Abstract
Context.State-of-the-art planet formation models are now capable of accounting for the full spectrum of known planet types. This comes at the cost of an increasing complexity of the models, which calls into question whether established links between their initial conditions and the calculated planetary observables are preserved.Aims.In this paper, we take a data-driven approach to investigate the relations between clusters of synthetic planets with similar properties and their formation history.Methods.We trained a Gaussian mixture model on typical exoplanet observables computed by a global model of planet formation to identify clusters of similar planets. We then traced back the formation histories of the planets associated with them and pinpointed their differences. Using the cluster affiliation as labels, we trained a random forest classifier to predict planet species from properties of the originating protoplanetary disk.Results.Without presupposing any planet types, we identified four distinct classes in our synthetic population. They roughly correspond to the observed populations of (sub-)Neptunes, giant planets, and (super-)Earths, plus an additional unobserved class we denote as “icy cores”. These groups emerge already within the first 0.1 Myr of the formation phase and are predicted from disk properties with an overall accuracy of >90%. The most reliable predictors are the initial orbital distance of planetary nuclei and the total planetesimal mass available. Giant planets form only in a particular region of this parameter space that is in agreement with purely analytical predictions. IncludingN-body interactions between the planets decreases the predictability, especially for sub-Neptunes that frequently undergo giant collisions and turn into super-Earths.Conclusions.The processes covered by current core accretion models of planet formation are largely predictable and reproduce the known demographic features in the exoplanet population. The impact of gravitational interactions highlights the need forN-body integrators for realistic predictions of systems of low-mass planets.
Highlights
We aim to explore which distinct planet species emerge from our planet formation model and how they compare to observedplanet types
That we have identified the solid disk mass and the initial orbital separation of a planetary embryo as the most important features, we investigate the regions that different planet types occupy in the space that these parameters span
We have investigated how different properties of protoplanetary disks relate to the emergence of different planet types in a planetesimal-based core accretion context
Summary
One of the most remarkable findings in recent years of exoplanetology has been the enormous diversity of planetary systems (e.g., Ribas & Miralda-Escudé 2007; Howard et al 2012; Fressin et al 2013; Petigura et al 2013; Mulders et al 2015; Hobson & Gomez 2017; Brewer et al 2018; Owen & MurrayClay 2018; Hsu et al 2019; Bryan et al 2019; He et al 2020). Studied mechanisms include the evolution of accretion disks (e.g., Lüst 1952; Lynden-Bell & Pringle 1974; Pringle 1981), their interaction with embedded planets that may result in orbital migration (e.g., Goldreich & Tremaine 1979; Tanaka et al 2002; D’Angelo et al 2003; Paardekooper et al 2011; Dittkrist et al 2014), how these protoplanets form and grow by accreting solid components and gas (e.g., Bodenheimer & Pollack 1986; Ida & Makino 1993; Pollack et al 1996; Thommes et al 2003; Fortier et al 2013), their gravitational interaction among each other (e.g., Chambers et al 1996; Raymond et al 2009), photoevaporation of both protoplanetary disks (Hollenbach et al 1994; Clarke et al 2001; Alexander et al 2014) and planetary atmospheres (Lammer et al 2003; Owen & Jackson 2012; Jin et al 2014), and the long-term evolution of planets and their atmospheres (e.g., Bodenheimer & Pollack 1986; Guillot 2005; Fortney & Nettelmann 2010; Mordasini et al 2012c).
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