[astro-ph.EP] The JWST Rocky Worlds Director’s Discretionary Time (RWDDT) program uses secondary eclipse observations to determine the conditions under which rocky exoplanets around M dwarfs retain their atmospheres.
We present RWDDT MIRI/F1500W observations of LTT 1445 A b and c, the transiting rocky siblings in the nearest such system (6.9 pc), comprising two eclipses of each planet across three visits. We perform three independent reductions and a joint fit that includes all existing JWST data and 220 radial velocities, improving the precision on the planetary radii, masses, and orbital parameters by factors of 1.6-2.3.
We measure eclipse depths of 157+18−17 ppm and 154+54−37 ppm for planets b and c, corresponding to 15 μm brightness temperatures of 528+39−29 K and 625+109−79 K, both consistent with the dark, bare rock limit.
For planet b, combining these data with the archival MIRI/LRS spectrum in a Bayesian retrieval limits the CO2 partial pressure to <0.6 mbar and the surface pressure to ≲0.1 bar, while forward models exclude thick aerosol layers that could mimic a bare surface in transmission. We conclude that planet b has no substantial atmosphere.
For planet c, a potentially grazing eclipse and overlapping events leave the depth poorly constrained; the data rule out only thick, CO2-bearing atmospheres (≳1 bar with ≥1% CO2). LTT 1445 A b is the planet closest to the canonical XUV-driven Cosmic Shoreline observed in eclipse so far. Its lack of a substantial atmosphere implies that the Cosmic Shoreline cannot lie much farther toward the airless side than nominally predicted.
The observed eclipse spectrum of LTT 1445 A b against various HELIOS forward models of atmospheres and surfaces. Each panel focuses on: bare surfaces (top right), pure CO2 atmospheres of varying pressures (top left), models with different types of aerosols (bottom left), and 0.1 bar atmospheres with single absorbers (bottom right). The σ values are calculated from a two-sided χ 2 distribution with dof = 17. The surface models show a range of surface roughness from RMS 0–50 deg with the center at 30 deg, wherein RMS=0 corresponds to a smooth sphere. We show the F1500W bandpass function weighted by the stellar spectrum. The blackbody emission at the predicted temperature is shown for a grid of Bond albedo values for bare surfaces and no redistribution (f = 2/3) and full redistribution (f = 1/4) for atmospheres, which do not consider any spectral effects; the small wiggles in these curves are from the stellar spectrum. We do not show this plot in brightness temperature as the 1σ uncertainties in the LRS eclipse spectrum are too large. — [astro-ph.EP]
Same as Figure 10 but for LTT 1445 A c. The large observed uncertainty allows for a wide range of atmosphere and surface scenarios; we show the models that disagree at > 2σ in the bottom right panel, which are generally thick atmospheres. Note that the measured eclipse depth is slightly non-Gaussian (see Figure 7); the σ values are calculated from the equivalent percentiles to a Gaussian distribution.– [astro-ph.EP]
Jegug Ih, Shubham Kanodia, Nicholas Wogan, Mark Fortune, Natalie H. Allen, Hannah Diamond-Lowe, Mike Greklek-McKeon, Måns Holmberg, Jonathan Brande, Reza Ashtari, Katherine A. Bennett, Jacob Lustig-Yaeger, Megan Weiner Mansfield, Andrea S.J. Lin, Caleb I. Cañas, Te Han, Jessica Libby-Roberts, Suvrath Mahadevan
Comments: Submitted to AAS Journals, comments welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2611.17300 [astro-ph.EP] (or arXiv:2611.17300v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2611.17300
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Submission history
From: Nicholas Wogan
[v1] Thu, 12 Nov 2026 18:55:27 UTC (8,987 KB)
https://arxiv.org/abs/2611.17300
Astrobiology, Exoplanet,
