A transiting rocky super-Earth and a non-transiting sub-Neptune orbiting the M dwarf TOI-771

Context. The origin and evolution of the sub-Neptune population is a highly debated topic in the exoplanet community. With the advent of JWST, atmospheric studies can now put unprecedented constraints on the internal composition of this population. In this context, the <ns0:monospace>THIRSTEE</ns0:monospace> project aims to investigate the population properties of sub-Neptunes with a comprehensive and demographic approach, providing a homogeneous sample of precisely characterised sub-Neptunes across stellar spectral types. Aims. We present here the precise characterisation of the planetary system orbiting one of the <ns0:monospace>THIRSTEE</ns0:monospace> M-dwarf targets, TOI-771 (d = 25 pc, V = 14.9 mag), known to host one planet, TOI-771 b, which has been statistically validated using TESS observations. Methods. We use TESS, SPECULOOS, TRAPPIST, and M-Earth photometry together with 31 high-precision ESPRESSO radial velocities to derive the orbital parameters and investigate the internal composition of TOI-771 b, as well as exploring the presence of additional companions in the system. Results. We derived the precise mass and radius for TOI-771 b, a super-Earth with Rb = 1.36 ± 0.10 R⊕ and Mb = 2.47−0.31+0.32M⊕ orbiting every 2.3 days around its host star. Its composition is consistent with an Earth-like planet, and it adds up to the rocky population of subNeptunes lying below the density gap identified around M dwarfs. With a ∼ 13% precision in mass, a∼ 7% radius precision, and a warm equilibrium temperature of Teq =543K, TOI-771 b is a particularly interesting target for atmospheric characterisation with JWST, and it is indeed one of the targets under consideration for the Rocky World DDT programme. Additionally, we discover the presence of a second, non-transiting planet in the system, TOI-771 c, with a period of 7.61 days and a minimum mass of Mp sin i = 2.87−0.38+0.41M⊕. Even though the inclination is not directly constrained, the planet likely belongs to the temperate sub-Neptune population, with an equilibrium temperature of ∼ 365K.