I am interested in studies and/or publications about planets that wander in space and are not orbiting around a star or its remnants. These dark and frozen worlds should've got ripped away from their respective system by a large passing body (another star? passing black hole) or any other astronomical event (which one?).
I am curious, whether there exist statistical estimates on the number of such planets and the rate at which they are produced. I am also interested in reading about the statistical models that can produce those estimates and encode researchers' beliefs. If there are no such models, why?
A provoking thought: According to Greek, such planets are really fitting their name πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers"). It is frightening to imagine such world that drifts deeper and deeper into empty space. Forever.