1:45 PM - 2:00 PM
[MIS08-01] The 3D effects on thermal emission spectra of directly-imaged rocky planets and the
implication for the presence of ocean
Keywords:Exoplanets, Habitability, planetary atmospheres
Constraining the presence of surface liquid water on exoplanets is a crucial step in astrobiological investigations but remains a challenging task. While reflected-light observations have been proposed as a method to detect oceans based on the characteristic scattering properties of surface liquid water (such as specular reflection, increased reflectivity at grazing angles, and otherwise low reflectivity) each of these signatures has its own detection limitations.
Observations of planetary thermal emission provide a complementary approach. Previous studies have shown that the thermal phase variation (i.e., time variation in disk-integrated planetary thermal emission due to rotation) is significantly flatter for tidally-locked planets globally covered with liquid water than for desert planets with otherwise similar properties (Yang et al., 2013). This difference arises from the cloud decks near the substellar point as well as the relatively homogeneous surface temperature, the latter driven by efficient heat transport via water vapor and, depending on the orbit, oceanic heat transport. Conversely, the large temperature gradient of the planetary surface would indicate the absence of surface liquid water, providing a useful criterion for prioritizing targets for in-depth observations and contextualizing atmospheric composition analyses.
While thermal phase curves of rocky planets have been successfully observed for transiting planets using combined star-plus-planet flux measurements, characterizing those of non-transiting planets may require direct imaging. Direct imaging of nearest potentially habitable planets in thermal emission has been planned with the extremely large ground-based telescopes (Bowens et al., 2021). Additionally, the Large Interferometer For Exoplanets (LIFE; Quanz et al., 2022), a space-based infrared nulling interferometer, is designed to survey several dozen nearby potentially habitable planets across a range of stellar spectral types. However, assessments of the scientific potential of these direct imaging projects have so far focused on detecting atmospheric species, often using one-dimensional (1D) models, leaving the detectability of horizontal temperature gradients largely unexplored.
This study investigates the potential of future direct imaging missions to constrain the thermal gradients of potentially habitable planets. To this end, we first calculate the three-dimensional atmospheric structures of a benchmark exoplanet, Teegarden’s Star b, with and without a global ocean, using the general circulation model (GCM) ROCKE-3D (Way et al., 2017). We then compute the disk-integrated thermal emission spectra with varying geometrical configurations, and estimate observational noise levels assuming the LIFE mission using LIFEsim (Dannert et al., 2022).
Our results show that for Teegarden’s Star b with an orbital inclination of 60 degree, the thermal phase variation at 8–9 µm reaches 200% for a 1-bar clear CO2 atmosphere, whereas for an ocean-covered planet, the variation is only a few tens of percent. Such difference is easier to detect compared to detailed spectral features and can be used to filter promising targets. Furthermore, the three-dimensional temperature structure modulates snapshot spectra in two key ways: (1) deviation of the continuum from the expected blackbody curve due to horizontal temperature gradients and (2) emission features near the edges of absorption bands resulting from vertical temperature inversions away from the substellar point. These spectral signatures, which can be detected in high-SNR snapshot spectra alongside atmospheric species, provide an alternative method for studying temperature structures when phase curves are not well constrained—for instance, in near-face-on orbits or when absolute planetary flux measurements are uncertain. Finally, we extend our detectability analysis to a broader range of targets and assess observational yields under various mission configurations.
Observations of planetary thermal emission provide a complementary approach. Previous studies have shown that the thermal phase variation (i.e., time variation in disk-integrated planetary thermal emission due to rotation) is significantly flatter for tidally-locked planets globally covered with liquid water than for desert planets with otherwise similar properties (Yang et al., 2013). This difference arises from the cloud decks near the substellar point as well as the relatively homogeneous surface temperature, the latter driven by efficient heat transport via water vapor and, depending on the orbit, oceanic heat transport. Conversely, the large temperature gradient of the planetary surface would indicate the absence of surface liquid water, providing a useful criterion for prioritizing targets for in-depth observations and contextualizing atmospheric composition analyses.
While thermal phase curves of rocky planets have been successfully observed for transiting planets using combined star-plus-planet flux measurements, characterizing those of non-transiting planets may require direct imaging. Direct imaging of nearest potentially habitable planets in thermal emission has been planned with the extremely large ground-based telescopes (Bowens et al., 2021). Additionally, the Large Interferometer For Exoplanets (LIFE; Quanz et al., 2022), a space-based infrared nulling interferometer, is designed to survey several dozen nearby potentially habitable planets across a range of stellar spectral types. However, assessments of the scientific potential of these direct imaging projects have so far focused on detecting atmospheric species, often using one-dimensional (1D) models, leaving the detectability of horizontal temperature gradients largely unexplored.
This study investigates the potential of future direct imaging missions to constrain the thermal gradients of potentially habitable planets. To this end, we first calculate the three-dimensional atmospheric structures of a benchmark exoplanet, Teegarden’s Star b, with and without a global ocean, using the general circulation model (GCM) ROCKE-3D (Way et al., 2017). We then compute the disk-integrated thermal emission spectra with varying geometrical configurations, and estimate observational noise levels assuming the LIFE mission using LIFEsim (Dannert et al., 2022).
Our results show that for Teegarden’s Star b with an orbital inclination of 60 degree, the thermal phase variation at 8–9 µm reaches 200% for a 1-bar clear CO2 atmosphere, whereas for an ocean-covered planet, the variation is only a few tens of percent. Such difference is easier to detect compared to detailed spectral features and can be used to filter promising targets. Furthermore, the three-dimensional temperature structure modulates snapshot spectra in two key ways: (1) deviation of the continuum from the expected blackbody curve due to horizontal temperature gradients and (2) emission features near the edges of absorption bands resulting from vertical temperature inversions away from the substellar point. These spectral signatures, which can be detected in high-SNR snapshot spectra alongside atmospheric species, provide an alternative method for studying temperature structures when phase curves are not well constrained—for instance, in near-face-on orbits or when absolute planetary flux measurements are uncertain. Finally, we extend our detectability analysis to a broader range of targets and assess observational yields under various mission configurations.