Trends in Atmospheric Properties of Neptune-Size Exoplanets

TL;DR

This study analyzes atmospheric data of warm Neptune-sized exoplanets, revealing correlations between spectral feature amplitudes and planetary parameters, which could inform target selection for future JWST observations.

Contribution

It identifies new correlations between atmospheric spectral features and planet properties, and provides an analytic tool to optimize JWST observing strategies.

Findings

Spectral feature amplitude correlates with equilibrium temperature and H/He mass fraction.

Lower T_eq and higher metallicity atmospheres may have more hazes.

Potential reduction in JWST targets by up to a factor of eight.

Abstract

Precise atmospheric observations have been made for a growing sample of warm Neptunes. Here we investigate the correlations between these observations and a large number of system parameters to show that, at 95% confidence, the amplitude of a warm Neptune's spectral features in transmission correlates with either its equilibrium temperature (T_eq) or its bulk H/He mass fraction (f_HHe) --- in addition to the standard kT/mg scaling. These correlations could indicate either more optically-thick, photochemically-produced hazes at lower T_eq and/or higher-metallicity atmospheres for planets with smaller radii and lower f_HHe. %Since hazes must exist in some of these planets, we favor the former explanation. We derive an analytic relation to estimate the observing time needed with JWST/NIRISS to confidently distinguish a nominal gas giant's transmission spectrum from a flat line. Using this tool, we show that these possible atmospheric trends could reduce the number of expected TESS planets accessible to JWST spectroscopy by up to a factor of eight. Additional observations of a larger sample of planets are required to confirm these trends in atmospheric properties as a function of planet or system quantities. If these trends can be confidently identified, the community will be well-positioned to prioritize new targets for atmospheric study and eventually break the complex degeneracies between atmospheric chemistry, composition, and cloud properties.