Impact of Variable Photospheric Radius on Exoplanet Atmospheric Retrievals

TL;DR

This paper investigates how assuming a constant photospheric radius in exoplanet atmospheric retrieval models can lead to biases, especially for low gravity planets, and recommends accounting for radius variability in future analyses.

Contribution

It demonstrates the bias introduced by ignoring wavelength-dependent radius variations in emission spectrum retrievals, particularly for low gravity exoplanets.

Findings

Bias occurs in low gravity planets when using constant radius models.

High gravity planets are less affected by radius assumption biases.

Accounting for variable radius improves retrieval accuracy.

Abstract

Inverse techniques are used to extract information about an exoplanet's atmosphere. These techniques are prone to biased results if the appropriate forward model is not used. One assumption used in a forward model is to assume that the radius of the planet is constant with wavelength, however a more realistic assumption is that the photospheric radius varies with each wavelength. We explore the bias induced when attempting to extract the molecular abundance from an emission spectrum which was generated with a variable radius. We find that for low gravity planets, the retrieval model is not able to fit the data if a constant radius model is used. We find that biased results are obtained when studying a typical hot Jupiter in the MIRI LRS wavelength range. Finally, we show that high gravity planets do not suffer a bias. We recommend that future spectral retrievals that interpret exoplanet emission spectra should take into account a variable radius.