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. Astronomers call this gravitational lensing, and it's exactly how we caught a planet that would otherwise be invisible.
Two telescopes (one in Korea, one in Chile) imaged the star every minute. Such frequent observations helped pinpoint the planet's location. By measuring the star's color, which was also captured, they calculated the planet weighs about as much as forty Earths. Its host star is small and dim, only one-third the mass of the Sun. The pair lies about seven and a half thousand light-years away.
This planet-hunting method proved so sensitive that scientists tested it on a computer. It turns out it can even find lone planets that don't orbit a star but drift through space on their own. In the center of our galaxy, the technique could spot a rogue planet as massive as Mars; closer to us, even one as small as the Moon.
The lens that helped us see a planet today could tomorrow reveal a whole swarm of invisible wanderers. And then we'll know how many solitary worlds drift between the stars.
A gravitational lens works like eyeglass glass, but instead of glass, it's the planet's gravity. The more massive the planet, the more it bends the light of a star behind it.