CNET writes:
In just a few decades, we’ve gone from knowing of no planets beyond the solar system to thousands. Here’s how it happened.
The now retired spacecraft, Kepler, has been instrumental in the discovery of over 4,000 planets beyond our solar system. Referenced here: CNET article and NASA video.
Check It Out: How the Spacecraft, Kepler, Discovered Over 4,000 Exoplanets