TRAPPIST-1c, also designated as 2MASS J23062928-0502285 c, is a mainly rocky exoplanet orbiting around the ultracool dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs) away from Earth in the constellation Aquarius.

[2] Initial estimates suggested that TRAPPIST-1c has a lower density (4.89 g/cm3) and gravity (0.966g) than Earth, consistent with a rock-based composition and a thick, Venus-like atmosphere.

[3][9] However, an observation of the secondary eclipse of TRAPPIST-1c by the James Webb Space Telescope, announced in 2023, suggests against a thick CO2 atmosphere, however this does not exclude a thick abiotic oxygen dominated atmosphere as is hypothesized to be Common around Red dwarf stars, with a measured surface temperature of 380 K (107 °C; 224 °F).

The combined transmission spectrum of TRAPPIST-1 b and c rules out a cloud-free hydrogen-dominated atmosphere for each planet, so they are unlikely to harbor an extended gas envelope.

[11][3] An observation of the secondary eclipse of TRAPPIST-1c by the James Webb Space Telescope, announced in 2023 rules out a thick carbon dioxide atmosphere like that of Venus.

[7][6] This is similar to JWST results on the inner planet TRAPPIST-1b announced earlier the same year, which suggest that it does not have a thick CO2 dominated atmosphere.