Understanding the Effects of Systematics in Exoplanetary Atmospheric Retrievals

TL;DR

This paper investigates how correlated noise and systematics affect exoplanet atmospheric retrievals, revealing biases and limitations in current methods, especially with different observational instruments.

Contribution

It demonstrates the impact of correlated noise on retrieval accuracy, highlighting the importance of accounting for systematics in exoplanet atmospheric analysis.

Findings

Correlated noise can cause overfitting and bias in atmospheric parameters.

Higher precision data may lead to larger errors due to correlated noise.

Distinguishing correlated noise is challenging with HST but possible with JWST.

Abstract

Retrieval of exoplanetary atmospheric properties from their transmission spectra commonly assumes that the errors in the data are Gaussian and independent. However, non-Gaussian noise can occur due to instrumental or stellar systematics and merging discrete datasets. We investigate the effect of correlated noise and constrain the potential biases incurred in the retrieved posteriors. We simulate multiple noise instances of synthetic data and perform retrievals to obtain statistics of goodness-of-retrieval for varying noise models. We find that correlated noise allows for overfitting the spectrum, thereby yielding better goodness-of-fit on average but degrading the overall accuracy of retrievals. In particular, correlated noise can manifest as an apparent non-Rayleigh slope in the optical range, leading to an incorrect estimate of cloud/haze parameters. We also find that higher precision causes correlated results to be further off from the input values in terms of estimated errors. As such, we emphasize that caution must be taken in analyzing retrieved posteriors and that estimated parameter uncertainties are best understood as lower limits. Finally, we show that while correlated noise cannot be be reliably distinguished with HST observations, inferring its presence and strength may be possible with JWST observations.