Quantitative Criteria for Defining Planets

TL;DR

This paper proposes quantitative, unified criteria for defining planets and exoplanets, addressing vagueness in current definitions by using clustering analysis and dynamical dominance principles.

Contribution

It introduces two new frameworks for planetary classification, one aligned with IAU standards and a simpler mass-based approach, improving clarity and applicability.

Findings

Satellites are distinct from planets in clustering analysis.

Dynamical dominance is a key organizing principle.

Proposed frameworks unify definitions for Solar System and exoplanets.

Abstract

The current IAU definition of "planet" is problematic because it is vague and excludes exoplanets. Here, we describe aspects of quantitative planetary taxonomy and examine the results of unsupervised clustering of Solar System bodies to guide the development of possible classification frameworks. Two unsurprising conclusions emerged from the clustering analysis: (1) satellites are distinct from planets and (2) dynamical dominance is a natural organizing principle for planetary taxonomy. To generalize an existing dynamical dominance criterion, we adopt a universal clearing timescale applicable to all central bodies (brown dwarfs, stars, and stellar remnants). Then, we propose two quantitative, unified frameworks to define both planets and exoplanets. The first framework is aligned with both the IAU definition of planet in the Solar System and the IAU working definition of an exoplanet. The second framework is a simpler mass-based framework that avoids some of the difficulties ingrained in current IAU recommendations.