Formation of planetary populations – I. Metallicity and envelope opacity effects

We present a comprehensive body of simulations of the formation of exoplanetary populations that incorporate the role of planet traps in slowing planetary migration. The traps we include in our model are the water ice line, the disc heat transition, and the dead zone outer edge. We reduce our model parameter set to two physical parameters: the opacity of the accreting planetary atmospheres (κenv) and a measure of the efficiency of planetary accretion after gap opening (fmax). We perform planet population synthesis calculations based on the initial observed distributions of host star and disc properties – their disc masses, lifetimes, and stellar metallicities. We find the frequency of giant planet formation scales with disc metallicity, in agreement with the observed Jovian planet frequency–metallicity relation. We consider both X-ray and cosmic ray disc ionization models, whose differing ionization rates lead to different dead zone trap locations. In both cases, Jovian planets form in our model out to 2–3 au, with a distribution at smaller radii dependent on the disc ionization source and the setting of envelope opacity. We find that low values of κenv (0.001–0.002 cm2 g−1) and X-ray disc ionization are necessary to obtain a separation between hot Jupiters near 0.1 au, and warm Jupiters outside 0.6 au, a feature present in the data. Our model also produces a large number of super Earths, but the majority are outside of 2 au. As our model assumes a constant dust-to-gas ratio, we suggest that radial dust evolution must be taken into account to reproduce the observed super Earth population.