A Continuum of Planet Formation Between 1 and 4 Earth Radii

TL;DR

This study investigates the relationship between star metallicity and small planet sizes, finding no evidence for distinct categories and supporting a continuum of planet formation between 1 and 4 Earth radii.

Contribution

The paper challenges previous claims of distinct planet categories based on radius boundaries, demonstrating a continuous formation process through three independent analyses.

Findings

No evidence for a radius boundary at 1.7 R_Earth.

A single relationship between radius and metallicity fits the data.

Planet sizes between 1 and 4 R_Earth form a continuum.

Abstract

It has long been known that stars with high metallicity are more likely to host giant planets than stars with low metallicity. Yet the connection between host star metallicity and the properties of small planets is only just beginning to be investigated. It has recently been argued that the metallicity distribution of stars with exoplanet candidates identified by Kepler provides evidence for three distinct clusters of exoplanets, distinguished by planet radius boundaries at 1.7 R_Earth and 3.9 R_Earth. This would suggest that there are three distinct planet formation pathways for super-Earths, mini-Neptunes, and giant planets. However, as I show through three independent analyses, there is actually no evidence for the proposed radius boundary at 1.7 R_Earth. On the other hand, a more rigorous calculation demonstrates that a single, continuous relationship between planet radius and metallicity is a better fit to the data. The planet radius and metallicity data therefore provides no evidence for distinct categories of small planets. This suggests that the planet formation process in a typical protoplanetary disk produces a continuum of planet sizes between 1 R_Earth and 4 R_Earth. As a result, the currently available planet radius and metallicity data for solar-metallicity F and G stars give no reason to expect that the amount of solid material in a protoplanetary disk determines whether super-Earths or mini-Neptunes are formed.