The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets (1703.10375v2)

Abstract: The size of a planet is an observable property directly connected to the physics of its formation and evolution. We used precise radius measurements from the California-Kepler Survey (CKS) to study the size distribution of 2025 $\textit{Kepler}$ planets in fine detail. We detect a factor of $\geq$2 deficit in the occurrence rate distribution at 1.5-2.0 R${\oplus}$. This gap splits the population of close-in ($P$ < 100 d) small planets into two size regimes: R$_P$ < 1.5 R${\oplus}$ and R$P$ = 2.0-3.0 R${\oplus}$, with few planets in between. Planets in these two regimes have nearly the same intrinsic frequency based on occurrence measurements that account for planet detection efficiencies. The paucity of planets between 1.5 and 2.0 R${\oplus}$ supports the emerging picture that close-in planets smaller than Neptune are composed of rocky cores measuring 1.5 R${\oplus}$ or smaller with varying amounts of low-density gas that determine their total sizes.

• The paper identifies a statistically significant gap in the radius distribution of small exoplanets between 1.5 and 2.0 Earth radii using a sample of 2025 planets.
• A sophisticated methodology employing weighted kernel density estimation and completeness corrections underpins the robust classification of rocky and gaseous planets.
• Findings support photoevaporation models and provide practical guidance for future exoplanet surveys and planetary composition studies.

Analysis of the Radius Gap in Small Exoplanets Detected by Kepler

The paper undertaken by Fulton et al. investigates a significant feature in the exoplanet size distribution, known as the "radius gap," by leveraging data from the California-Kepler Survey (CKS). This analysis is rooted in precise radius measurements derived from the CKS, which enhances the understanding of exoplanet demographics in comparison to earlier assessments that relied on more approximate stellar radii.

Methodology and Data Analysis

The authors examine a large sample of Kepler planets, totaling 2025, with a specific focus on those within a short orbital period (P < 100 days). The research identifies a notable deficit in planet occurrence rates between 1.5 and 2.0 Earth radii (R⊕), effectively splitting the population of close-in small planets into two primary size regimes: planets with radii less than 1.5 R⊕ and those between 2.0 and 3.0 R⊕.

The paper's methodology involves numerous comprehensive simulations to improve detection completeness corrections and to assess the impact of systematic errors in stellar radius measurements. Furthermore, the analysis applies weighted kernel density estimation (wKDE) to calculate the completeness-corrected planet radius distribution. The inclusion of observational data combined with sophisticated statistical analyses has solidified the statistical significance of this gap, suggesting it is not an artifact of observational biases.

Theoretical Implications

The detected radius gap provides key insights into the compositional differentiation of small exoplanets. The gap supports a model where small planets (< 1.6 R⊕) are predominantly rocky with little to no gaseous envelope, whereas larger planets (2.0–3.0 R⊕) possess substantial gaseous envelopes. These characteristic sizes hint at varying core formation processes and volatile retention capabilities.

This feature aligns with theoretical models of planet formation that include photoevaporation—a process where intense stellar radiation strips away the gaseous envelopes of close-in, low-mass planets—thereby leaving behind rocky cores. This interpretation is supported by the observed bifurcation in planet size distributions that correlates with different irradiation environments, as well as the apparent dependency of the gap's location on the stellar and orbital parameters.

Practical Implications

Discovering the radius gap offers practical implications for future exoplanetary research and characterization. This work provides a framework for distinguishing between super-Earths and sub-Neptunes based on their size, significantly aiding the classification of exoplanetary systems. Moreover, it informs the focus of both ground-based and space telescopic surveys, facilitating the exploration and modeling of planetary composition and formation history in the context of stellar neighborhood effects.

Future Research Directions

To further explicate the causes behind the radius gap, future research should aim to extend the sample of precisely characterized planets, particularly through upcoming missions such as TESS and PLATO, which are expected to provide additional mass and radius data for a large number of exoplanets. Additionally, integrating data on planetary atmospheres via spectroscopic analysis—such as that which can be performed with JWST—will be crucial for constraining models of gaseous envelope accretion and loss.

In conclusion, this paper expands the astrophysical understanding of small exoplanets, unveiling distinct compositional regimes that have far-reaching implications for planetary formation theories. The radius gap represents a pivotal observational feature that challenges existing models, propelling forward the discourse on planet formation and evolution.