A compact multi-planet system transiting HIP 29442 (TOI-469) discovered by TESS and ESPRESSO Radial velocities lead to the detection of transits with low signal-to-noise ratio

Context. One of the goals of the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) Guaranteed Time Observations (GTO) consortium is the precise characterisation of a selected sample of planetary systems discovered by TESS. One such target is the K0V star HIP 29442 (TOI-469), already known to host a validated sub-Neptune companion TOI-469.01, which we followed-up with ESPRESSO.Aims. We aim to verify the planetary nature of TOI-469.01 by obtaining precise mass, radius, and ephemeris, and constraining its bulk physical structure and composition.Methods. Following a Bayesian approach, we modelled radial velocity and photometric time series to measure the dynamical mass, radius, and ephemeris, and to characterise the internal structure and composition of TOI-469.01.Results. We confirmed the planetary nature of TOI-469.01 (now renamed HIP 29442 b), and thanks to the ESPRESSO radial velocities we discovered two additional close-in companions. Through an in-depth analysis of the TESS light curve, we could also detect their low signal-to-noise transit signals. We characterised the additional companions, and conclude that HIP 29442 is a compact multi-planet system. The three planets have orbital periods P-orb,P- b = 13.63083 +/- 0.00003, P-orb,P- c = 3.53796 +/- 0.00003, and (Porb, d) = 6.42975+0.00009 -0.00010 days, and we measured their masses with high precision: m(p, b) = 9.6 +/- 0.8 M-circle plus, m(p, c) = 4.5 +/- 0.3 M-circle plus, and m(p, d) = 5.1 +/- 0.4 M-circle plus. We measured radii and bulk densities of all the planets (the 3 sigma confidence intervals are shown in parentheses): R-p,R- b = 3.48(-0.08 (-0.28))(+0.07(+0.19)) R-circle plus and rho(p, b) = 1.3 +/- 0.2 (0.3) g cm(-3); R-p,R- c = (1.58+0.10(+0.30))(-0.11(-0.34)) R-circle plus and rho(p, c) = 6.3(-1.3(-2.7))(+1.7(+6.0)) g cm(-3); R-p,R- d = 1.37 +/- 0.11((-0.43))((+0.32)) R-circle plus and rho(p, d) = 11.0(-2.4(-6.3))(+3.4(+21.0)) g cm(-3). Due to noisy light curves, we used the more conservative 3 sigma confidence intervals for the radii as input to the interior structure modelling. We find that HIP 29442 b appears as a typical sub-Neptune, likely surrounded by a gas layer of pure H-He with a mass of 0.27(-0.17)(+0.24) M-circle plus and a thickness of 1.4 +/- 0.5 R-circle plus. For the innermost companions HIP 29442 c and HIP 29442 d, the model an Earth-like composition.Conclusions. The compact multi-planet system orbiting HIP 29442 offers the opportunity to study simultaneously planets straddling the gap in the observed radius distribution of close-in small-size exoplanets. High-precision photometric follow-up is required to obtain more accurate and precise radius measurements, especially for planets c and d. This, together with our determined high-precision masses, will provide the accurate and precise bulk structure of the planets, and enable an accurate investigation of the system's evolution.