Exoplanets, -moons, -comets

The Persistent Thermal Anomalies In Rocky Worlds

By Keith Cowing

Status Report

astro-ph.EP

January 9, 2026

Filed under astro-ph.EP], atmosphere, exoplanet, Hubble, imaging, JWST, M-dwarf, Nancy Grace Roman Space Telescope, Rocky Worlds Director’s Discretionary Time Program, Spectroscopy, telescope

The Persistent Thermal Anomalies In Rocky Worlds — Dayside temperature scaling factor, R = Tday/Tday,max, as a function of irradiation temperature, T_irr, for (a) all observed rocky exoplanets and (b) planets with T_irr > 1500 K, which may have rock vapor atmospheres. Observed exoplanets (Table 1), excluding LHS 1478 b, are shown as black error bars. Two fits are shown: (cyan) linear fit for M-Earths shows a positive trend, while (blue) BPL fit shows a positive trend for M-Earths with T_irr < T_b = 1790 K and a negative trend for five extremely hot planets with T_irr > T_b. Both fits are better than the null hypothesis (red line in panel a), suggesting an underlying physical mechanism that produces both thermal emission excess and deficit. In (a), the melting temperatures of silicates (orange and magenta vertical lines) are annotated. In (b), R of exoplanets with rock vapor atmospheres derived from a scaling relation (D. D. B. Koll 2022) are shown in red, green, and orange for surface Bond albedo of 0, 0.3, and 0.5, respectively. Thick atmospheres with Ps much greater than the equilibrium pressure Peq are required to explain the observed thermal deficit. — astro-ph.EP

Observing the dayside thermal emissions of rocky exoplanets provides essential insights into their compositions and the presence of atmospheres.

Even though no conclusive evidence has been found for atmospheres on small rocky exoplanets orbiting M dwarfs, recent JWST observations identified puzzling thermal emission excesses: some rocky exoplanets orbiting M dwarfs have dayside emission temperatures higher than the theoretical maximum.

Theoretical maximum temperatures assume stellar irradiation as the sole energy source, implying that these planets may have internal heat sources. In this work, we simulate three possible planetary internal processes that may generate excessive heat in addition to stellar irradiation: residual heating from formation, tidal heating, and induction heating due to interactions with the stellar magnetic field.

We found that these mechanisms, even when combined, cannot explain the observed thermal emission excesses, nor can they explain a tentative positive trend in the brightness temperature scaling factor as a function of irradiation temperature.

Our results imply that planetary internal processes are unlikely to generate remotely detectable heat, so the observed thermal excesses, if astrophysical, are likely caused by stellar contamination, surface processes, or other internal processes not considered in this study. The ongoing JWST-HST Rocky Worlds Director’s Discretionary Time Program and the upcoming Nancy Grace Roman Space Telescope will provide more insights into the thermal emission of rocky exoplanets.

Zifan Lin, Tansu Daylan

Comments: 19 pages, 11 figures, 1 table, submitted to the AAS journals
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2601.00412 [astro-ph.EP] (or arXiv:2601.00412v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2601.00412
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Submission history
From: Zifan Lin
[v1] Thu, 1 Jan 2026 17:59:01 UTC (2,259 KB)
https://arxiv.org/abs/2601.00412
Astrobiology, Astrogeology, exoplanet,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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