An upper limit for the growth of inner planets?

The exotic range of known planetary systems has provoked an equally exotic range of physical explanations for their diverse architectures. However, constraining formation processes requires mapping the observed exoplanet population to that which initially formed in the protoplanetary disc. Numerous results suggest that (internal or external) dynamical perturbation alters the architectures of some exoplanetary systems. Isolating planets that have evolved without any perturbation can help constrain formation processes. We consider the Kepler multiples, which have low mutual inclinations and are unlikely to have been dynamically perturbed. We apply an adaption of previous modelling efforts, accounting for the two-dimensionality of the radius (R-pl = 0.3-20 R-circle plus) and period (P-orb = 0.5-730 d) distribution. We find that an upper limit in planet mass of the form M-lim proportional to a(pl)(beta) exp(-a(in)/a(pl)), for semimajor axis a(pl) and a broad range of a(in) and beta, can reproduce a distribution of P-orb, R-pl that is indistinguishable from the observed distribution by our comparison metric. The index is consistent with beta = 1.5, expected if growth is limited by accretion within the Hill radius. This model is favoured over models assuming a separable PDF in P-orb, R-pl. The limit, extrapolated to longer periods, is coincident with the orbits of RV-discovered planets (a(pl) > 0.2 au, M-pl > 1 M-J) around recently identified low density host stars, hinting at isolation mass limited growth. We discuss the necessary circumstances for a coincidental age-related bias as the origin of this result; such a bias is possible but unlikely. We conclude that, in light of the evidence suggesting that some planetary systems have been dynamically perturbed, simple models for planet growth during the formation stage are worth revisiting.

Automated summary

Observations suggest that some planetary systems may have been dynamically perturbed. Isolating planets that have evolved without any perturbation can help constrain formation processes. Revisiting simple models for planet growth during the formation stage is worth considering.