As missions like Kepler and TESS discovered more rocky exoplanets in recent years, scientists looked forward to the launch of the JWST. The powerful space telescope has the ability gather infrared spectra of exoplanet atmospheres, a key need in understanding the planets being discovered. It was hoped that these atmospheric characterizations would advance our understanding of habitability.
As we’ve seen, the JWST has lived up to its promise. It’s studied many exoplanet atmospheres and even found some intriguing, potential signs of biological activity, though nothing unequivocal.
The JWST’s latest exoplanet atmosphere target is GJ 3929 b. Astronomers discovered it in TESS data in 2022. “Our analysis confirms the planetary nature of the transiting planet GJ 3929 b,” the authors of the 2022 paper wrote. “Its mass and radius (~ 0.43 Earth masses and ~1.15 Earth radii) put it into the regime of small Earth-sized planets.”
Earth-like planets attract attention for obvious reasons.
Now the JWST’s results are in, and the once-promising planet appears to be barren. The observations, results, and conclusion are in a new paper titled “The JWST Rocky Worlds DDT Program reveals GJ 3929b to likely be a bare rock.” The lead author is Qiao Xue from the Department of Astronomy and Astrophysics at the University of Chicago.
“We report first results from the JWST Rocky Worlds Director’s Discretionary Time program,” the authors write. The DDT program combines Hubble and JWST observations to search for atmospheres on rocky exoplanets orbiting red dwarfs. Astronomers target these planets for several reasons. Red dwarfs are very plentiful, and make up about 75% of stars in the Milky Way, so they likely host the majority of exoplanets.
They’re long-lived stars, meaning life would have ample time to develop on their planets. Rocky planets around red dwarfs are abundant, and are the most common potentially habitable planets in the galaxy. They’re also easier to study, since the habitable zones around red dwarfs are so small. Their transits are easier to observe, and their atmospheres are well-lit by their nearby stars and are amenable to spectroscopy.
The new observations refined the mass and size measurements for GJ 3929b, as well as its surface temperature, which matches that of a bare, black rock. “These results rule out CO2-rich atmospheres thicker than 100 mbar at > 3σ, suggesting that GJ 3929b has lost any significant secondary atmosphere,” the researchers write.
This figure illustrates some of the results of the JWST’s observations of GJ 3929 b. The panel on the left shows emission spectra for different atmospheric scenarios. The black point represents the joint-fit eclipse depth, whereas the red error bars are the individual visits. Triangles represent the F1500W bandpass-integrated model eclipse depths. The right panel shows a temperature-pressure profile for each atmospheric scenario, with individual atmospheric layers indicated by horizontal lines. The disk-averaged surface temperatures for bare rock surfaces are indicated by triangles. “Our results rule out a variety of CO2-rich atmospheres thicker than 100 mbar,” the researchers explain. Earth’s sea level atmospheric pressure is just over 1000 millibars. Image Credit: Xue et al. 2025.
“Our forward modeling results support the conclusion that GJ 3929b lacks a substantial atmosphere,” the researchers write. “This adds a valuable data point to evaluate the cosmic shoreline hypothesis, which posits that secondary atmosphere presence/absence is largely controlled by the cumulative XUV radiation received by a planet
over its lifetime.”
The cosmic shoreline hypothesis is based on the fact that x-ray and UV emissions from stars can heat planetary atmospheres. If they’re heated enough, the atmospheric particles gain enough energy to escape a planet’s atmosphere. That process is called photoevaporation or atmospheric escape.
The researchers compared GJ 3929b to three versions of the cosmic shoreline, and came out on the airless side of all three.
This figure compares GJ 3929 b to three different versions of the cosmic shoreline. Other exoplanets orbiting stars with between 0.2 to 0.4 Earth masses are shown for comparison. “GJ 3929b lies well beyond all three shorelines we consider here, indicating it resides in a regime where atmospheric erosion is expected to outpace replenishment,” the researchers explain. Image Credit: Xue et al. 2025.
“The Rocky Worlds DDT program’s first two visits of GJ 3929b yield no evidence for a thick CO2- or H2O–rich atmosphere, instead pointing to a dark, bare-rock world,” the authors write in their conclusion. “GJ 3929b lies well on the airless side of many proposed versions of the cosmic shoreline.”
The results indicate that the planet has lost any original atmosphere it may have had. It has also lost any atmosphere replenished by outgassing.
“In comparison with the other presumed bare rocks, TRAPPIST-1b and c, GJ 1132b, and GJ 486b, GJ 3929b is the farthest from the shoreline, underscoring that it is highly unlikely to sustain a CO2 atmosphere,” the researchers explain.
Though it’s always disappointing when another Earth-like world is revealed to be without an atmosphere, these results still have scientific value. The JWST will likely revisit this world, and many other rocky worlds, in the JWST’s Director’s Discretionary Time program.
“Together, these data will build a statistical framework for atmospheric retention, surface composition, and XUV-driven erosion on these small planets, transforming our understanding of how they form, evolve, and either keep or lose their atmospheres in the intense radiation environments,” the authors conclude.