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

Abstract

JWST is transforming our ability to characterise 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$\pm$45 ppm, somewhat lower than expected for a blackbody. We test a wide range of data reduction and analysis assumptions and provide new insights into MIRI detector settling behaviour that will benefit future observations. We model a suite of airless surfaces with varied compositions, textures, and degrees of space weathering, as well as idealised atmospheric scenarios including the possibility of atmospheric collapse. Both atmospheric and bare-rock interpretations remain consistent with the data, but we exclude thick CO$_2$ 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

Figures (11)

• Figure 1: Settling behaviour of the point-spread function (PSF) across the four visits. Visits 1–3 exhibit an increase in PSF width over time, while visit 4 shows the opposite trend. The two images display the flux ratio between the average of the first 20 integrations and the average of the final quarter of integrations for visits 1 and 4, respectively, prior to background subtraction. In visits 1–3, the central pixels are initially brighter and the surrounding pixels fainter, resulting in a narrow PSF at the start of the observations. In visit 4, the opposite pattern is observed, with a broader initial PSF that narrows over time.
• Figure 2: Phase-folded and systematics-corrected MIRI data and secondary eclipse model of GJ 3473 b. The unbinned and binned data are shown in light grey and purple, respectively. To detrend the data, we subtracted the predicted GP models and divided by the systematic models. The pink curve corresponds to an eclipse model with parameters from Table \ref{['tab:MIRI_params']}, corresponding to our canonical case.
• Figure 3: MIRI F1500W light curves of GJ 3473 b at secondary eclipse. The raw and binned data are shown in light grey and purple, respectively. The pink curves correspond to the median model predictions from the joint fit, while the brown data points depict the binned model. The model shown here corresponds to the TovarMendoza2022 flare model case, as described in Appendix \ref{['app:flare']}, used to model the flare-like feature during the second visit. The dotted vertical lines show the nominally masked region in the second visit. Compared to Figure \ref{['fig:miri_phase_folded']}, this figure displays the data without correcting for systematics.
• Figure 4: Single-scattering albedo of Basalt powder at varying degrees of space weathering via the formation of nanophase metallic iron particles. No added space weathering is represented by 0% iron. The spectra of the stellar irradiation and planetary flux, with 0.5% iron, are represented by solid light grey and dashed grey lines, respectively; however, these are not to scale for clarity.
• Figure 5: The span of eclipse depths of GJ 3473 b in the MIRI F1500W bandpass for different bare-rock and atmosphere scenarios, assuming that the planet is tidally locked in a 1:1 spin–orbit resonance. In the bare-rock cases, the limits arise from different materials, textures, and varying degrees of space weathering, as described in Section \ref{['sec:surface']}. In contrast, in the atmospheric scenarios, the limits are determined by varying surface pressures, which range from 0.1 mbar to 100 bar, as outlined in Section \ref{['sec:atmosphere']}. The two shades of color represent Bond albedos of 0 (darker) and 0.3 (lighter), respectively. The stars indicate the critical surface pressure (0.6-1 mbar) below which a CO$_2$ atmosphere is susceptible to collapse, as discussed in Section \ref{['sec:collapse']}. The dashed vertical line represents the eclipse depth of a blackbody. The measured average eclipse depth of GJ 3473 b is shown in black at the bottom of the figure.