Neptune’s closest neighbor is Mercury

There is information that we have stored in our heads since we learned it, such as prepositions or the planets that make up the Solar System. And that has its handicaps: having to enumerate the list to get to what interests you or if you are already a few years old, finishing the string of planets with Pluto. Old spoiler: Pluto was demoted in 2006, despite the fact that there are scientists who they question the definition of a planet and therefore, its appearance or not on that list.

What is the closest planet to Earth? Faced with that question and with the temptation to recite the burned-in list, many people will probably say Venus and just as many will say Mars. Reality has its substance and although the situation changes frequently, it is generally considered that the correct answer is Venus. In fact, taking a look at the distances between each pair of planets We would reach that same conclusion. Well, yes, but no.

Mercury is the winning horse. To NASA It refers to Venus as “our closest planetary neighbor” and although this is true if we stick to which planet is closest to Earth, it is not true if we are interested in knowing which planet is closest on average. Here things change and have a new winner: Mercury. Mercury is the innermost planet in the solar system, but on average it spends more time near Earth than Venus. What’s more, Mercury is on average the closest planet to all the other planets in the Solar System.

How is the proximity between planets considered?. The usual method is limited to subtracting the mean radius of the inner orbit from that of the outer orbit. Thus, the average distance between Earth (1 AU) and Venus (0.72 AU) would be 0.28 AU. When they are furthest away, Venus is 1.72 AU from Earth. Although it is intuitive to consider the average distance between each point of two concentric ellipses as the difference of their radii, in reality that difference only determines the average distance of the closest points of the ellipses.

A more precise mathematical method that considers time. The average of both scenarios above improves the calculation, but is still imprecise, they explain scientists Tom Stockman, Gabriel Monroe and Samuel Cordner. So the American Institute of Physics devised a more precise mathematical method that averages the distance over time of the planets and in this scenario everything changes and not only for the Earth, but also for all the planets.

The method in question is called point-circle (PCM) and models orbits as concentric and coplanar circles. Since the planets spend the same time at each point in their orbit, the average distance can be calculated by integrating all possible positions. Using this method, Venus is an average of 1.14 AU from Earth and Mercury is only 1.04 AU. As they explain:

“We observe that the distance between two orbiting bodies is minimum when the inner orbit is the smallest. That observation gives rise to what we call the whirly-dirly corollary (a reference to an episode of the series Rick and Morty): for two bodies with approximately coplanar, concentric and circular orbits, the average distance between them decreases as the radius of the inner orbit decreases.”

The checks. This research team ran a simulation that calculated the position of the eight planets over 10,000 years and recorded their distance. The results differed by 300% compared to the traditional method, but less than 1% compared to the point-circle method.

Mercury is the closest to all. This discovery not only affects Earth. In fact, it can be generalized to any pair of bodies with approximately circular, concentric and coplanar orbits. With this method, the average distance between two bodies depends on the radius of the inner orbit and the smaller the inner orbit, the smaller the average distance. In short: Mercury is the closest planet to Earth, but also to Neptune and even the degraded Pluto. This finding, beyond changing the paradigm of how to consider distances between planets, may also be useful for estimating communications with satellites.

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Cover | NASA Hubble Space Telescope