Imagine Earth plucked from the solar system and tossed into the freezing dark with no sun at all. That nightmare is a routine reality across the Milky Way, which may hold billions or even trillions of rogue planets drifting unbound to any star. This deep dive explains the two ways these worlds are born, either as failed stars collapsing from gas clouds too small to ignite fusion, or as planets violently ejected from young solar systems through planet-planet scattering and stellar flybys, with objects as small as 0.6 Jupiter masses found forming alone in the Trapezium Cluster.

We then cover the ingenious ways astronomers find them: gravitational microlensing events that revised population estimates from nearly two Jupiter-mass rogues per star in 2011 to about 0.25 in 2017, the 42-minute event that revealed the first Earth-mass rogue in 2020, ...