Newton’s laws enabled the scientists to understand and decode the movement of objects in the solar system. It worked every time, everywhere. Everywhere except Mercury. Something strange was going on with our solar system’s innermost planet.
It turns out that every time the Mercury orbits around the Sun, its ellipse precesses ever so slightly. So far so good, no problem there. Albeit, when scientists tried to calculate how quickly this should precess and then they observed how fast this procession was, the numbers did not match. There was a discrepancy of 43 seconds of arc per century (one second of arc=1/3600 degrees). This is small, very small. Nevertheless, it is still a discrepancy.
Newton’s law could not explain this discrepancy and nobody knew what was cause it. Until 1859. Urbain-Jean Joseph Le Verrier, the man who discovered Neptune and sought out opportunities to extend his knowledge, thought he had found the solution to the Mercury’s anomaly.
“….a planet, or if one prefers a group of smaller planets circling in the vicinity of Mercury’s orbit, would be capable of producing the anomalous perturbation felt by the latter planet… According to this hypothesis, the mass sought should exist inside the orbit of Mercury.”
A planet, or a belt or asteroids inside the orbit of Mercury. It sounded exciting.