On the Origin of Hd149026b

The high density of the recently discovered close-in extrasolar planet HD149026b suggests the presence of a huge core in the planet, which challenges planet formation theory. We first derive constraints on the amount of heavy elements and hydrogen/helium present in the planet: We find that preferred values of the core mass are between 50 and 80M ⊕ , although a minimum value of the core mass is ∼ 35M ⊕ in the extreme case of formation of the planet at > 0.5 AU, followed by late inward migration after > 1 Ga and negligible reheating due to tidal dissipation. We then investigate the possibility of subcritical core accretion as envisioned for Uranus and Neptune. We show that a massive core surrounded by an envelope in hydrostatic equilibrium with the gaseous disk may indeed grows beyond 30M ⊕ provided the core accretion rate remains larger than ∼ 2 × 10 −5 M ⊕ yr −1. However, we find the subcritical accretion scenario is very unlikely in the case of HD149026b for at least two reasons: (i) Subcritical planets are such that the ratio of their core mass to their total mass is above ∼ 0.7, in contradiction with constraints for all but the most extreme interior models of HD149026b; (ii) High accretion rates and large isolation mass required for the formation of a subcritical > 35M ⊕ core are possible only at specific orbital distances in a disk with a surface density of dust equal to at least 10 times that of the minimum mass solar nebula. This value climbs to 30 when considering a 50M ⊕ core. These facts point toward two main routes for the formation of this planet: (i) Gas accretion that is limited by a slow viscous inflow of gas in an evaporating disk; (ii) A significant modification of the composition of the planet after gas accretion has stopped. These two routes are not mutually exclusive. Illustrating the second route, we show that for a wide range of impact parameters, giant impacts lead to a loss of the gas component of the planet and thus may lead to – 2 – planets that are highly enriched in heavy elements. Alternatively, the planet may be supplied with heavy elements by planetesimals by secular perturbations. Both in the giant impact and the secular perturbation scenarios, we expect an outer giant planet to be present. Observational studies by imaging, astrometry …