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.
as written by the authors arXiv:2605.26216
Recent ground-based microlensing surveys suggest that our Galaxy may abound with small free floating planets, potentially up to $$\sim$$21 such planets per star. We explore the implication of such possibility on the mass budget for planet formation. When the microlensing planets, both bound and free-floating, are taken into account, along with the short-period planets, T Tauri disks have insufficient mass to source the mass of known planets, even if all the solids convert into planetary bodies. Younger Class 0/I disks can help resolve the problem but generally fall short of the required mass when variable planet formation efficiency from pebble or planetesimal accretion is taken into consideration. If the free-floating planet mass function is as bottom-heavy as reported, heavier Class 0/I disks may be necessary. Alternatively, free-floaters may preferentially form in the most massive disks around massive stars consuming the majority of the mass budget, leading to a decrease in the bound planet occurrence rate for higher mass stars, which is observed. Precise constraints on the bottom of planet mass function are necessary: a peaked mass function may eliminate the missing mass problem; by contrast, verifying a bottom-heavy function could spell a crisis in planet formation.
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.