On the thick-disc exoplanet host subgiant HD 155358

The nearby subgiant HD 155358 has very recently been announced by Cochran et al. to harbour two Jovian-mass planetary companions and to be the lowest metallicity exoplanet host star yet found. Here, we present a model atmosphere analysis and stellar evolutionary tracks for HD 155358, and demonstrate that it is actually a member of the ancient thick-disc population of the Milky Way (τ≥ 12 Gyr). As such, HD 155358 is considerably rich in its α-chain nuclei, and hence only about a factor of 2 below the solar abundance in terms of these species. Yet, as a precursor to the thin disc and residing on the metal-poor end of the thin-disc metal abundance distribution, HD 155358 gives way to the principal possibility that any star of the thin-disc population of the Galaxy can be a potential exoplanet host. As opposed to this, the formation of planets and even planetary systems for HD 155358 and HD 37124 (another previously known thick-disc exoplanet host) is mentionable, as there is strong evidence for a vigorous starburst phase in the early Milky Way, and secondly, the direct census of an unbiased nearby thick-disc sample implies a minimum fraction of no less than 30 per cent multiple star systems (N≥ 3) to originate from that epoch.