The cosmic accounting has hit a snag. The bookkeeper is balancing the books: in one column, all the planets astronomers have counted in the galaxy; in the other, the stockpile of building material in the dusty cocoons of newborn stars. The numbers don't line up. Even if you gathered every last grain of dust and molded it into planets, there wouldn't be enough material.
Recent sky surveys using gravitational microlensing (where an invisible body acts as a natural lens, brightening the light of a distant star) have shown that free-floating planets, unattached to any star, could be staggeringly numerous: up to two dozen for every star. Add these wandering worlds to the ordinary planets in orbit, and the total mass goes off the charts. T Tauri dust disks, gas-and-dust "pancakes" around young suns, contain far less material than needed for such a crowd.
The researchers tested an idea: maybe earlier disks, around Class 0 and I stars, which hold more dust, could solve the problem? But even their reserves fall short when you account for the variable efficiency of planet formation — sometimes greedily gathering pebbles, sometimes stalling for long stretches. It seems either we're miscounting the smallest rogue planets, or they're mostly born around the most massive stars, gobbling up the lion's share of the budget. In that case, such stars should have fewer planets left in orbit, and that is indeed observed. The mystery of the planetary deficit isn't closed yet, but the galaxy's ledger is already bursting at the seams.
Rogue planets can be ejected from their systems by the gravitational kicks of larger neighbors, like a bully tossing a little kid out of the sandbox.
In Jack London's novel "The Star Rover," the soul travels without a body — much like lonely planets drifting through interstellar darkness.