  The TRAPPIST-1 system is remarkable for its seven planets that are similar in
size, mass, density, and stellar heating to the rocky planets Venus, Earth, and
Mars in our own Solar System (Gillon et al. 2017). All TRAPPIST-1 planets have
been observed with the transmission spectroscopy technique using the Hubble or
Spitzer Space Telescopes, but no atmospheric features have been detected or
strongly constrained (Ducrot et al. 2018; de Wit et al. 2018; Zhang et al.
2018; Garcia et al. 2022). TRAPPIST-1 b is the closest planet to the system's M
dwarf star, and it receives 4 times as much irradiation as Earth receives from
the Sun. This relatively large amount of stellar heating suggests that its
thermal emission may be measurable. Here we present photometric secondary
eclipse observations of the Earth-sized TRAPPIST-1 b exoplanet using the F1500W
filter of the MIRI instrument on JWST. We detect the secondary eclipse in each
of five separate observations with 8.7-$\sigma$ confidence when all data are
combined. These measurements are most consistent with the re-radiation of the
TRAPPIST-1 star's incident flux from only the dayside hemisphere of the planet.
The most straightforward interpretation is that there is little or no planetary
atmosphere redistributing radiation from the host star and also no detectable
atmospheric absorption from carbon dioxide (CO$_2$) or other species.
