Abstract
Ribas and collaborators have recently proposed that an additional, ~5 M_earth planet orbits the transiting planet host star GJ436. Long-term dynamical interactions between the two planets leading to eccentricity excitation might provide an explanation for the transiting planet's unexpectedly large orbital eccentricity. In this paper we examine whether the existence of such a second planet is supported by the available observational data when the short-term interactions that would result from its presence are accounted for. We find that the model for the system suggested by Ribas and collaborators lead to predictions that are strongly inconsistent with the measured host star radial velocities, transiting planet primary and secondary eclipse times, and transiting planet orbital inclinations. A search for an alternative two planet model that is consistent with the data yields a number of plausible solutions, although no single one stands out as particularly unique by giving a significantly better fit to the data than the nominal single planet model. We conclude from this study that Ribas and collaborator's general hypothesis of an additional short-period planet in the GJ436 system is still plausible, but that there is not sufficient evidence to support their claim of a planet detection.
Highlights
The GJ 436 system is unique among the nearly 250 extrasolar planetary systems identified so far1
We find that the model for the system suggested by Ribas and collaborators lead to predictions that are strongly inconsistent with the measured host star radial velocities, transiting planet primary and secondary eclipse times, and transiting planet orbital inclinations
The planet has an orbital eccentricity e = 0.15 ± 0.01 as determined from analyzing the radial velocities of the host star with the constraint provided by the observed time of the planet’s secondary eclipse that was observed with Spitzer (Deming et al 2007; Demory et al 2007)
Summary
The GJ 436 system is unique among the nearly 250 extrasolar planetary systems identified so far. The second planet in the RFB model has an orbital period, P = 5.1859 d, which puts it close to a 2:1 orbital resonance with the transiting planet, and minimum mass, M sin i = 4.7 M⊕ They proposed that this second planet exists based on the success of their model for providing a source for the transiting planet’s eccentricity, a reason for the non-detection of transits by Butler et al (2004), and a fit to the radial velocities. One particular aspect of the RFB study that merits further investigation is the possible sensitivity of observables to gravitational interactions between the two planets in their model These two planets would be in close, moderately eccentric, and possibly non-coplanar orbits and so their mutual perturbations might be significant on short timescales, in addition to the long timescales that RFB only considered. Transit timings for a planet near to a low-order resonance, as RFB propose for GJ 436b, are sensitive to very lowmass planets (Steffen & Agol 2005)
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