Up until 1991, it was assumed that we would NEVER, EVER, find any planets outside of our solar system. They are really small in comparison to stars. They are really really far away. They do not give off any light. The sun/star they orbit is so bright that we will never see past the brightness to see something so small and unlit.
In 1992, that all changed.
It is a long and difficult process that requires measurements that no one thought possible.
We have now found over 4,000 planets outside of our solar system.