeplerian orbital dynamics have been observed (see, e.g., GJ876; Rivera et al 2010;Nelson et al 2016;Millholland et al 2018). For transiting systems, the i sin ambiguity is negligible and RVs constrain planet mass and bulk composition directly.…”