A revisit of the Mass-Metallicity Trends in Transiting Exoplanets

TL;DR

This study investigates the relationship between planetary mass and atmospheric metallicity in transiting exoplanets, highlighting current data limitations and proposing future observational strategies to better understand planet formation processes.

Contribution

The paper provides a detailed analysis of host star and planetary atmospheric abundances, demonstrating the current dataset's limitations and recommending sample sizes for future studies to detect mass-metallicity trends.

Findings

Current data cannot distinguish between different abundance-mass models.

A minimum of 58 planets with water measurements is needed for strong trend detection.

Future JWST data will significantly improve trend detection capabilities.

Abstract

The two prevailing planet formation scenarios, core-accretion and disk instability, predict distinct planetary mass-metallicity relations. Yet, the detection of this trend remains challenging due to inadequate data on planet atmosphere abundance and inhomogeneities in both planet and host stellar abundance measurements. Here we analyze high-resolution spectra for the host stars of 19 transiting exoplanets to derive the C, O, Na, S, and K abundances, including planetary types from cool mini-Neptunes to hot Jupiters ($T_{\rm eq}\ \sim$ 300 - 2700 K; planet radius $\sim$ 0.1 - 2 $R_{\rm J}$). Our Monte Carlo simulations suggest that the current dataset, updated based on Welbanks et al. 2019, is unable to distinguish between a linear relation and an independent distribution model for the abundance-mass correlation for water, Na, or K. To detect a trend with strong evidence (Bayes factor > 10) at the 2$\sigma$ confidence interval, we recommend a minimum sample of 58 planets with HST measurements of water abundances coupled with [O/H] of the host stars, or 45 planets at the JWST precision. Coupled with future JWST or ground-based high resolution data, this well-characterized sample of planets with precise host star abundances constitute an important ensemble of planets to further probe the abundance-mass correlation.