GJ 3929 b: Key to Unlocking the Atmospheres of Rocky Exoplanets ✛ ✦
Context
Rocky exoplanets around M-dwarfs are the most common type of planet in the galaxy, yet their ability to hold onto an atmosphere is fiercely debated. Intense X-ray and ultraviolet radiation from the star, along with tidal locking, can completely strip a planet of its gaseous envelope. The ‘cosmic shoreline’ hypothesis—the line dividing worlds with and without atmospheres—can be tested by measuring a planet’s thermal emission during secondary eclipse. The presence of carbon dioxide or heat redistribution to the nightside are reliable indicators of an atmosphere.
Methods
The team used the MIRI instrument on JWST in the F1500W filter (15 µm) for four observations of secondary eclipses of exoplanet GJ 3929 b. Aperture photometry extracted light curves, which were then cleaned of systematic effects using two methods: a standard polynomial approach and a new technique—Frame-Normalized Principal Component Analysis (FN-PCA). In FN-PCA, principal component analysis is applied to each frame to isolate inter-pixel variations, making the result more robust to aperture size choices. They also compared against models of bare rock surfaces and atmospheres with varying CO2 content, computed with the JESTER and SCARLET codes.
Results
A combined analysis of all four eclipses yields a depth of 118±22 ppm, about 1.5σ lower than the preliminary result from the first two sessions (160+26-27 ppm). The dayside brightness temperature is 641+59-64 K, and the ratio of the maximum bare-rock temperature to the measured temperature, R=0.87±0.09, is consistent with both an airless rocky body and a thin atmosphere. Dense carbon dioxide atmospheres with full heat redistribution are ruled out at >3σ. Eclipse depth variability of up to 2.5σ between individual visits was detected, hinting at imperfect tidal locking or a transient atmosphere. A slight orbital eccentricity was confirmed: e cos ω = −0.0225±0.0010, corresponding to a 54±2 minute offset of the eclipse center from a circular orbit.
Implications
The result strengthens the cosmic shoreline hypothesis: most warm rocky exoplanets around M-dwarfs likely lack dense atmospheres. For the first time in the Rocky Worlds DDT survey, it’s shown that even four JWST visits may not give a definitive answer if a planet has moderate albedo or a thin atmosphere. This underscores the need for population studies and combining photometry with other techniques, such as spectroscopy over a broader wavelength range.
Future development
The topic will evolve as data accumulate from the Rocky Worlds DDT survey, which will study dozens more temperate exoplanets. Statistical analysis is expected to map out the cosmic shoreline from both sides. Additionally, improving detrending methods like FN-PCA and cross-validation between different pipelines will boost confidence in the conclusions. In the future, similar observations with JWST and upcoming telescopes (such as the Habitable Worlds Observatory) could directly detect biosignatures in the atmospheres of Earth-like planets.
Impact
This work directly impacts our understanding of the habitability of rocky worlds in the galaxy and the strategy for searching for biosignatures. The results will be used in planning future missions and ground-based surveys.
Next steps
Plans include continued monitoring of GJ 3929 b to better characterize eclipse variability and independent data analysis with other pipelines. Observations of the remaining targets in the Rocky Worlds DDT survey have begun.
Key open problems
The study directly addresses the problem of how terrestrial planets acquire and retain atmospheres, and tests the cosmic shoreline hypothesis, which predicts the fate of volatiles based on the level of stellar radiation and a planet’s gravity.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study measured the secondary eclipse depth of GJ 3929 b using MIRI/JWST at 15 μm, obtaining a value of 118±22 ppm. Comparison with models of bare rocks and atmospheres with CO2 showed that a thick CO2 atmosphere is ruled out at >3σ, and the data are consistent with an airless surface or a thin atmosphere. Variability in the eclipse depth between visits was detected, and a slight orbital eccentricity was confirmed.
Where its strength is
A strong point is the application of the new FN-PCA method to remove systematics, which improves the reliability of the photometry, as well as the joint analysis of four eclipses, which allowed refining the planet's parameters and constraining the atmospheric properties.
Where it could go next
- It may be interesting to compare the obtained constraints with results for other rocky planets around M dwarfs from the Rocky Worlds DDT survey, to test the 'cosmic shoreline' hypothesis at the population level.based on: Ultrahot planet hides under a gas blanket arXiv:2509.17231
- Related work on a dark, cooled planet without volcanoes discusses the link between the absence of an atmosphere and geological activity; it might be worth considering similar scenarios for GJ 3929 b.based on: Dark, Chilled Planet Without Volcanoes: JWST’s Gaze arXiv:2605.00100
- To test the thin atmosphere hypothesis, it may be useful to look at approaches to modeling atmospheres with volcanic outgassing, as described in the work on hydrogen atmospheres of hot planets.based on: Hydrogen Atmospheres on Scorching Planets: The Volcano Secret arXiv:2510.07378
Worth trying
- Conduct additional observations in other MIRI filters to refine the temperature contrast and search for CO2 spectral features.
- Apply the FN-PCA method to archival data of other planets to test its effectiveness in reducing systematics.
- Model light curves accounting for possible orbital eccentricity and incomplete tidal locking to explain the variability.
Why this may matter
Understanding which rocky planets around M dwarfs retain atmospheres is important for assessing the prevalence of potentially habitable worlds and for planning future exoplanet characterization missions.
Nearby works in our archive
- Ultrahot planet hides under a gas blanketsimilarity 0.64
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- Chemical Portrait of an Invisible Worldsimilarity 0.63
- Carbon Giant by a Dead Starsimilarity 0.63
- How a Planet's Breath Shapes Its Fatesimilarity 0.62
- Hydrogen Atmospheres on Scorching Planets: The Volcano Secretsimilarity 0.62
The closest work in the archive sits at 0.64 by meaning — the usual level for our corpus.
Original abstract
as written by the authors arXiv:2606.07511v1
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 $$\mu$$m 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$$_\oplus$$ and instellation flux of 17.3+/-0.7 S$$_\oplus$$. 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 3$$\sigma$$. We also continue with lessons-learned in robustly analyzing these kind of high-precision JWST/MIRI 15 $$\mu$$m 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.
The dayside of GJ 3929 b is heated to 640 kelvins, about the melting point of tin, and its orbital period is just 2.6 Earth days, meaning a year there flies by faster than a week on Earth.
Key numbers
- Eclipse Depth: 118±22 ppm
- Dayside Brightness Temperature: 641+59-64 K
- Orbital Period: 2.6 days
- Planet Mass: 1.75 M⊕
- Insolation Flux: 17.3 S⊕