The survival of dense secondary atmospheres around temperate rocky planets orbiting low-mass red dwarfs remains an important open question. Here we show that the thermal phase curves of TRAPPIST-1 b and TRAPPIST-1 c, measured with the James Webb Space Telescope at 15 μm, are consistent with bare rocky surfaces rather than thick atmospheres. TRAPPIST-1 b exhibits a high dayside brightness temperature (490 ± 17 K), no significant nightside emission and no phase offset—features indicative of a dark, airless surface. TRAPPIST-1 c shows a cooler dayside (369 ± 23 K) and a similarly cold nightside, consistent with either a tenuous, oxygen-rich atmosphere or an equally airless, more reflective surface. Models with surface pressures above ~1 bar are strongly disfavoured for both planets. These results suggest divergent evolutionary pathways or atmospheric loss processes despite their similar compositions.
