GPT4 seems to think it is a 3 body problem. Mercury and Earth are of similar size and exert gravitational effects on each other. Take away mercury and maybe the earth slowly drifts a little.
The point is that a very small difference in starting conditions yield wildly different results.
We discovered half the planets by doing the math to predict the orbits based on the known distribution of math in the solar system.
General Relativity was initially validated by predicting mercury's orbit accurately.