  If confirmed, the Neptune-size exomoon candidate in the Kepler 1625 system
will be the first natural satellite outside our Solar System. Its
characteristics are nothing alike we know for a satellite. Kepler 1625b I is
expected to be as massive as Neptune and to orbit at 40 planetary radii around
a ten Jupiter mass planet. Because of its mass and wide orbit, this satellite
was firstly thought to be captured instead of formed in-situ. In this work, we
investigated the possibility of an in-situ formation of this exomoon candidate.
To do so, we performed N-body simulations to reproduce the late phases of
satellite formation and use a massive circum-planetary disc to explain the mass
of this satellite. Our setups started soon after the gaseous nebula
dissipation, when the satellite embryos are already formed. Also for selected
exomoon systems we take into account a post-formation tidal evolution. We found
that in-situ formation is viable to explain the origin of Kepler 1625b I, even
when different values for the star-planet separation are considered. We show
that for different star-planet separations the minimum amount of solids needed
in the circum-planetary disc to form such a satellite varies, the wider is this
separation more material is needed. In our simulations of satellite formation
many satellites were formed close to the planet, this scenario changed after
the tidal evolution of the systems. We concluded that if the Kepler1625 b
satellite system was formed in-situ, tidal evolution was an important mechanism
to sculpt its final architecture.
