Rogue Planets 101: Roman will use the technique called microlensing to "see" rogue planets. when a rogue planet lines up with a star from our vantage point, it will bend and amplify the light from the star. allowing us to see this lonely planet.
Rogue Planets 101: Fun fact, the universe could be teeming with rogue planets and we wouldn’t even know it. We would never find out without undertaking a thorough, space-based microlensing survey like Roman is going to do.