Occurrence rates of Earth-like planets around very low-mass stars
To put our results in context, we compared the currently known planets around the stars within our stellar sample with predictions from state-of-the-art planet formation models. We simulated mock observations of planets drawn from a synthetic planet population around M dwarfs based on the standard core accretion scenario (Burn et al. 2021), and weighted them by our estimated detection probabilities. The observed orbital periods are shorter and the minimum masses are smaller than predicted. In the framework of standard core accretion, such a deviation could in principle be explained by a reduced planetesimal surface density in models. While such a correction was not needed based on previous occurrence rate studies, we probed here the very low-mass star regime. Therefore, the dependency of available mass in planet building blocks on stellar mass needs to be further investigated. Apart from that, the observed lower masses could also be explained by alternative formation scenarios such as accretion of dry pebbles within the snowline or mass loss during giant impacts.
Altogether, our results and the discussion show the importance of differentiation in stellar masses when discussing planet occurrences and, ultimately, planet formation processes. Once the CARMENES survey of M dwarfs is completed and at least 50 RV epochs have been obtained for all targets, we will conclude our analysis of occurrence rates on the entire sample.