Zhixing Li

One night, a star at the edge of the sky suddenly flickered and brightened for a few days. It wasn't a supernova, and no cloud passed by — an invisible planet drifted between the star and Earth. Its gravity bent the light of the distant star like a pair of glasses focusing a sunbeam.

A pebble drops into a night lake, and ripples spread across the water. You can't see the pebble itself, but the ripples give it away. Astronomers use a similar trick when hunting for dark bodies in space: these objects betray themselves by the way they warp the light of distant stars.