How do astronomers find planets in other solar systems?

It was almost 100 years ago, Clyde W. Tombaugh discovered the planet Pluto. This was the last planet discovered until 1992, when humans discovered another planet. But this new planet was not in our solar system, but was orbiting another star. We call this an exoplanet, or an “exoplanet” for short.

Since then, astronomers have cataloged more than 6,000 exoplanets. If you think it’s hard to remember the names of our planets, try everyone Planets, with names like HD 189733b. (A joyful place where rain falls and molten glass, and the wind blows at a speed of 9,000 kilometers per hour).

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

Why are they so hard to find?

The problem is that you can’t take your best telescope and start looking around at the sky. Telescopes have limited resolving power, the smallest angular size they can “see”. For the Hubble Space Telescope, this distance is 0.05 arcsecond, which is very small — about 1/72,000 of a degree. HST can detect a giant Jupiter-sized planet at a distance of 590 billion kilometers. This is amazing, but it is only 0.06 light-years away, and the closest star to us, Proxima Centauri, is 4.25 light-years away.

Another problem is the opacity of the planets. Jupiter is certainly easier to see in our night sky, due to sunlight reflecting off its surface. But you can’t see Jupiter at all during the day, because the reflected light is much weaker than direct sunlight. It’s the same for exoplanets. When we look at the light from a star, the planets surrounding it are not bright enough to be distinguished.

Fortunately, there are other methods, and I will explain the two methods that have been used to find most of the exoplanets we know today. There’s a bunch of cool physics here, so let’s get started!

Orbits, rocking stars and blue transitions

What happens when a planet moves around a star? First, there is the gravitational interaction that pulls the planet toward 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 (p) between them:

Illustration: Rhett Allen