Most 1.6 Earth-Radius Planets are not Rocky (1407.4457v2)

Abstract: The Kepler Mission, combined with ground based radial velocity (RV) follow-up and dynamical analyses of transit timing variations, has revolutionized the observational constraints on sub-Neptune-size planet compositions. The results of an extensive Kepler follow-up program including multiple Doppler measurements for 22 planet-hosting stars more than doubles the population of sub-Neptune-sized transiting planets that have RV mass constraints. This unprecedentedly large and homogeneous sample of planets with both mass and radius constraints opens the possibility of a statistical study of the underlying population of planet compositions. We focus on the intriguing transition between rocky exoplanets (comprised of iron and silicates) and planets with voluminous layers of volatiles (H/He and astrophysical ices). Applying a hierarchical Bayesian statistical approach to the sample of Kepler transiting sub-Neptune planets with Keck RV follow-up, we constrain the fraction of close-in planets (with orbital periods less than ~50 days) that are sufficiently dense to be rocky, as a function of planet radius. We show that the majority of 1.6 Earth-radius planets are too low density to be comprised of Fe and silicates alone. At larger radii, the constraints on the fraction of rocky planets are even more stringent. These insights into the size demographics of rocky and volatile-rich planets offer empirical constraints to planet formation theories, and guide the range of planet radii to be considered in studies of the occurrence rate of "Earth-like" planets, $\eta_{Earth}$.

• The paper demonstrates that most exoplanets with ~1.6 Earth radii have low density and are unlikely to be rocky, based on precise mass-radius measurements.
• It employs a hierarchical Bayesian analysis of Kepler and RV data from 22 stars to robustly assess the transition from rocky to volatile-rich planets.
• The study informs planet formation theories by refining the upper size limit for rocky planets and guiding future observational strategies.

Overview of "Most 1.6 Earth-Radius Planets are not Rocky"

The paper, authored by Leslie A. Rogers, provides a detailed examination of the composition of sub-Neptune-size exoplanets derived from transit and radial velocity (RV) data, with a concentrated effort on identifying the proportion of these planets that are dense enough to be rocky. Utilizing data obtained from the Kepler Mission and ground-based follow-ups, the paper applies hierarchical Bayesian statistical methods to assess the underlying composition characteristics within a statistical sample of exoplanets. This investigation is central to understanding the mass-radius relationship, particularly at the intriguing boundary where planets transition from rocky bodies to those with substantial volatile envelopes.

Key Findings and Statistical Analysis

The paper's central finding is that most exoplanets with radii of approximately 1.6 Earth radii ($R_{\oplus}$) are of low density and consequently not rocky. The evidence supporting this conclusion derives from analyzing Kepler's extensive data set, which includes 22 planet-hosting stars, facilitating more than double the previously available sub-Neptune planets with RV mass constraints.

A hierarchical Bayesian model is applied to this unprecedented dataset, allowing for a statistically robust assessment of the mass-radius distribution for these celestial bodies. The analysis meticulously defines the parameter space for planets that are "potentially rocky." Such planets are characterized by mass and radius combinations dense enough to preclude significant quantities of hydrogen/helium (H/He) or astrophysical ices from contributing to their transit radii. The model suggests that planets greater than ~1.6 $R_{\oplus}$ are statistically likely to possess substantial volatiles.

Implications for Planet Formation Theories

This paper's empirical constraints on planet size demographics have significant implications for planet formation theories. By identifying an upper size limit for rocky planets at shorter orbital periods, Rogers's findings inform models of planet evolution and the mechanisms behind core accretion and volatile retention. The absence of rocky planets beyond this size threshold supports theoretical predictions from core-nucleated accretion models, where substantial gas envelopes are accreted by planetary embryos of sufficient mass during their formation within protoplanetary environments.

Moreover, these findings refine the operational parameters that should define "Earth-like" within planetary classification and occurrence studies. The empirical evidence suggesting that most exoplanets above 1.6 $R_{\oplus}$ are not purely rocky advises caution when designating such planets as "Earth-like" in astrophysical contexts, such as calculating the frequency of potentially habitable worlds ($\eta_{\oplus}$).

Future Research Directions and Observational Strategies

Future research can explore the mass-radius distribution by expanding the sample size of comparably measured exoplanets and enhancing the precision of these measurements using updated methodologies or more sensitive observational technologies, such as the Transiting Exoplanet Survey Satellite (TESS) and further RV monitoring.

The paper also highlights the significance of structured approaches for selecting candidates for RV follow-up, emphasizing the importance of mass-radius surveys with well-defined criteria in expounding underlying demographic compositions. An algorithmic strategy to select transiting planet candidates might bolster the statistical reliability of subsequent studies, thereby leveraging the full potential of RV non-detections to derive population-level insights.

In conclusion, this paper serves as a pivotal reference point for interpreting sub-Neptune-size exoplanet compositions observed by initiatives such as Kepler. It underscores the nuanced interplay between planetary size, density, and composition, offering a comprehensive framework for understanding the compositional diversity and evolution within our galaxy.