What Is The Smallest Planet In The Solar System?

Our solar system is home to eight planets, all of which have different sizes, masses, and densities. The smallest planet in terms of size in our solar system is Mercury, which also happens to be the closest planet to the sun. Mercury is the smallest planet with a diameter of 3,032-miles (4,879-kilometres).

How Small Is Mercury?

Mercury vs Earth
Size comparison of Mercury and Earth. Image credit: NASA

To get an idea of just how small Mercury is, let us compare it to Earth’s moon. The moon has a diameter of 2,159-miles (3,475-kilometres). By dividing Mercury’s diameter by the moon’s diameter, we get how much larger Mercury is than the moon. If we divide 3,032-miles by 2,159-miles, we get 1.4, meaning that Mercury is only 1.4 times larger than Earth’s moon. If we subtract the two numbers, then we get by exactly how much Mercury is larger, which ends up being 873-miles. 

Why Is Mercury So Small?

Image of Mercury taken by the Messenger spacecraft. Image credit: NASA

Our solar system follows an interesting pattern for the first three planets. Starting with Mercury, the planets increase in size through Venus and Earth, and interesting enough, this is probably not a coincidence. When the sun first formed, bursts of energy pushed material away from the sun and towards the outer regions of the solar system, and so as you move further away from the sun, planets tend to get bigger up until a certain point. That’s why the inner solar system contains the small rocky planets, while the outer solar system contains the gas giants. Since Mercury is the closest planet to the sun, it was doomed to be the smallest. 

Not Everything About Mercury Is Small

Multi-colored image of Mercury taken by Messenger. Image credit: NASA

Mercury may be the smallest planet in the solar system, yet it is actually one of the densest planets in the solar system. In fact, the only planet denser than Mercury is Earth, which makes Mercury the second densest planet in the solar system. However, if we measure density relative to size, then Mercury is actually denser than Earth. How is such a small planet so dense? Interestingly, all the rocky planets are significantly more dense than the gas giants, and this is due to the fact that rocky planets contain a much higher amount of heavy metals such as iron and nickel. In the case of Mercury, it has an abnormally large iron core compared to the other planets. It is this iron core that makes Mercury so dense. 


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