The Eclipse Mapping Null Space: Comparing Theoretical Predictions with Observed Maps

TL;DR

This paper explores the mathematical limitations of exoplanet eclipse mapping, specifically the null space, and proposes methods to compare theoretical models with observations more accurately, enhancing our understanding of exoplanet atmospheres with JWST data.

Contribution

It introduces a detailed analysis of the eclipse-mapping null space, connects it with eigenmapping, and demonstrates how to incorporate null-space uncertainties into atmospheric map retrievals.

Findings

Null space affects the observability of map patterns.

Transforming GCM models to observable modes improves comparison.

Null-space uncertainties can be incorporated into retrievals.

Abstract

High-precision exoplanet eclipse light curves, like those possible with JWST, enable flux and temperature mapping of exoplanet atmospheres. These eclipse maps will have unprecedented precision, providing an opportunity to constrain current theoretical predictions of exoplanet atmospheres. However, eclipse mapping has unavoidable mathematical limitations because many map patterns are unobservable. This ``null space'' has implications for making comparisons between predictions from general circulation models (GCMs) and the observed planet maps, and, thus, affects our understanding of the physical processes driving the observed maps. We describe the eclipse-mapping null space and show how GCM forward models can be transformed to their observable modes for more appropriate comparison with retrieved eclipse maps, demonstrated with applications to synthetic data of an ultra-hot Jupiter and a cloudy warm Jupiter under JWST-best-case- and extreme-precision observing scenarios. We show that the effects of the null space can be mitigated and manipulated through observational design, and JWST exposure times are short enough to not increase the size of the null space. Furthermore, we show the mathematical connection between the null space and the ``eigenmapping'' method, demonstrating how eigenmaps can be used to understand the null space in a model-independent way. We leverage this connection to incorporate null-space uncertainties in retrieved maps, which increases the uncertainties to now encompass the ground truth for synthetic data. The comparisons between observed maps and forward models that are enabled by this work, and the improved eclipse-mapping uncertainties, will be critical to our interpretation of multidimensional aspects of exoplanets in the JWST era.