Are there intergalactic planets without solar systems?
There are planets without solar systems.
A planet can get ejected from a solar system by the gravitational pull of its sun, another planet or another star passing through its solar system.
There are problems with our understanding of our solar system. Jupiter must have been formed much closer to the Sun and has since migrated to its current position. Jupiter and Saturn are both capable of ejecting a planet from the solar system, and simulations indicate that they should have destabilised the solar system.
There are theories that there was once another gas giant between Saturn and Uranus. Adding this planet makes simulations as to how our solar system got to its current configuration more likely. If this planet did exist it would have been expelled from the solar system by an encounter with either Jupiter or Saturn.