  Seven rocky planets orbit the nearby dwarf star TRAPPIST-1, providing a
unique opportunity to search for atmospheres on small planets outside the Solar
System (Gillon et al., 2017). Thanks to the recent launch of JWST, possible
atmospheric constituents such as carbon dioxide (CO2) are now detectable
(Morley et al., 2017, Lincowski et al., 2018}. Recent JWST observations of the
innermost planet TRAPPIST-1 b showed that it is most probably a bare rock
without any CO2 in its atmosphere (Greene et al., 2023). Here we report the
detection of thermal emission from the dayside of TRAPPIST-1 c with the
Mid-Infrared Instrument (MIRI) on JWST at 15 micron. We measure a
planet-to-star flux ratio of fp/fs = 421 +/- 94 parts per million (ppm) which
corresponds to an inferred dayside brightness temperature of 380 +/- 31 K. This
high dayside temperature disfavours a thick, CO2-rich atmosphere on the planet.
The data rule out cloud-free O2/CO2 mixtures with surface pressures ranging
from 10 bar (with 10 ppm CO2) to 0.1 bar (pure CO2). A Venus-analogue
atmosphere with sulfuric acid clouds is also disfavoured at 2.6 sigma
confidence. Thinner atmospheres or bare-rock surfaces are consistent with our
measured planet-to-star flux ratio. The absence of a thick, CO2-rich atmosphere
on TRAPPIST-1 c suggests a relatively volatile-poor formation history, with
less than 9.5 +7.5 -2.3 Earth oceans of water. If all planets in the system
formed in the same way, this would indicate a limited reservoir of volatiles
for the potentially habitable planets in the system.
