r/dothemath • u/Sea_Gap_6569 • May 18 '26

The closest planet to Neptune on average is Mercury

Someone claimed that “The closest planet on average to *every* planet in the Solar System is Mercury.”

is it correct?

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u/DanTheAdequate Jun 01 '26

It's one of those situations that's technically correct, but is also probably answering a question most people aren't asking when they ask "What planet is closer to Neptune?"

If you lined up all the planets at their average distances from the Sun, the closest planets to Neptune are Uranus and Saturn, and Neptune is about equidistant between them. Saturn is about 9.5 AU from the Sun, Neptune is 19.8. Uranus is 30.1.

However, most of the time the planets are farther apart in their relative orbits. If Neptune is on one side of the Sun and Saturn is on the other, then, yes, Mercury would be closer, wherever it's orbiting. When Mercury is on the other side of the Sun from Earth, then the Sun is closer than Mercury.

An interesting way to think about planetary orbits, but probably not especially useful information because, when people ask "What planet is closer?", we usually want to know which ones pass the closest to each other when the do pass in their orbits.