Habitable Zones & Global Climate

Evaluating The Evidence For Water World Populations Using Mixture Models

By Keith Cowing
May 2, 2022
Filed under
Evaluating The Evidence For Water World Populations Using Mixture Models
A diagram of the models developed in this paper. The legend on the left shows the six planet subpopulations included in these models, where these subpopulations are distinguished by their core composition (rocky or icy), and formation/evolution history (gaseous, evaporated, or intrinsically rocky/icy). The colored border around each subpopulation in the legend is consistent with the color scheme used in the following figures in this paper. Models in the table increase in the number of subpopulations (labeled at the top), and thus the complexity, towards the right. The top row of models, taken from NR20, only include planet subpopulations with rocky-core compositions. The models introduced in this paper, on the bottom row, incorporate a mix of rocky and icy-core compositions in different combinations. Our most complex model, Model Icy6, includes all six subpopulations of planets listed in the legend.

Water worlds have been hypothesized as an alternative to photo-evaporation in order to explain the gap in the radius distribution of Kepler exoplanets.

We explore water worlds within the framework of a joint mass-radius-period distribution of planets fit to a sample of transiting Kepler exoplanets, a subset of which have radial velocity mass measurements. We employ hierarchical Bayesian modeling to create a range of ten mixture models that include multiple compositional subpopulations of exoplanets. We model these subpopulations – including planets with gaseous envelopes, evaporated rocky cores, evaporated icy cores, intrinsically rocky planets, and intrinsically icy planets – in different combinations in order to assess which combinations are most favored by the data.

Using cross-validation, we evaluate the support for models that include planets with icy compositions compared to the support for models that do not, finding broad support for both. We find significant population-level degeneracies between subpopulations of water worlds and planets with primordial envelopes. Among models that include one or more icy-core subpopulations, we find a wide range for the fraction of planets with icy compositions, with a rough upper limit of 50%. Improved datasets or alternative modeling approaches may better be able to distinguish between these subpopulations of planets.

Andrew R. Neil, Jessica Liston, Leslie A. Rogers

Comments: 31 pages, 10 figures, accepted for publication in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2205.00006 [astro-ph.EP] (or arXiv:2205.00006v1 [astro-ph.EP] for this version)
Submission history
From: Leslie Rogers PhD
[v1] Thu, 28 Apr 2022 22:49:23 UTC (2,752 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻