It was almost 100 years ago that Clyde W. Tombaugh discovered Pluto. That was the last planet found until 1992, when humans found another one. But this new planet wasn’t in our solar system—it was orbiting another star. We call this an extrasolar planet, or “exoplanet” for short.

Since then, astronomers have cataloged more than 6,000 exoplanets. If you thought it was hard to remember the names of our own planets, try all the planets, with names like HD 189733b. (A jolly place where it rains molten glass and the wind blows 9,000 kilometers per hour.)

Even the closest exoplanets are more than 4 light years away (36 trillion miles), which makes it doubtful that we’ll ever visit one—so why bother? The reason is, it helps us answer an age-old question: Are we alone in the universe? As far as we understand, you need a planet to have life, and the race is on to locate one with Earth-like qualities.

Why Are They Hard to Find?

The problem is, you can’t just take your best telescope and start looking around the sky. Telescopes have a limited resolving power—the smallest angular size they can “see.” For the Hubble Space Telescope that’s 0.05 arc second, which is incredibly tiny—about 1/72,000th of a degree. The HST could make out a giant, Jupiter-size planet at a distance of 590 billion kilometers. That’s amazing, but it’s just 0.06 light year, and the nearest star, Proxima Centauri, is 4.25 light years away.

Another problem is the dimness of planets. Sure, Jupiter is easy to see in our own night sky, because of the sunlight reflecting off its surface. But you can’t see Jupiter at all during the day, because that reflected light is much dimmer than direct sunlight. It’s the same for exoplanets. When we’re looking at the light from a star, the planets around it just aren’t bright enough to be discernable.

Luckily, there are other methods, and I’m going to explain the two that were used to find most of the exoplanets we know today. There’s a bunch of cool physics here, so let’s go!

Orbits, Jiggly Stars, and Blue Shifts

What happens when a planet moves around a star? First, there’s a gravitational interaction that pulls the planet in the direction of the star. The magnitude of this force (F_G) depends on the mass of the star (M) and the planet (m), as well as the distance (r) between them:

Illustration: Rhett Allain