Exoplanets, -moons, -comets

GJ 3929 b as the First Complete Rocky Worlds DDT Data Set

By Keith Cowing
Status Report
astro-ph.EP
June 8, 2026
Filed under , , , , ,
GJ 3929 b as the First Complete Rocky Worlds DDT Data Set
Simulated emission spectra of GJ 3929 b for various surface (left) and atmospheric (right) compositions compared to the measured eclipse depths from our FN-PCA calculation, alongside the previously reported result using only the first two visits by Xue et al. (2025). The F1500W filter transmission is shown in the dotted line at the bottom. The simulated MIRI 1500W eclipse depths for each of the models are shown as square markers in each panel, with error bars indicating the estimated model uncertainty (Section 4.4, Monaghan et al. (2026)). Small horizontal offsets between points are added for clarity. The ultramafic dunite, granite, and basalt model spectra were generated using the dunite xenolith, dalmatian granite, and K1919 basalt spectral data from Paragas et al. (2025). The thin atmosphere models were generated assuming zero heat redistribution to the night-side, and the thick atmosphere models assume full heat redistribution. Additional models and σ confidences are reported in Table 4. — astro-ph.EP

Despite their large abundance, it is still unknown whether and under what conditions rocky planets around M dwarf stars can host atmospheres.

This open question motivated the on-going Rocky Worlds DDT survey focused on searching for atmospheres on relatively low-temperature rocky exoplanets by systematically probing for the presence of day-night heat redistribution and CO2 absorption through JWST/MIRI 15 μm eclipse observations.

Here we present the analysis of the first full data set from this survey, consisting of four observations of the warm Earth-size exoplanet GJ 3929 b, with a planetary mass of 1.75+0.44-0.45 M⊕ and instellation flux of 17.3+/-0.7 S⊕.

In our analysis, we include two previously unpublished eclipse observations and find an overall eclipse depth of 118+/-22 ppm and a dayside surface brightness temperature of 641+59-64 K. This is marginally lower than the eclipse depth of 160+26-27 ppm previously reported based on only the first two observations. While the full data set remains consistent with bare rock scenarios, it also leaves more room for thin atmosphere scenarios.

Only thick CO2 atmospheres without thermal inversion remain ruled out at greater than 3σ. We also continue with lessons-learned in robustly analyzing these kind of high-precision JWST/MIRI 15 μm eclipse observations. Notably, we find that the Frame Normalized Principal Component Analysis (FN-PCA) method appears more robust against the choice of extraction aperture size, which otherwise can have a significant impact on the inferred eclipse depth and scientific conclusions when using a standard polynomial baseline detrending method.

Nicholas J. Connors, Christopher Monaghan, Bjorn Benneke, Lisa Dang, Pierre-Alexis Roy

Comments: 20 pages, 9 figures + 5 in Appendix, submitted to The Astrophysical Journal Letters. This version addresses all comments from the first review by the referee
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2606.07511 [astro-ph.EP] (or arXiv:2606.07511v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2606.07511
Focus to learn more
Submission history
From: Nicholas Connors
[v1] Fri, 5 Jun 2026 17:59:10 UTC (4,530 KB)
https://arxiv.org/abs/2606.07511

Astrobiology,

Keith Cowing

Biologist, Explorers Club Fellow, ex-NASA Space Biologist and Payload integrator, Editor of NASAWatch.com and Astrobiology.com, Lapsed climber, Explorer, Synaesthete, Former Challenger Center board member 🖖🏻

Follow on Twitter
MORE FROM Exoplanets, -moons, -comets