Orbital structure of the GJ876 extrasolar planetary system, based on the latest Keck and HARPS radial velocity data

TL;DR

This paper analyzes the orbital configuration of the GJ876 exoplanet system using advanced data fitting techniques that account for correlated noise, revealing more accurate orbital parameters and potential non-coplanarity.

Contribution

It introduces a fast method for fitting perturbed orbital configurations and incorporates red noise modeling, improving the accuracy of orbital parameter estimation.

Findings

Red noise significantly affects orbital parameter estimates.

The non-zero eccentricity of planet d may be due to noise misinterpretation.

The outer planet's eccentricity is poorly constrained, bounded by ~0.2.

Abstract

We use full available array of radial velocity data, including recently published HARPS and Keck observatory sets, to characterize the orbital configuration of the planetary system orbiting GJ876. First, we propose and describe in detail a fast method to fit perturbed orbital configuration, based on the integration of the sensitivity equations inferred by the equations of the original $N$-body problem. Further, we find that it is unsatisfactory to treat the available radial velocity data for GJ876 in the traditional white noise model, because the actual noise appears autocorrelated (and demonstrates non-white frequency spectrum). The time scale of this correlation is about a few days, and the contribution of the correlated noise is about 2 m/s (i.e., similar to the level of internal errors in the Keck data). We propose a variation of the maximum-likelihood algorithm to estimate the orbital configuration of the system, taking into account the red noise effects. We show, in particular, that the non-zero orbital eccentricity of the innermost planet \emph{d}, obtained in previous studies, is likely a result of misinterpreted red noise in the data. In addition to offsets in some orbital parameters, the red noise also makes the fit uncertainties systematically underestimated (while they are treated in the traditional white noise model). Also, we show that the orbital eccentricity of the outermost planet is actually ill-determined, although bounded by $\sim 0.2$. Finally, we investigate possible orbital non-coplanarity of the system, and limit the mutual inclination between the planets \emph{b} and \emph{c} orbits by $5^\circ-15^\circ$, depending on the angular position of the mutual orbital nodes.