Tidal downsizing model - III. planets from sub-earths to brown dwarfs: Structure and metallicity preferences

Abstract

We present population synthesis calculations of the Tidal Downsizing (TD) hypothesis for planet formation. Our models address the following observations: (i) most abundant planets being super-Earths; (ii) cores more massive than ~5-15M⊕ are enveloped by massive atmospheres; (iii) the frequency of occurrence of close-in gas-giant planets correlates strongly with metallicity of the host star; (iv) no such correlation is found for sub-Neptune planets; (v) presence of massive cores in giant planets; (vi) gas-giant planets are overabundant in metals compared to their host stars; (vii) this overabundance decreases with planet's mass; (viii) a deep valley in the planet mass function between masses of ~10-20M⊕ and ~100M⊕. A number of observational predictions distinguish the model from Core Accretion: (a) composition of the massive cores is always dominated by rocks not ices; (b) the core mass function is smooth with no minimum at ~3M⊕ and has no ice-dominated cores; (c) gas giants beyond 10 au are insensitive to the host-star metallicity; (d) objects more massive than ~10MJ do not correlate or even anticorrelate with metallicity. The latter prediction is consistent with observations of low-mass stellar companions. TD can also explain formation of planets in close binary systems. TD model is a viable alternative to the Core Accretion scenario in explaining many features of the observed population of exoplanets.