Newton's laws were used later to predict the existence of the planet Neptune based on changes in the orbit of Uranus, and again to predict the existence of another planet closer to the Sun than Mercury. When this was done, it was learned that his theory was not entirely correct. These mistakes in his theory were corrected by Albert Einstein's theory of General Relativity. Newton's theory is still commonly used for many things because it is simpler and is accurate enough for many uses.