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A low-mass companion to the two-solar mass star HIP65426 has recently beendetected by SPHERE at around 100 au from its host. Explaining the presence ofsuper-Jovian planets at large separations, as revealed by direct imaging, iscurrently an open question.  We want to derive statistical constraints on the mass and initial entropy ofHIP65426b and to explore possible formation pathways of directly imaged objectswithin the core-accretion paradigm, focusing on HIP65426b.  Constraints on the planet's mass and post-formation entropy are derived fromits age and luminosity combined with cooling models. For the first time, theresults of population synthesis are also used to inform the results. Then, aformation model that includes N-body dynamics with several embryos per disc isused to study possible formation histories and the properties of possibleadditional companions. Finally, the outcomes of two- and three-planetscattering in the post-disc phase are analysed, taking tides into account.  The mass of HIP65426b is found to be Mp = 9.9 +1.1 -1.8 MJ using the hotpopulation and Mp = 10.9 +1.4 -2.0 MJ with the cold-nominal population. Coreformation at small separations from the star followed by outward scattering andrunaway accretion at a few hundred AU succeeds in reproducing the mass andseparation of HIP65426b. Alternatively, systems having two or more giantplanets close enough to be on an unstable orbit at disc dispersal are likely toend up with one planet on a wide HIP65426b-like orbit with a relatively higheccentricity (>~ 0.5).  If this scattering scenario explains its formation, HIP65426b is predicted tohave a high eccentricity and to be accompanied by one or several roughlyJovian-mass planets at smaller semi-major axes, which also could have a higheccentricity. This could be tested by further direct-imaging as well asradial-velocity observations.

Exploring the formation by core accretion and the luminosity evolution of directly imaged planets: The case of HIP 65426 b