Bayesian re-analysis of the Gliese 581 exoplanet system

Abstract

A re-analysis of Gliese 581 HARPS and HIRES precision radial velocity data was carried out with a Bayesian multiplanet Kepler periodogram (from one to six planets) based on a fusion Markov chain Monte Carlo algorithm. In all cases, the analysis included an unknown parametrized stellar jitter noise term. For the HARPS data set, the most probable number of planetary signals detected is five with a Bayesian false alarm probability of 0.01. These include the 3.1498 ± 0.0005, 5.3687 ± 0.0002, 12.927+0.006− 0.004 and 66.9 ± 0.2 d periods reported previously plus a 399+14− 16 d period. Their orbital eccentricities are 0.0+0.2− 0.0, 0.00+0.02− 0.00, 0.10+0.06− 0.10, 0.33+0.09− 0.10 and 0.02+0.30− 0.01, respectively. The semimajor-axis and M sin i of the five planets are (0.0285 ± 0.0006 au, 1.9 ± 0.3 M⊕), (0.0406 ± 0.0009 au, 15.7 ± 0.7 M⊕), (0.073 ± 0.002 au, 5.3 ± 0.4 M⊕), (0.218 ± 0.005 au, 6.7 ± 0.8 M⊕) and (0.7 ± 0.2 au, 6.6+2.0− 2.7M⊕), respectively. The analysis of the HIRES data set yielded a reliable detection of only the strongest 5.37 and 12.9 d periods. The analysis of the combined HIRES/HARPS data again only reliably detected the 5.37 and 12.9 d periods. The detection of four planetary signals with the periods of 3.15, 5.37, 12.9 and 66.9 d was only achieved by including an additional unknown but parametrized Gaussian error term added in quadrature to the HIRES quoted errors. The marginal distribution for the σ of this additional error term has a well-defined peak at 1.8 ± 0.4 m s−1. It is possible that this additional error arises from unidentified systematic effects. We did not find clear evidence for a fifth planetary signal in the combined HIRES/HARPS data set. Based on the available data, our analysis does not support the claimed detection of a sixth planet Gliese 581g.