  Located at the bottom of the main sequence, ultracool dwarf stars are
widespread in the solar neighbourhood. Nevertheless, their extremely low
luminosity has left their planetary population largely unexplored, and only one
of them, TRAPPIST-1, has so far been found to host a transiting planetary
system. In this context, we present the SPECULOOS project's detection of an
Earth-sized planet in a 17 h orbit around an ultracool dwarf of M6.5 spectral
type located 16.8 pc away. The planet's high irradiation (16 times that of
Earth) combined with the infrared luminosity and Jupiter-like size of its host
star make it one of the most promising rocky exoplanet targets for detailed
emission spectroscopy characterization with JWST. Indeed, our sensitivity study
shows that just ten secondary eclipse observations with the Mid-InfraRed
Instrument/Low-Resolution Spectrometer on board JWST should provide strong
constraints on its atmospheric composition and/or surface mineralogy.
