The hidden depths of planetary atmospheres

TL;DR

This paper investigates how atmospheric refraction creates a hidden boundary in planetary transmission spectra, affecting the visibility of spectral features and varies with planetary and observational parameters, aiding future telescope studies.

Contribution

It combines refractive theory with spectral analysis to estimate the refractive boundary's location, providing a quick method to assess refraction effects in exoplanet atmospheres.

Findings

More than four extra scale heights are hidden in exoplanet transits compared to solar system planets.

Refraction effects vary with atmospheric composition, temperature, and host star type.

Refraction can significantly flatten spectra, especially in terrestrial CO2-rich atmospheres.

Abstract

Atmospheric regions below a refractive boundary are hidden in limb observations. Refraction thus creates a gray continuum in the planet's transmission spectrum which can hide spectral features associated with sources of atmospheric opacity. We combine refractive theory with recent analytical advances describing the effects of surfaces and clouds on transmission spectra, to express the location of this boundary in atmospheric opacity space, both for atomic and molecular extinction, as well as collision-induced absorption. This allows one to quickly estimate how refraction affects spectral features in well-mixed atmospheres. We show that differences in the geometry of limb observations between solar system planets and exoplanets leads to different locations of this boundary, and that more than four extra scale heights of atmosphere are hidden in exoplanet transits compared to solar system observations of cold gas giants. We explore how the location of this refractive boundary in exoplanet transits changes in a well-mixed isothermal atmosphere with its temperature and composition, the spectral type of the planet's host star, and the size of the planet. We demonstrate that five extra scale heights of atmosphere are hidden in a terrestrial planet with a CO$_2$ atmosphere compared to a helium atmosphere, resulting in a flatter spectrum than from its smaller scale height alone. We provide results for a few exoplanets, notably those in the TRAPPIST-1 system, to help the scientific community estimate the impact of refraction on the size of spectral features without radiative transfer calculations, and thus help refine planned James Web Space Telescope observations.