  Transiting planets around young stars are key benchmarks for our
understanding of planetary systems. One of such candidates was identified
around the K dwarf HD 18599 by TESS, labeled as TOI-179. We present the
confirmation of the transiting planet and the characterization of the host star
and of the TOI-179 system over a broad range of angular separations. To this
aim, we exploited the TESS photometric time series, intensive radial velocity
monitoring performed with HARPS, and deep high-contrast imaging observations
obtained with SPHERE and NACO at VLT. The inclusion of Gaussian processes
regression analysis is effective to properly model the magnetic activity of the
star and identify the Keplerian signature of the transiting planet. The star,
with an age of 400+-100 Myr, is orbited by a transiting planet with period
4.137436 days, mass 24+-7 Mearth, radius 2.62 (+0.15-0.12) Rearth, and
significant eccentricity (0.34 (+0.07-0.09)). Adaptive optics observations
identified a low-mass companion at the boundary between brown dwarfs and very
low mass stars (mass derived from luminosity 83 (+4-6) Mjup) at a very small
projected separation (84.5 mas, 3.3 au at the distance of the star). Coupling
the imaging detection with the long-term radial velocity trend and the
astrometric signature, we constrained the orbit of the low mass companion,
identifying two families of possible orbital solutions. The TOI-179 system
represents a high-merit laboratory for our understanding of the physical
evolution of planets and other low-mass objects and of how the planet
properties are influenced by dynamical effects and interactions with the parent
star.
