[Paper Review] Planet Size Distribution from the Kepler Mission and its Implications for Planet Formation

Li Zeng, S. B. Jacobsen+9 more|arXiv (Cornell University)|Jun 15, 2018

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper analyzes the size distribution of exoplanets from NASA's Kepler mission, revealing a bimodal distribution: rocky planets with radii <2 Earth radii and water-rich planets with radii >2 Earth radii, often with gaseous envelopes. The findings suggest distinct formation pathways, with rocky planets forming in hotter, dry regions and water-rich planets in cooler, icy zones, supporting core accretion models with radial segregation.

ABSTRACT

The size distribution of exoplanets is a bimodal division into two groups: Rocky planet (&lt;2 Earth radii) and water-rich planet (&gt;2 Earth radii) with or without gaseous envelope.

Motivation & Objective

• To characterize the size distribution of exoplanets detected by the Kepler space telescope.
• To investigate the physical and formation implications of the observed bimodal size distribution.
• To determine whether the size distribution reflects distinct planetary formation pathways.
• To explore the role of radial temperature gradients and volatile content in shaping planetary composition.
• To link observed size distributions to core accretion and migration models in planet formation.

Proposed method

• Analysis of Kepler mission data to identify and classify exoplanets by radius.
• Categorization of planets into two distinct size groups: <2 Earth radii (rocky) and >2 Earth radii (water-rich with or without gaseous envelopes).
• Use of statistical modeling to identify bimodal structure in the size distribution.
• Comparison of observed size distribution with theoretical models of planet formation, particularly core accretion and radial migration.
• Incorporation of thermal and compositional constraints to infer formation environments.
• Application of planetary interior modeling to assess internal structure and volatile content.

Experimental results

Research questions

• RQ1 What is the underlying structure of the exoplanet size distribution as revealed by Kepler data?
• RQ2 Why does the size distribution exhibit a bimodal peak rather than a continuous distribution?
• RQ3 What physical processes in protoplanetary disks lead to the formation of two distinct planetary classes?
• RQ4 How do temperature gradients and volatile availability influence planetary composition and size?
• RQ5 To what extent do core accretion and migration models explain the observed size distribution?

Key findings

• The exoplanet size distribution from Kepler data shows a clear bimodal structure with peaks at rocky planets (<2 Earth radii) and water-rich planets (>2 Earth radii).
• Planets with radii greater than 2 Earth radii are predominantly water-rich, often with substantial gaseous envelopes, indicating formation in cooler, icy regions of protoplanetary disks.
• The bimodal distribution suggests a physical divide in planetary formation, with rocky planets forming in hotter, dry zones and volatile-rich planets in colder, icy zones.
• The observed size distribution supports core accretion models where planetary cores grow to critical mass and trigger gas runaway accretion in specific thermal environments.
• The transition at ~2 Earth radii corresponds to a threshold in planetary composition and internal structure, likely tied to the ice line in protoplanetary disks.
• The data imply that radial migration and disk temperature profiles play a key role in determining the final size and composition of planets.