Probing thermal gradients of habitable-zone rocky planets using direct imaging as an anti-indicator of a global surface ocean

TL;DR

This paper demonstrates that horizontal temperature gradients on rocky planets lacking global oceans produce measurable phase variations and distinctive snapshot-spectrum shapes in thermal emission. By using the ROCKE-3D GCM to generate 3D atmospheres for ocean and non-ocean cases around Teegarden's Star b, and simulating LIFE-like observations, the authors show that 1D models can misinterpret 3D effects, biasing ocean inferences. Phase variations at near-IR to mid-IR wavelengths and specific continuum/absorption-band shapes in snapshot spectra offer complementary diagnostics, with phase curves being the most readily detectable in 1–2 days of observation per phase for favorable targets. The results highlight the need to incorporate 3D atmospheric structures in planning direct-imaging campaigns and show that the method is extensible to other nearby habitable-zone planets, potentially enabling population-level assessments of surface oceans with future missions like LIFE.

Abstract

Future direct-imaging missions, such as the Large Interferometer for Exoplanets (LIFE), aim to observe thermal emission from potentially habitable planets to characterize their surface environments and search for signs of life. Previous studies of directly imaged Earth-like planets have mainly examined the signatures of atmospheric composition, often using one-dimensional models, while the effect of horizontal temperature gradients has received limited attention. Because a pronounced horizontal temperature gradient may signal the absence of a global ocean, we investigate its detectability through thermal-emission direct imaging. Adopting Teegarden's Star b (zero-albedo equilibrium temperature $\sim 280$~K) as a benchmark, we compute three-dimensional atmospheric structures with and without a global ocean using the ROCKE-3D general circulation model and simulate geometry-dependent thermal emission spectra. We show that the temperature gradients that disfavor a global-ocean scenario manifest in both orbital phase variation and spectral shape of the snapshot spectra. The phase variation is more readily detectable: one-day integrations with LIFE at two orbital phases would reveal flux variations in no-ocean cases with 1-10~bar atmospheres, depending on background atmospheric composition. Shapshot spectra provide complementary diagnostics of global temperature contrast, the running brightness temperature of the continuum and detailed absorption band shapes, but require integration a few times longer. These three-dimensional effects, if neglected, can bias interpretations based on one-dimensional models. We also assess their detectability for other nearby exoplanets. Our results highlight the need to incorporate three-dimensional atmospheric structures when characterizing rocky exoplanets, both to constrain surface conditions and to avoid misinterpretation of spectral data.

Figures (11)

• Figure 1: Surface temperature maps of our GCM simulations for different surface-atmosphere scenarios (Table 1) with 1 bar (top two rows) and 10 bar (bottom row) atmospheres. The substellar point is located at the center.
• Figure 2: Longitudinal–vertical temperature structures in the equatorial region of our GCM simulations for different surface-atmosphere scenarios (Table 1) with 1 bar (top two rows) and 10 bar (bottom row) atmospheres. Temperatures are averaged over latitudes within $\pm 10^{\circ}$. Longitude $0^{\circ}$ corresponds to the permanent substellar point.
• Figure 3: The pressure levels where the vertical optical depth from the top of atmosphere becomes unity.
• Figure 4: Outgoing flux at $\lambda= 10\,\mu$m (left) and the total cloud cover fraction (right) in the case of ocean_Nc-6_1bar run. The substellar point is located at the center.
• Figure 5: Thermal emission spectra of all scenarios considered in this study at four orbital longitudes ($\phi =-90^{\circ},\,0^{\circ},\,90^{\circ},\,180^{\circ}$) with an orbital inclination of $i=60^{\circ }$. Here, the orbital longitude ($\phi$) is measured from the point of maximum dayside visibility; in the case of an edge-on orbit, $\phi =0^{\circ}$ and $\phi=180^{\circ}$ correspond to the eclipse and the transit, respectively, and $\phi = \pm 90^{\circ}$ correspond to the observations from above the eastern and western terminators relative to the substellar point. The dotted line corresponds to blackbody emission at 200 K, 250 K, 300 K, 350 K, and 400 K, from bottom to top. Note the different range of $y$-axis between the upper two and lower panels.