Planet Horizon is a metaphor, representing that the main source of the Stelodeor Crystals are unattainable. The crystals are always on the planet's horizon, which is a position relative to each person. One can never reach the horizon, but it is still there.

Planet Horizon Diogram

A Diagram showing the core of a planet before and after the event of the Planet Horizon.

"Planet Horizon" also refers to the point in time that this event took place, which was not until after the Great Galactic War.