Super-Earth formation in systems with cold giants

Around our Sun, terrestrial planets did not grow beyond Earth in mass, while super-Earths are found to orbit approximately every other solar-like star. It remains unclear what divides these super-Earth systems from those that form terrestrial planets, and what role wide-orbit gas giants play in this process. 

To address this problem we use a 1D pebble accretion semi-analytical model to simulate the simultaneous growth of inner embryos and outer giant planets, focusing on the effect of mutual pebble filtering between outer and inner embryos. We assume pebble sizes limited both by fragmentation and radial drift and also investigate the significant influence of the pebble scale height and assumed fragmentation velocity on the pebble accretion efficiency. The initial seed planetesimals for the embryo growth are taken from the top of the streaming instability mass distribution. In our simulations we include two different disc models: one whose temperature profile is entirely set by stellar irradiation and another one that includes viscous heating.

We show that the key uncertainty in determining the system’s final architecture is the degree of viscous heating in the inner disc. In systems with maximally efficient viscous heating, pebble accretion in the terrestrial region is suppressed. More realistic levels of viscous heating, at higher elevation, allow both super-Earth and terrestrial embryo formation at Earth-like orbits. We also find that the role of the water iceline in preventing super-Earth formation is minor, except in cases involving extreme volatile loss and a significant reduction in pebble sizes.

Furthermore, we show that in systems with gas-giant formation, the role of mutual pebble filtering by outer pebble-accreting embryos is limited, unless some mechanism of delaying inner disc growth, like viscous heating or the presence of an iceline, is simultaneously employed. This latter point appears to be consistent with the fact that no strong suppression is seen in the occurrence rate of super-Earths in systems with known gas giants in wider orbits. We conclude that the diversity in inner-disc systems may largely be driven by complex, and as-of-yet poorly understood, disc accretion physics inside the water ice line.