The formation of mini-Neptunes

TL;DR

This study explores how mini-Neptunes form beyond the iceline, highlighting the importance of envelope enrichment, accretion rates, and disk conditions, and discusses the challenges in forming Neptune-like planets.

Contribution

It provides new insights into the formation pathways of mini-Neptunes, emphasizing the roles of envelope enrichment, accretion rates, and disk properties in their formation.

Findings

Envelope enrichment increases mini-Neptune formation frequency.

Low solid accretion rates (~10^{-6} Earth masses/year) favor mini-Neptune formation.

Formation of Neptune-like planets remains challenging for current theories.

Abstract

Mini-Neptunes seem to be common planets. In this work we investigate the possible formation histories and predicted occurrence rates of mini-Neptunes assuming the planets form beyond the iceline. We consider pebble and planetesimal accretion accounting for envelope enrichment and two different opacity conditions. We find that the formation of mini-Neptunes is a relatively frequent output when envelope enrichment by volatiles is included, and that there is a "sweet spot" for mini-Neptune formation with a relatively low solid accretion rate of ~10^{-6} Earth masses per year. This rate is typical for low/intermediate-mass protoplanetary disks and/or disks with low metallicities. With pebble accretion, envelope enrichment and high opacity favor the formation of mini-Neptunes, with more efficient formation at large semi-major axes (~30 AU) and low disk viscosity. For planetesimal accretion, such planets can form also without enrichment, with the opacity being a key aspect in the growth history and favorable formation location. Finally, we show that the formation of Neptune-like planets remains a challenge for planet formation theories.