The Lick-Carnegie Exoplanet Survey: A Uranus-Mass Fourth Planet for Gj 876 in an Extrasolar Laplace Configuration

Abstract

Continued radial velocity monitoring of the nearby M4V red dwarf star GJ 876 with Keck/HIRES has revealed the presence of a Uranus-mass fourth planetary companion in the system. The new planet has a mean period of P e = 126.6 days (over the 12.6-year baseline of the radial velocity observations), and a minimum mass of m e sin i e = 12.9 ± 1.7 M ⊕ . The detection of the new planet has been enabled by significant improvements to our radial velocity data set for GJ 876. The data have been augmented by 36 new high-precision measurements taken over the past five years. In addition, the precision of all of the Doppler measurements have been significantly improved by the incorporation of a high signal-to-noise template spectrum for GJ 876 into the analysis pipeline. Implementation of the new template spectrum improves the internal RMS errors for the velocity measurements taken during 1998-2005 from 4.1 m s −1 to 2.5 m s −1 . Self-consistent, N-body fits to the radial velocity data set show that the four-planet system has an invariable plane with an inclination relative to the plane of the sky of i = 59.5 • . The fit is not significantly improved by the introduction of a mutual inclination between the planets "b" and "c," but the new data do confirm a non-zero eccentricity, e d = 0.207 ± 0.055 for the innermost planet, "d." In our best-fit coplanar model, the mass of the new component is m e = 14.6 ± 1.7 M ⊕ . Our best-fitting model places the new planet in a 3-body resonance with the previously known giant planets (which have mean periods of P c = 30.4 and P b = 61.1 days). The critical argument, ϕ Laplace = λ c − 3λ b + 2λ e , for the Laplace resonance librates with an amplitude of ∆ϕ Laplace = 40±13 • about ϕ Laplace = 0 • . Numerical integration indicates that the four-planet system is stable for at least a billion years (at least for the coplanar cases). This resonant configuration of three giant planets orbiting an M-dwarf primary differs from the well-known Laplace configuration -3of the three inner Galilean satellites of Jupiter, which are executing very small librations about ϕ Laplace = 180 • , and which never experience triple conjunctions.The GJ 876 system, by contrast, comes close to a triple conjunction between the outer three planets once per every orbit of the outer planet, "e."

Dynamical Interactions Make Hot Jupiters in Open Star Clusters

“…We are unaware of a close analog (yet!) among the known exoplanets to such a system, but we note the orbital similarities presented by the brown dwarf HD 41004 B b (a=0.0177 AU and e=0.081) (Zucker et al 2004), and by the super-Earth GJ 876 d (a=0.0208 AU and e=0.207) (Rivera et al 2010).…”

A High-Precision Near-Infrared Survey for Radial Velocity Variable Low-Mass Stars Using Cshell and a Methane Gas Cell

“…Correia et al (2010) predicted the possible existence of a low-mass (<2 M ⊕ ) planet in 4:1 orbital resonance with GJ876b that could explain how the high eccentricity (e=0.14) of the orbit of GJ876d could have survived for more than ≈1 Myr, but the existence of such a planet has not been confirmed yet. Finally, Rivera et al (2010) confirmed the existence of a fourth planet, GJ876e, on a 126.6 day orbit and a minimum mass of =  M i sin 12.9 1.7 p M ⊕ . We followed GJ876 as part of the nearby sample over 17 nights spanning >700 days with a typical S/N ≈ 170 per night and recovered it as an RV-variable target with V=196±19 m s −1 , which is consistent within 1.6σ with the RV amplitude measured by Marcy et al (1998).…”

“…The method of Doppler radial velocity (RV) variations has proven itself fruitful in the last decades both for the identification of new exoplanets (e.g., Mayor & Queloz 1995;Marcy et al 1998Marcy et al , 2001Delfosse et al 1998a;Cochran et al 2002;Endl et al 2003;Butler et al 2004;Rivera et al 2005bRivera et al , 2010Meschiari et al 2011;Dumusque et al 2012;Montet et al 2014;Tuomi et al 2014) and for the confirmation of exoplanets detected by the method of transit (e.g., Kepler-78b; Akeson et al 2013;Pepe et al 2013;Sanchis-Ojeda et al 2013). Recent developments have shown that cool (3800 K) stellar hosts in the M spectral class represent valuable targets for the identification of new Earthmass planetary companions in the habitable zone using the RV method due to their smaller mass and significantly larger population (Henry et al 2006).…”

Trapping in three-planet resonances during gas-driven migration

“…At present, at least three extrasolar systems are believed to be in multiple-planet resonance: HR8799 (e.g. Reidemeister et al 2009) and Gl876 (Rivera et al 2010) in a Laplace configuration, and the KOI500 five-planet exosystem that shows two three-body resonances (Lissauer et al 2011). Moreover, simulations on a possible trapping of HD82943 exosystem in a Laplace resonance were also performed by Beaugé et al (2008).…”