Impact of Climate States and Seasons on Future Exo-Earth Observations

Abstract

Many planetary parameters impact the climate state of Earth-like exoplanets and could vary significantly from those on Earth. However, some of these parameters may be impossible to observe, causing ambiguity in determining exoplanet climate and characterizing their atmospheric features. We explore how distinct planetary climate states impact their reflectance spectra to reduce uncertainty in the interpretation of future direct imaging observations, such as with the Habitable Worlds Observatory. We find that worlds with the same atmospheric composition but distinct climate states have notable differences in apparent albedos and feature detectability. An additional consequence is that the exposure time required to detect atmospheric features and biosignatures, such as O$_2$, will depend on climate state, with icier worlds being more favorable for biosignature detection while ice-limited worlds may be more habitable. We find that clouds improve the strength and detectability of atmospheric features in reflected light, especially for ice-limited low albedo worlds. We find temporal variation in the strength of spectra at different seasons on high obliquity worlds, causing the required time to resolve atmospheric features to vary between the equinoxes and solstices. This abiogenic seasonality could be detectable through repeated direct imaging observations and may help inform the planetary climate state, especially in combination with constraints on inclination and mass. Our work elevates the importance of astrometry performed concurrently with direct imaging for characterizing climate state and planetary habitability of exoplanets. Interpretation of future spectroscopic observations must also account for temporal variations created by obliquity when searching for biosignatures.