As reported by Gizmodo:
Rogue planets, as they’re informally known, likely formed from a protoplanetary disc around a host star but were then tossed out into interstellar space by the gravitational perturbations of larger planets. These wanderers range in size from Earth-like through to Jupiter-scale behemoths. Fascinatingly, they might be exceptionally abundant, with some scientists estimating trillions of them in the Milky Way. Their true population size remains a mystery, however...As Einstein famously predicted, foreground objects can warp the light emanating from background stars. This results in the temporary magnification of the incoming light from our perspective, and the effect can last from just a few hours to a few days. Microlensing events are relatively rare, and those produced by rogue planets are even rarer...The new paper suggests the presence of a large population of Earth-sized rogue planets in the Milky Way. It’s becoming clear that free-floating planets are common. McDonald said his team is currently working to come up with a more precise estimate for how many of them might exist.