Formation of Close-in Neptunes Around Low-Mass Stars Through Breaking Resonant Chains

Conventional planet formation theories predict a paucity of massive planets around small stars, especially very low-mass ($0.1 - 0.3 \ M_{\odot}$) mid-to-late M dwarfs. Such tiny stars are expected to form planets of terrestrial sizes, but not much bigger. However, this expectation is challenged by the recent discovery of LHS 3154 b, a planet with period of 3.7 days and minimum mass of $13.2 \ M_{\oplus}$ orbiting a $0.11 \ M_{\odot}$ star. Here, we propose that close-in Neptune-mass planets like LHS 3154 b formed through an anomalous series of mergers from a primordial compact system of super-Earths. We perform simulations within the context of the "breaking the chains" scenario, in which super-Earths initially form in tightly-spaced chains of mean-motion resonances before experiencing dynamical instabilities and collisions. Planets as massive and close-in as LHS 3154 b ($M_p \sim 12 - 20 \ M_{\oplus}$, $P < 7$ days) are produced in $\sim$1% of simulated systems, in broad agreement with their low observed occurrence. These results suggest that such planets do not require particularly unusual formation conditions but rather are an occasional byproduct of a process that is already theorized to explain compact multi-planet systems. Interestingly, our simulated systems with LHS 3154 b-like planets also contain smaller planets at around $\sim 30$ days, offering a possible test of this hypothesis.