Ask anyone which planet is the closest to the Earth, and the most popular answer will be Venus. Surprisingly, this answer is correct and wrong at the same time.  We discussed this question with our planetarium learners and they came up with the following reasoning:

• Look at the order of the planets. The closest planet to the Earth should be either the previous planet (Venus), or the following (Mars) 

• Look up the mean radii of these planets orbits

• To find the average distance between Earth & Mars and Earth & Venus, subtract the smaller radius from the bigger one for each pair

• The smaller distance gives us the closest planet. That’s Venus!

Can you see what the problem with this reasoning is? What we actually calculate this way is the minimal possible separation between the two planets, i.e. the closest their orbits will allow them to get to one another. But here is the catch. Any two planets only stay close for a small part of their orbits. Therefore to answer “the closest planet” question we need to look at the full picture and take into account the planets mutual positions through the rest of their orbital journeys. 

So is it Venus?

If you answered Venus to the question above, you probably had in mind that Venus can get closer to the Earth than any other Solar System planet. That is correct. Approximately once every 1 ½ Earth years the two planets get within 25 million miles of one another. No other planet ever gets quite as close to us. But is Venus always the closest planet to the Earth? Absolutely NOT! A big part of Venus’ orbit takes the planet far away from the Earth. At the maximum separation, that is when Venus is on the opposite side from the Sun than the Earth, Venus is a whooping 160 million miles away.

So when Venus is far, what other planet is the closest to us?

Sometimes, like today, for example, the closest planet to the Earth is Mars. The smallest possible distance between us and the Red Planet is 33.9 million miles (although the next time Mars will be that close to us is in the year 2287). In 2020 Mars will make its close approach to the Earth on October 6. On that day it will be 38.6 million miles away.   But on average, the planet that spends THE MOST TIME close to the Earth is not Venus or Mars. It’s Mercury! Of all the planets in the Solar System, Mercury has the smallest orbit. So although it never gets quite as close to the Earth as Venus or Mars, it never gets far away from us also!   In fact, Mercury is the closest – for most of the time- planet not only to the Earth, but also to Mars and Venus and… (things are getting weirder, are you ready?)… Jupiter, Saturn, Uranus and Neptune. The logic here is the same: while Mercury is never really close to any of these planets, its tiny (in space distances) orbit never takes Mercury far away from any of them. Just think of the outer planets. As their orbits are so big, it makes the separation between the planets when they are at a maximum distance from one another, i.e. on the opposite sides from the Sun, absolutely ginormous. This brilliant video illustrates the point perfectly!

The bottom line is…

Mercury is the closest planet to the Earth A LOT of the time. So if somebody asks you – Which planet is the closest to the Earth today? – go for Mercury. Most of the time this would be the correct answer. If you want to learn more cool facts about the Solar System planets, here are our other space blog posts you might like: How come Jupiter has a fuzzy core? How long is a day on the inner planets? The ice giants and why they are so different Rings around gas giants in the Solar System Exploring planet Mercury Any questions or comments? Let our Portable Planetarium team know!