The HARPS search for southern extra-solar planets: XIX. Characterization and dynamics of the GJ 876 planetary system

• Published in: Astronomy and astrophysics (Berlin) 2010-02, Vol.511, p.A21
• Main Authors: CORREIA, A. C. M, COUETDIC, J, PEPE, F, PERRIER, C, QUELOZ, D, UDRY, S, LASKAR, J, BONFILS, X, MAYOR, M, BERTAUX, J.-L, BOUCHY, F, DELFOSSE, X, FORVEILLE, T, LOVIS, C
• Format: Article
• Language: English
• Subjects: Astronomy; Astrophysics; Earth, ocean, space; Exact sciences and technology; Physics
• ISSN: 0004-6361; 1432-0746; 1432-0756
• DOI: 10.1051/0004-6361/200912700

Precise radial-velocity measurements for data acquired with the HARPS spectrograph infer that three planets orbit the M 4 dwarf star GJ 876. 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 M⊕. 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 ib = 48.9◦ ± 1.0◦ and ic = 48.1◦ ± 2.1◦, which infers the true planet masses of Mb = 2.64 ± 0.04 MJup and Mc = 0.83 ± 0.03 MJup, 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◦, 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.