Before they could see Neptune, what evidence led scientists to conclude that it existed?

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Explanation:

The mass of Neptune's rings is low when compared to some of our system moons. ... what made the scientist think that Neptune must exist? After the discovery of Uranus, scientists were having trouble figuring out the planet's orbit. They realized that there must be another planet farther out than Uranus

answer; i believe that the correct answer is (the explosion of a nearby star(supernova);