What are interstellar planets?
Something of a contradiction in terms, an interstellar planet is a planet-like object that is not in orbit around a star but roaming through interstellar space.
Interstellar panets are believed to be things that started out as regular planets. But they got too close to another, large planet and the orbit was upset by the gravitational interaction. Under some conditions this planet-planet gravitational interaction can put enough energy into one planet's motion to escape the original star. Then that planet becomes interstellar.
It could have happened in our own Solar System (https://en.wikipedia.org/wiki/Five-planet_Nice_model). An artificial version occurs when we send a space probe into the outer Solar System by using the gravity and motion of the giant planets to pull the space probe outward (https://en.wikipedia.org/wiki/Gravity_assist). Voyager 1, for instance, has been boosted all the way out of thw Solar System and thus might be called an artificial "interstellar planet".