Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307

Abstract

Context. The K2.5 dwarf HD 40307 has been reported to host three super-Earths. The system lacks massive planets and is therefore a potential candidate for having additional low-mass planetary companions. Aims. We re-derive Doppler measurements from public HARPS spectra of HD 40307 to confirm the significance of the reported signals using independent data analysis methods. We also investigate these measurements for additional low-amplitude signals. Methods. We used Bayesian analysis of our radial velocities to estimate the probability densities of different model parameters. We also estimated the relative probabilities of models with differing numbers of Keplerian signals and verified their significance using periodogram analyses. We investigated the relation of the detected signals with the chromospheric emission of the star. As previously reported for other objects, we found that radial velocity signals correlated with the S-index are strongly wavelength dependent. Results. We identify two additional clear signals with periods of 34 and 51 days, both corresponding to planet candidates with minimum masses a few times that of the Earth. An additional sixth candidate is initially found at a period of 320 days. However, this signal correlates strongly with the chromospheric emission from the star and is also strongly wavelength dependent. When analysing the red half of the spectra only, the five putative planetary signals are recovered together with a very significant periodicity at about 200 days. This signal has a similar amplitude as the other new signals reported in the current work and corresponds to a planet candidate with M sin i ∼ 7 M ⊕ (HD 40307 g). Conclusions. We show that Doppler measurements can be filtered for activity-induced signals if enough photons and a sufficient wavelength interval are available. If the signal corresponding to HD 40307 g is a genuine Doppler signal of planetary origin, this candidate planet might be capable of supporting liquid water on its surface according to the current definition of the liquid water habitable zone around a star and is not likely to suffer from tidal locking. Also, at an angular separation of ∼46 mas, HD 40307 g would be a primary target for a future space-based direct-imaging mission.

Up to four planets around the M dwarf GJ 163

“…As a set of reference priors, we use those presented in Tuomi (2012) because this choice is a rather uninformative one and does not constrain the model parameters strongly (e.g. Anglada-Escudé & Tuomi 2012;Tuomi 2012;Tuomi et al 2013a). The functional forms of the priors and the limits of the respective parameter spaces of the model parameters are shown in Table 1.…”

“…In addition to these Keplerian parameters, we also include a constant reference velocity (γ), a radial velocity jitter (σ J ; amount of excess white noise in the data on top of the estimated instrument uncertainties), and a correlation coefficient between the noise of subsequent epochs (φ) as defined in e.g. Tuomi et al (2013a) and Tuomi et al (2013b).…”

“…Gaussian white noise and a simple red noise model. We perform the analyses using posterior samplings (following Tuomi 2012; Tuomi & Jones 2012;Tuomi et al 2013a) and calculate estimates for the Bayesian evidences using the truncated posterior mixture estimate (Tuomi & Jones 2012). As a result, we report our best estimates for the number and properties of planetary signals in the RV data of GJ 163 using the different prior and noise model choices.…”

“…Ford & Gregory (2007) defined a set of rather uninformative prior densities for analyses of radial velocity data using the typical statistical models consisting of Keplerian signals and white noise. These priors were modified slightly in Tuomi (2012) and Tuomi et al (2013a) but the effects different prior choices have on the obtained results have not been studied. We study these effects in this article by comparing the effects of prior models on the obtained results when analysing the RV data of GJ 163.…”

Evolution of the habitable zone of low-mass stars

“…Hart 1978;Kasting et al 1993;Underwood et al 2003;Selsis et al 2007;Pierrehumbert & Gaidos 2011;Kopparapu et al 2013). Several potentially habitable planets are already reported in literature (see among others Udry et al 2007;Pepe et al 2011;Borucki et al 2011Borucki et al , 2012Anglada-Escudé et al 2012;Barclay et al 2013;Tuomi et al 2013;Batista et al 2014), and their number is expected to increase with time.…”