How are free-floating planets detected through gravitational microlensing?

Free-floating planets, also known as rogue planets, are detected through gravitational microlensing by observing the temporary brightening of a more distant star.12

This technique relies on the principles of general relativity, which state that a massive object can act as a lens by bending the light of a background source. When a free-floating planet passes almost perfectly in front of a distant star from the perspective of an observer on Earth, the planet's gravitational field focuses the star's light, causing the star to appear to grow brighter.12

Because these planets do not orbit a star, they cannot be detected through methods like radial velocity or transit photometry, which require a host star's motion or light-blocking events. Microlensing allows astronomers to identify these wandering bodies—even those as small as 0.3 to 2 Earth masses—by monitoring the characteristic light-curve signatures produced during these rare alignments.23