Final answer:
The study of our planetary system has progressed significantly in the past few decades due to space exploration missions that have allowed astronomers to directly study and gather data from every planet in our solar system.
Step-by-step explanation:
The most important reason that astronomers have learned more about our planetary system in the last 30-40 years than all of history before then is the advent of space exploration missions. Through spacecraft such as Voyager, Pioneer, Curiosity, and Pathfinder, astronomers have been able to send robots to fly past, orbit, or land on every planet in our solar system, gathering images and data that have greatly expanded our knowledge. For the first time, astronomers have been able to directly study the objects they want to understand, providing valuable insights.