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Hot Rocks Survey. V. Secondary Eclipse Photometry of GJ 3473 b with JWST/MIRI

Holmberg, Måns ; Diamond-Lowe, Hannah ; Mendonça, João M. ; Kitzmann, Daniel ; Espinoza, Néstor ; Allen, Natalie H. ; August, Prune C. ; Fortune, Mark ; Gressier, Amélie and Ih, Jegug , et al. (2026) In Astronomical Journal 171(4).
Abstract

JWST is transforming our ability to characterize small exoplanets, from sub-Neptunes to rocky worlds. A key open question is whether highly irradiated rocky planets can retain atmospheres or are stripped bare by stellar irradiation—a boundary that remains to be mapped observationally. Here we present the first JWST secondary eclipse observations of the rocky exoplanet GJ 3473 b, obtained with MIRI F1500W photometry. Using four visits, we confidently detect the eclipse at an average depth of 186 ± 45 ppm, somewhat lower than expected for a blackbody. We test a wide range of data reduction and analysis assumptions and provide new insights into MIRI detector settling behavior that will benefit future observations. We model a suite of... (More)

JWST is transforming our ability to characterize small exoplanets, from sub-Neptunes to rocky worlds. A key open question is whether highly irradiated rocky planets can retain atmospheres or are stripped bare by stellar irradiation—a boundary that remains to be mapped observationally. Here we present the first JWST secondary eclipse observations of the rocky exoplanet GJ 3473 b, obtained with MIRI F1500W photometry. Using four visits, we confidently detect the eclipse at an average depth of 186 ± 45 ppm, somewhat lower than expected for a blackbody. We test a wide range of data reduction and analysis assumptions and provide new insights into MIRI detector settling behavior that will benefit future observations. We model a suite of airless surfaces with varied compositions, textures, and degrees of space weathering, as well as idealized atmospheric scenarios including the possibility of atmospheric collapse. Both atmospheric and bare-rock interpretations remain consistent with the data, but we exclude thick CO2 atmospheres, placing a 95% credible upper limit of 1.2–6.5 bar on the surface pressure. We also find tentative evidence for visit-to-visit variability in eclipse depth (33–371 ppm), though additional data are required to confirm this. Our results highlight the challenges and intrinsic degeneracies in interpreting MIRI F1500W eclipse measurements of rocky exoplanets, indicating that such observations alone may not uniquely distinguish between bare-rock and atmospheric scenarios. Future spectroscopic or phase-curve observations will be required to determine whether or not GJ 3473 b hosts a substantial atmosphere.

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type
Contribution to journal
publication status
published
subject
in
Astronomical Journal
volume
171
issue
4
article number
251
publisher
IOP Publishing
external identifiers
  • scopus:105034222544
ISSN
0004-6256
DOI
10.3847/1538-3881/ae4c45
language
English
LU publication?
yes
id
c8c898a3-7d7c-4673-8c91-94bd93221f65
date added to LUP
2026-04-30 12:12:02
date last changed
2026-04-30 12:12:33
@article{c8c898a3-7d7c-4673-8c91-94bd93221f65,
  abstract     = {{<p>JWST is transforming our ability to characterize small exoplanets, from sub-Neptunes to rocky worlds. A key open question is whether highly irradiated rocky planets can retain atmospheres or are stripped bare by stellar irradiation—a boundary that remains to be mapped observationally. Here we present the first JWST secondary eclipse observations of the rocky exoplanet GJ 3473 b, obtained with MIRI F1500W photometry. Using four visits, we confidently detect the eclipse at an average depth of 186 ± 45 ppm, somewhat lower than expected for a blackbody. We test a wide range of data reduction and analysis assumptions and provide new insights into MIRI detector settling behavior that will benefit future observations. We model a suite of airless surfaces with varied compositions, textures, and degrees of space weathering, as well as idealized atmospheric scenarios including the possibility of atmospheric collapse. Both atmospheric and bare-rock interpretations remain consistent with the data, but we exclude thick CO<sub>2</sub> atmospheres, placing a 95% credible upper limit of 1.2–6.5 bar on the surface pressure. We also find tentative evidence for visit-to-visit variability in eclipse depth (33–371 ppm), though additional data are required to confirm this. Our results highlight the challenges and intrinsic degeneracies in interpreting MIRI F1500W eclipse measurements of rocky exoplanets, indicating that such observations alone may not uniquely distinguish between bare-rock and atmospheric scenarios. Future spectroscopic or phase-curve observations will be required to determine whether or not GJ 3473 b hosts a substantial atmosphere.</p>}},
  author       = {{Holmberg, Måns and Diamond-Lowe, Hannah and Mendonça, João M. and Kitzmann, Daniel and Espinoza, Néstor and Allen, Natalie H. and August, Prune C. and Fortune, Mark and Gressier, Amélie and Ih, Jegug and Valdés, Erik Meier and Zgraggen, Merlin and Buchhave, Lars A. and Demory, Brice Olivier and Fisher, Chloe and Gibson, Neale P. and Heng, Kevin and Prinoth, Bibiana and Burgasser, Adam J.}},
  issn         = {{0004-6256}},
  language     = {{eng}},
  number       = {{4}},
  publisher    = {{IOP Publishing}},
  series       = {{Astronomical Journal}},
  title        = {{Hot Rocks Survey. V. Secondary Eclipse Photometry of GJ 3473 b with JWST/MIRI}},
  url          = {{http://dx.doi.org/10.3847/1538-3881/ae4c45}},
  doi          = {{10.3847/1538-3881/ae4c45}},
  volume       = {{171}},
  year         = {{2026}},
}