Increased understanding of the gravitational pull

While Isaac Newton's description of gravity was good enough to detect Neptune's existence in the mid-1800s, Newton's law was not perfect. Astronomers discovered a mismatch between his law and the laws of nature.

This mismatch was resolved in 1915 when Albert Einstein published his theory of general relativity. Before this theory, physicists knew how to calculate a planet's gravitational pull, but they didn't understand why gravity behaved this way.