TRAPPIST-1b is a terrestrial, Earth-sized exoplanet orbiting around the ultra-cool dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs) away from Earth in the constellation of Aquarius.

[3] An observation of the secondary eclipse of TRAPPIST-1b by the James Webb Space Telescope, announced in 2023, suggests that the planet does not have any significant atmosphere, with a measured surface temperature of about 503 K (230 °C; 446 °F),[8][9] and a low albedo.

The combined transmission spectra of TRAPPIST-1 b and c rule out cloud-free hydrogen-dominated atmospheres for both planets, so they are unlikely to harbor extended gas envelopes.

A water vapor atmosphere would need to have a scale height of >100 km (62 mi) and a temperature >1,800 K (1,530 °C; 2,780 °F) to produce the variations seen in the planet's transit depths and its transmission spectrum, and would be vulnerable to photodissociation where CO2 would not be.

[10][3] An observation of the secondary eclipse of TRAPPIST-1b by the James Webb Space Telescope, announced in March 2023, suggested that the planet does not have any significant atmosphere.

[5] Further studies of the exoplanet by transmission spectroscopy (primary eclipse), reported in September 2023, also confirmed the absence of a hydrogen-rich atmosphere, but due to stellar contamination were unable to determine the presence or absence of other types of atmospheres based on the transmission spectroscopy data alone.

These results are also compatible with a young ultramafic surface and no atmosphere, which the authors of the 2024 paper slightly favor.

The difference in eclipse depths between the two wavelengths indicates the surface is geologically fresh (≤1000 yrs) and not significantly impacted by space weathering.

The presence of fresh ultramafic rock on TRAPPPIST-1 b could indicate recent crustal reprocessing, from volcanic resurfacing or plate tectonics.