  The origin of Uranus and Neptune remains a challenge for planet formation
models. A potential explanation is that the planets formed from a population of
a few planetary embryos with masses of a few Earth masses which formed beyond
Saturn's orbit and migrated inwards. These embryos can collide and merge to
form Uranus and Neptune. In this work we revisit this formation scenario and
study the outcomes of such collisions using 3D hydrodynamical simulations. We
investigate under what conditions the perfect-merging assumption is
appropriate, and infer the planets' final masses, obliquities and rotation
periods, as well as the presence of proto-satellite disks. We find that the
total bound mass and obliquities of the planets formed in our simulations
generally agree with N-body simulations therefore validating the
perfect-merging assumption. The inferred obliquities, however, are typically
different from those of Uranus and Neptune, and can be roughly matched only in
a few cases. In addition, we find that in most cases the planets formed in this
scenario rotate faster than Uranus and Neptune, close to break-up speed, and
have massive disks. We therefore conclude that forming Uranus and Neptune in
this scenario is challenging, and further research is required. We suggest that
future planet formation models should aim to explain the various physical
properties of the planets such as their masses, compositions, obliquities,
rotation rates and satellite systems.
