Astronomical discoveries have significantly expanded our understanding of the universe. Key milestones include the identification of the four largest moons of Jupiter by Galileo Galilei in 1610, which provided evidence that not everything orbits the Earth. The discovery of exoplanets in the 1990s revealed that many stars have their own planetary systems, suggesting that our solar system is not unique. Another major breakthrough was the detection of gravitational waves in 2015, confirming a prediction made by Albert Einstein over a century earlier. These waves, caused by massive cosmic events like merging black holes, opened a new way to observe the universe, allowing scientists to study phenomena that were previously invisible.