SPECULOOS programme selection: Left: estimated S/N in transmission spectroscopy with JWST/NIRSPEC (assuming an ‘Earth-like’ planet, 200 h of JWST/NIRSPEC time, and a spectral sampling of 100 nm) as a function of spectral type for the 14 164 M- and L-dwarfs within 40 pc. TRAPPIST-1 is shown as a magenta star. Red dashed line: S/N = 4, used to select Programme 1 targets. Colour coding: estimated S/N for detecting a single transit of an Earth-sized planet with one telescope of the SPECULOOS network, with yellow points representing a S/N ≥ 15. Light-blue histogram: distribution of all objects and their corresponding S/N. Right: estimated S/N for TESS to detect an Earth-sized planet with an irradiation four times larger than Earth. Colour coding: estimated S/N for detecting the transit of an Earth-sized planet with the SPECULOOS network, with yellow points representing a S/N ≥ 15. Red dashed line: S/N = 5, used to select Programme 2 targets. Light-blue histogram: distribution of all objects and their corresponding S/N.