Exoplanetology: Exoplanets & Exomoons

Population-level Hypothesis Testing with Rocky Planet Emission Data: A Tentative Trend in the Brightness Temperatures of M-Earths

By Keith Cowing
Status Report
astro-ph.EP
December 10, 2024
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Population-level Hypothesis Testing with Rocky Planet Emission Data: A Tentative Trend in the Brightness Temperatures of M-Earths
Simulated effects of ‘bare-rock’ fresh surface albedo profiles from Hu et al. (2012a), varying regolith grain sizes of basalt, and npFe0 /graphite space weathering on simulated emission observations in the MIRI LRS bandpass (5 − 12 µm). Brightness temperature ratios in the MIRI F1500W and Spitzer IRAC2 bandpasses show similar trends. Planets too hot for solid surfaces (Tirr ≳ 1250 K) are not shown. Results are dependent on the exact stellar properties, and we include results for a GJ 367-like M1 star and a TRAPPIST-1-like M7.5 star as bounding cases defining the width of each colored band. This roughly encompasses the spread expected for M-Earths. A trend of increasing brightness temperature ratio (R) with irradiation temperature (Tirr) can be explained by grain sizes increasing with temperature or stronger space weathering on closer-in planets, as explored in §3.2. — astro-ph.EP

Determining which rocky exoplanets have atmospheres, and why, is a key goal for JWST. So far, emission observations of individual rocky exoplanets orbiting M stars (M-Earths) have not provided definitive evidence for atmospheres.

Here, we synthesize emission data for M-Earths and find a trend in measured brightness temperature (ratioed to its theoretical maximum value) as a function of instellation. However, the statistical evidence of this trend is dependent on the choice of stellar model and we consider its identification tentative.

We show that this trend can be explained by either the onset of thin/tenuous atmospheres on colder worlds, or a population of bare rocks with stronger space weathering and/or coarser regolith on closer-in worlds. Such grain coarsening may be caused by sintering near the melting point of rock or frequent volcanic resurfacing.

We also find that fresh, fine-grained surfaces can serve as a false positive to the detection of moderate atmospheric heat redistribution. However, we argue that such surfaces are unlikely given the ubiquity of space weathering in the Solar System and the low albedo of Solar System airless bodies.

Furthermore, we highlight considerations when testing rocky planet hypotheses at the population level, including the choice of instrument, stellar modeling, and how brightness temperatures are derived. Emission data from a larger sample of M-Earths will be able to confirm or reject this tentative trend and diagnose its cause.

Brandon Park Coy, Jegug Ih, Edwin S. Kite, Daniel D.B. Koll, Moritz Tenthoff, Jacob L. Bean, Megan Weiner Mansfield, Michael Zhang, Qiao Xue, Eliza M.-R. Kempton, Kay Wolhfarth, Renyu Hu, Xintong Lyu, Christian Wohler

Comments: Submitted to AAS Journals, comments welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2412.06573 [astro-ph.EP] (or arXiv:2412.06573v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2412.06573
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Submission history
From: Brandon Park Coy
[v1] Mon, 9 Dec 2024 15:27:40 UTC (1,579 KB)
https://arxiv.org/abs/2412.06573

Astrobiology

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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