Probing Thermal Gradients Of Habitable-zone Rocky Planets Using Direct Imaging As An Anti-indicator Of A Global Surface Ocean

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 ∼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.

Relation between the elongation and the distance from Earth for the inner edge of the habitable zone around G2- (blue), M1- (gold) and M8-type (red) stars. The stellar radius, R⋆, and the effective temperature, T_eff, are assumed to be {R⋆, T⋆, eff} = {R⊙, 5800 K}, {0.5R_⊙, 3600 K}, {0.1R_⊙, 2500 K} for an G2-, M1-, and M8-type star, respectively. The inner edge here is calculated based on R. K. Kopparapu et al. (2013). Known planets are overlaid with labels and color-coded according to the effective temperature of their host stars. The vertical lines indicate the range of elongation as the planets move between 30◦ < α < 150◦ . The gray filled region indicates that the elongation is smaller than the inner working angle (IWA), assumed to be 10 mas at λ = 10 µm and 20 mas at λ = 20 µm. — astro-ph.EP

Yuka Fujii, Daniel Angerhausen, Taro Matsuo, Eric T. Wolf

Comments: 18 pages, 11 figures, 2 tables, under review
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2512.16575 [astro-ph.EP] (or arXiv:2512.16575v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2512.16575
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Submission history
From: Yuka Fujii
[v1] Thu, 18 Dec 2025 14:13:33 UTC (4,824 KB)
https://arxiv.org/abs/2512.16575
Astrobiology, Exoplanet,