The New Generation Planetary Population Synthesis (NGPPS)

Context. Planetary population synthesis is a tool that is used to better understand the key processes of planet formation at the statistical level. Aims. We seek to quantify the fidelity with which modern population syntheses reproduce observations in view of their use as predictive tools. Methods. We compared synthetic populations from the Generation 3 Bern Model of Planet Formation and Evolution (core accretion, solar-type host stars) and the HARPS/Coralie radial velocity sample. We biased the synthetic planet population according to the completeness of the observed data. We then performed quantitative statistical comparisons and systematically identified agreements and differences. Results. Our nominal population reproduces many of the main features of the HARPS planets, such as two main groups of planets in the mass-distance diagram (close-in sub-Neptunes and distant giants), a bimodal mass function with a less populated ‘desert’, an observed mean multiplicity of about 1.6, and several key correlations regarding the stellar metallicity dependency, the period ratio distribution, and the eccentricity distribution. Considering that the model was not optimised beforehand to reproduce any particular survey, this indicates that some of the important physical processes governing planetary formation could be captured. The remaining discrepancies that can be quantified thanks to the population synthesis approach point to areas that are not fully captured in the model. For instance, we find that the synthetic population has (1) in absolute terms too many planets by ≈70%, (2) a ‘desert’ that is too deep by ≈60%, (3) a relative excess of giant planets by ≈40%, (4) planet eccentricities that are on average too low by a factor of about two (median of 0.07 versus 0.15), and (5) a metallicity effect that is too weak. Finally, the synthetic planets are overall too close to the star compared to the HARPS sample. The differences allowed us to find model parameters that better reproduce the observed planet masses, for which we computed additional synthetic populations. We find that decreasing the planet formation efficiency by increasing the planetesimal size re-balances the number of sub-Neptunes versus giant planets. Changing the efficiency of gas-driven migration also affects the sub-Neptune to giant planet ratio, with lower migration rates resulting in more giant planets and fewer sub-Neptunes. Conclusions. However, only modifying the model parameters seems to be insufficient for the model to fully reproduce both the observed mass and distance distributions at the same time. Instead, physical processes appear to be missing. Planets may originate on wider orbits than our model predicts. Mechanisms leading to higher eccentricities and slower disc-limited gas accretion also seem necessary. We also advocate that theoretical models should make a quantitative, rather than merely a qualitative, comparison between the many current and future large surveys and theoretical results to better understand the origins of planetary systems.