In astronomical terms, such a planet could initially be considered a “rogue planet” or free-floating planet once it leaves the gravitational influence of its sun. When it is later captured by the gravity of a larger planet, it becomes a natural satellite of that larger planet, albeit temporarily. This scenario describes a process known as “planetary capture.”
As the captured planet continues its orbit around the larger celestial body, gravitational interactions — potentially including tidal forces and dynamical friction — could modify its trajectory. If these interactions cause the smaller planet to move progressively closer to the larger host planet, it could eventually be on a collision course, resulting in a merger or impact.
In this eventuality, one could refer to this entire sequence as an unstable orbit due to planetary capture leading to an eventual planetary collision. Such events, though hypothetically plausible, depend on numerous specific gravitational dynamics and orbital mechanics that can vary widely based on the involved bodies’ masses, velocities, distances, and compositions.