Which planet is closest to Earth on average?

Every school chart of the solar system shows Venus snuggled up next to Earth, so the answer to this one feels like a gift. Venus is our neighbour. Venus is our twin. Venus is closest. Except, when you average it out over time, it is not. The planet that spends the most time near us is little Mercury.

The short answer

On average, Mercury is the closest planet to Earth. In 2019, an article in Physics Today by Tom Stockman, Gabriel Monroe and Samuel Cordner calculated the average distance from Earth to Mercury as about 1.04 astronomical units, compared with about 1.14 AU to Venus. One astronomical unit is the average Earth-Sun distance.

The real story

The usual answer comes from subtracting orbit sizes. Earth orbits at about 1 AU, Venus at about 0.72 AU and Mercury at about 0.39 AU. Subtract and Venus comes out roughly 0.28 AU away, much nearer than Mercury. The trouble is that this measures the gap between two orbits, not between two planets. It only describes the moment when both planets line up on the same side of the Sun.

Planets do not sit still at their closest point. They race around the Sun at different speeds, and for long stretches Venus is on the opposite side of the Sun from us, more than 1.7 AU away. Mercury can never get that far, because its orbit is so small. It hugs the Sun, so it never strays very far from anyone.

The authors developed what they called the point-circle method, which averages the distance between every point on one orbit and every point on another. To check it, they ran a simulation covering 10,000 years of planetary motion, recording the distances every 24 hours. The simulation agreed with their method to within about 1 percent. And it threw up a bonus surprise: by the same measure, Mercury is, on average, the closest planet to every other planet in the solar system, including Neptune.

To be fair to Venus, it keeps two titles. Its orbit is still the closest to Earth’s orbit, and at its nearest it comes closer to us than any other planet ever does. It is just not around as much. The 2019 piece was a commentary rather than a peer-reviewed paper, but the maths is simple enough that you can check it yourself.

Why people get it wrong

Diagrams of the solar system draw the planets in a neat line, as if they were parked in order like cars in a car park. That picture quietly turns a question about moving objects into a question about static rings. Once you remember that the planets are constantly circling at different speeds, and often end up on opposite sides of the Sun, the averages change completely. Venus is the neighbour who lives next door but is always away on holiday.

The Sole’s take: “Venus is the closest when it bothers to show up. Mercury just never leaves. Every family’s got one.”