Gliese 876 - Characterization and Dynamics

Masses of gas giants revisited, as well as that for innermost Super-Earth (6.26 Me rather 5.7). Plus it's likely the presence of a fourth planet between d and c, within 0.083 AUs. No clear signal though, but it could be "hidden".

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

Quote :

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.

Emphasis mine.

Well this is definitely a neat result, similar in a way to the determination of the masses and orbital inclinations of the PSR B1257+12 system from timings alone. Good to see the Gliese 876 system is still under study.

The parameters in this new paper cannot unfortunately be directly compared with the ones in the previously-published radial velocity+astrometry fit, because the two sets of parameters are for times roughly 7 years apart, and the system evolves rapidly (here's an illustration of a 10-year integration I made using the radial velocity+astrometry fit parameters). This means that to do a direct comparison it is necessary to integrate the system to a specified date and reconstitute the orbital parameters from positions/velocities.

Note also that radial velocity is insensitive to the orientation of the system as a whole in the plane of the sky: the astrometry gives absolute values for the longitude of the ascending node Ω, whereas only relative values can be determined with radial velocities.

Another go at the system with updated data:

Orbital structure of the GJ876 extrasolar planetary system, based on the latest Keck and HARPS radial velocity data
http://arxiv.org/abs/1105.4696

Quote :

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.

(I've changed the thread title from "Gliese 876 - 2010 Update" to what it is now due to the latter nature of this new study).

Can GJ 876 host four planets in resonance?
http://arxiv.org/abs/1202.5865

Orbits between the hot super-Earth and the innermost giant planet are unstable.