Question

What is the evidence that suggests planetary systems are common in the universe?

Answer 1

By detecting planets using the transit method.

Explanation:

As far as we can tell, most sun-like stars have a planetary system.

The Kepler space telescope was tasked with finding planets using the transit method - observing periodic dips in the brightness of stars caused by planets moving in front of them. These dips in brightness are about `1` part in `10000`. To do this it has to keep focused on the same area of the sky for an extended period of time.

What is has observed is that the vast majority (I think `80%` plus) of sun-like stars have planets orbiting close to the star.