The HARPS search for southern extra-solar planets XIX. Characterization and dynamics of the GJ876 planetary system

TL;DR

This study uses precise radial-velocity data to characterize the GJ876 planetary system, confirming known planets, constraining their orbits and masses, and exploring the potential for additional planets in resonance.

Contribution

It provides detailed dynamical modeling of the GJ876 system, accurately determining orbital inclinations, masses, and resonance stability using only radial velocity data.

Findings

Confirmed the existence of planet d with a 1.94-day orbit and estimated its mass.

Determined the true masses and inclinations of planets b and c, revealing near coplanarity.

Suggested the possibility of a stable fourth planet in a 4:1 resonance with planet b.

Abstract

Precise radial-velocity measurements for data acquired with the HARPS spectrograph infer that three planets orbit the M4 dwarf star GJ876. In particular, we confirm the existence of planet "d", which orbits every 1.93785 days. We find that its orbit may have significant eccentricity (e=0.14), and deduce a more accurate estimate of its minimum mass of 6.3 Earth masses. Dynamical modeling of the HARPS measurements combined with literature velocities from the Keck Observatory strongly constrain the orbital inclinations of the "b" and "c" planets. We find that i_b = 48.9 degrees and i_c = 48.1 degrees, which infers the true planet masses of M_b = 2.64 Jupiter masses and M_c = 0.83 Jupiter masses, respectively. Radial velocities alone, in this favorable case, can therefore fully determine the orbital architecture of a multi-planet system, without the input from astrometry or transits. The orbits of the two giant planets are nearly coplanar, and their 2:1 mean motion resonance ensures stability over at least 5 Gyr. The libration amplitude is smaller than 2 degrees, suggesting that it was damped by some dissipative process during planet formation. The system has space for a stable fourth planet in a 4:1 mean motion resonance with planet "b", with a period around 15 days. The radial velocity measurements constrain the mass of this possible additional planet to be at most that of the Earth.