  The planets of our solar system formed from a gas-dust disk. However, there
are some properties of the solar system that are peculiar in this context.
First, the cumulative mass of all objects beyond Neptune (TNOs) is only a
fraction of what one would expect. Second, unlike the planets themselves, the
TNOs do not orbit on coplanar, circular orbits around the Sun, but move mostly
on inclined, eccentric orbits and are distributed in a complex way. This
implies that some process restructured the outer solar system after its
formation. However, some of TNOs, referred to as Sednoids, move outside the
zone of influence of the planets. Thus external forces must have played an
important part in the restructuring of the outer solar system. The study
presented here shows that a close fly-by of a neighbouring star can
simultaneously lead to the observed lower mass density outside 30 AU and excite
the TNOs onto eccentric, inclined orbits, including the family of Sednoids. In
the past it was estimated that such close fly-bys are rare during the relevant
development stage. However, our numerical simulations show that such a scenario
is much more likely than previously anticipated. A fly-by also naturally
explains the puzzling fact that Neptune has a higher mass than Uranus. Our
simulations suggest that many additional Sednoids at high inclinations still
await discovery, perhaps including bodies like the postulated planet X.
