Super-Earth formation in systems with cold giants

Abstract

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. Here, we show that the key uncertainty is the degree of viscous heating in the inner disc, which regulates the pebble accretion efficiency. In this parameter study, we assume pebble sizes limited by fragmentation and radial drift. The initial seed planetesimals for embryo growth are taken from the top of the streaming instability mass distribution. We then evaluate the important role of the pebble scale height and the assumed pebble fragmentation velocity. In systems with maximally efficient viscous heating, where all the accretion heating is deposited in the disc midplane, pebble accretion in the terrestrial region is suppressed. More realistic levels of viscous heating, at higher elevations, allow terrestrial embryo formation at Earth-like orbits. We also find that the role of the water iceline is minor, unless it is paired with extreme volatile loss and a change in the pebble fragmentation velocity. 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, such as 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 iceline.