Bayesian analysis of radial velocity data of GJ667C with correlated noise: evidence for only two planets

Abstract

GJ667C is the least massive component of a triple star system which lies at a\ndistance of about 6.8 pc (22.1 light-years) from Earth. GJ667C has received\nmuch attention recently due to the claims that it hosts up to seven planets\nincluding three super-Earths inside the habitable zone. We present a Bayesian\ntechnique for the analysis of radial velocity (RV) data-sets in the presence of\ncorrelated noise component ("red noise"), with unknown parameters. We also\nintroduce hyper-parameters in our model in order to deal statistically with\nunder or over-estimated error bars on measured RVs as well as inconsistencies\nbetween different data-sets. By applying this method to the RV data-set of\nGJ667C, we show that this data-set contains a significant correlated (red)\nnoise component with correlation timescale for HARPS data of order 9 days. Our\nanalysis shows that the data only provides strong evidence for the presence of\ntwo planets: GJ667Cb and c with periods 7.19d and 28.13d respectively, with\nsome hints towards the presence of a third signal with period 91d. The\nplanetary nature of this third signal is not clear and additional RV\nobservations are required for its confirmation. Previous claims of the\ndetection of additional planets in this system are due the erroneous assumption\nof white noise. Using the standard white noise assumption, our method leads to\nthe detection of up to five signals in this system. We also find that with the\nred noise model, the measurement uncertainties from HARPS for this system are\nunder-estimated at the level of ~50 per cent.\n