GJ 3929 b: Key to Unlocking the Atmospheres of Rocky Exoplanets ✦
As part of the Rocky Worlds DDT survey, transit eclipse observations of exoplanet GJ 3929 b (1.75 Earth masses, 17.3 Earth insolations) were carried out with the MIRI camera on JWST at 15 microns. From the complete data of four visits (two previously unpublished), the eclipse depth was measured at 118±22 ppm and the brightness temperature of the dayside hemisphere at 641+59/-64 K. This value is lower than previously reported (160+26/-27 ppm) based on the first two observations. The results are compatible with both a bare rocky surface and a thin atmosphere; thick CO₂ atmospheres without thermal inversion are ruled out with >3σ confidence. Additionally, it is shown that the FN-PCA method is more robust to the choice of extraction aperture size than standard polynomial detrending, which is critical for the reliability of the conclusions.
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 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⊕