Exoplanetology: Exoplanets & Exomoons

Reliable Detections of Atmospheres on Rocky Exoplanets with Photometric JWST Phase Curves

By Keith Cowing
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astro-ph.EP
September 9, 2024
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Reliable Detections of Atmospheres on Rocky Exoplanets with Photometric JWST Phase Curves
Comparing the ranges of possible F1500W day-side emission (for five observed eclipses) from the bare rock model, the atmospheric model with zero heat redistribution, and the atmospheric model with full heat redistribution. Each observation is modelled as the average of five secondary eclipses, with a baseline of four eclipse durations outside the eclipse. Each plotted range compiles the relevant models from the suites like those in Figure 2; for example, the left-most range shows the range of possible F1500W emission values for TRAPPIST-1 c due to all the different surface types in Table 1. The planets are ordered by equilibrium temperature. TRAPPIST-1 c and GJ 486 b are highlighted in bold, as they are plotted in more detail in Figure 2. The bare-rock surfaces can only be distinguished from the atmospheres with zero heat redistribution on GJ 806 b and LTT 1445 A b (that is, the uncertainties on their lower limits do not overlap). The bare-rock surfaces can be distinguished from some of the atmospheres with full heat redistribution on more planets, for a subset of atmospheres with both full heat redistribution and a low CO2 abundance. — astro-ph.EP

The distribution of different types of atmospheres and surfaces on rocky planets is one of the major questions in exoplanet astronomy, but there are currently no published unambiguous detections of atmospheres on any rocky exoplanets.

The MIRI instrument on JWST can measure thermal emission from tidally locked rocky exoplanets orbiting small, cool stars. This emission is a function of their surface and atmospheric properties, potentially allowing the detection of atmospheres.

One technique is to measure day-side emission to search for lower thermal emission than expected for a black-body planet due to atmospheric absorption features.

Another technique is to measure phase curves of thermal emission to search for night-side emission due to atmospheric heat redistribution. In this work we compare strategies for detecting atmospheres on rocky exoplanets using these techniques.

We simulate secondary eclipse and phase curve observations in the MIRI F1500W and F1280W filters, for a range of surfaces and atmospheres on thirty exoplanets selected for their F1500W signal-to-noise ratio. Our results show that secondary eclipse observations are highly degenerate between surfaces and atmospheres, given the wide range of potential surface albedos.

We also show that thick atmospheres can support emission consistent with a black-body planet in these filters.

These two results make it difficult to unambiguously detect or rule out atmospheres using their photometric day-side emission, except in a subset of CO2-dominated atmospheres. We suggest that an F1500W phase curve could instead be observed for a similar sample of planets, allowing the unambiguous detection of atmospheres by night-side emission.

A summary of the input data for the bare-rock and atmospheric models, alongside the thermal emission for the typical temperatures of star and planets in this paper. The top three panels show the spherical reflectances rS(λ) for each of the planetary surfaces in Table 1. The single-scattering albedo w for each surface is available at https://zenodo.org/records/13691960. The fourth panel shows the opacity of H2O and CO2 at 500K and 1 bar from DACE (Grimm et al. 2021). The fifth panel shows the throughputs of the F1280W and F1500W filters, as well as the normalised emission from a star at 2500 K, a planet at 1000 K, and a planet at 500K. — astro-ph.EP

Mark Hammond, Claire Marie Guimond, Tim Lichtenberg, Harrison Nicholls, Chloe Fisher, Rafael Luque, Tobias G. Meier, Jake Taylor, Quentin Changeat, Lisa Dang, Oliver Herbort, Johanna Teske

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2409.04386 [astro-ph.EP] (or arXiv:2409.04386v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2409.04386
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Submission history
From: Mark Hammond
[v1] Fri, 6 Sep 2024 16:25:57 UTC (829 KB)
https://arxiv.org/abs/2409.04386
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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