Giant planets around low-mass stars: a challenge for core accretion theory

Planet formation is sensitive to the environment in protoplanetary disks, for which scaling laws as a function of stellar mass are known. Can the observed population of planets around low-mass stars be explained by these trends or is there evidence for separate formation channels? We explored this question by confronting state-of-the-art planet formation theory with a sample of planets observed by the HARPS and CARMENES radial velocity (RV) surveys. To account for detection biases, we performed injection-and-retrieval experiments on the actual RV data to produce synthetic observations of planets that we simulated following the core accretion paradigm. We find that these simulations robustly reproduce the observed high occurrence of rocky worlds around low-mass stars. On the other hand, while planet formation theories consistently predict a strong stellar mass-giant planet occurrence correlation, we observe a population of giant planets around stars less massive than 0.4 solar masses that can not be reconciled with theory. This may point to a different giant planet formation channel around very low-mass stars