People have always been fascinated by the starry sky, and in ancient mythologies the sky was far from deserted, being populated by gods and heroes. But only the planets of our Solar System were known.
In 1995, the first planet orbiting around another Sun-like star was discovered. Since that pioneering discovery, many more have been found and we currently know of about 3000 of these “exoplanets”. Based on these discoveries, we now estimate that at least half of the stars in our galaxy have a planetary system, and that planet formation is a very common occurrence. All of these discoveries were a great surprise from the start!
In the early days of planet detection, we didn’t have any hard data, other than our solar system. People erroneously assumed (other) planetary systems would look like our own. The very first detection, 51 Pegasi, completely threw that notion out.
Every class of planets could be considered a surprise if you’re expecting something to look like our solar system. Nothing does. In that sense, we’re still in a state of constantly being surprised.