Astronomers can discover a planet not by noticing the planet’s gravitational effects on its parent star but rather on a star much farther away.
When one celestial object passes in front of another from our point of view, the closer object’s gravity can bend and magnify the light of the more distant object, causing the distant object to appear brighter than normal for a short time. This phenomenon is called gravitational microlensing.
For example, if a closer star passes in front of a farther star, the farther star will appear temporarily brighter and then return to its normal brightness after the closer star has passed.
But if, by chance, the closer star is accompanied by an unseen planet, the farther star’s brightness will increase a second time as the planet passes in front of it.
By watching for a distant star’s brightness to intensify twice, astronomers can find out whether the closer star has a secret traveling companion.