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A highly stylized rendition of our solar system NASA/JPL

If someone asked which planet was closest to Earth, you’d be pretty certain — it’s Venus. We measure distances in the solar system in terms of Astronomical Units (AU), where 1 AU is the distance between Earth and the Sun. So, with a orbit distance of 0.72 AU, you’d assume that Venus is just 0.28 AU away from us, right? Even NASA says that Venus is our closest planetary neighbor.

You’d be wrong and so is NASA, according to a new article in Physics Today. The authors argue that these measurements calculate the average distance of planets from the Sun, but they don’t take into account all of the time that the planets spend moving away from each other. They say the traditional model is “simple but wrong,” so they designed a new mathematical model which takes into account the movement of the planets relative to each other as well as to the Sun.

Using this model, they found the distance between two orbiting bodies is proportional to the radius of the inner orbit. “In other words,” the article explains, “Mercury is closer to Earth, on average, than Venus is because it orbits the Sun more closely.”

If you’re wondering how that works, it’s true that Venus is the closest planet to Earth when at its nearest point, as no other planet comes closer to us, but at times Venus and Earth are on opposite sides of the Sun and are 1.72 AU apart. When we consider the average distance between planets over time then our closet neighbor is actually Mercury which orbits at 0.39 AU.

Credit: Tom Stockman/Gabriel Monroe/Samuel Cordner

OK, sure, that makes sense. Mercury has a small orbit so we spend time close to it more often. But here’s the mind-bending part: Mercury is actually the closest planet to every other planet in the solar system, including Neptune and even quasi-planet Pluto. The same stipulations apply to other planets as they do to Earth — Mercury’s tiny orbit makes it closer on average to all other planets, as you can see in the table of average distances at the bottom of the article.

A simulation of the movements of bodies in the solar system was run for 10,000 years, and the traditional way of thinking about distances between planets gave results that differed from the simulation by up to 300 percent. The new method gave results that deviated by less than 1 percent, making it undeniably more accurate.

So next time someone asks which is the nearest planet to us, you’ll be in the know: it’s Mercury, as proven by mathematics.








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