How big is Neptune compared to Earth?

We all know that Earth isn’t the biggest planet in the solar system, but in comparison to those pretty close to it, it’s the largest of the terrestrial planets. But when you venture out past the Asteroid belt, which runs between Mars and Jupiter, the planets get much, much bigger. The furthest planet from the Sun is Neptune, and whilst it is much bigger than Earth, it may not be as big as you’d have thought.

How big is Neptune compared to Earth?

The answer is that Neptune is approximately four times the size of the planet Earth. Neptune has a radius of around 15,000 miles, and a diameter of approximately 30,000 miles. If we compare this to Earth, which has a radius of 4,000 miles and diameter of 16,000 miles, we can see that it’s a little less than 4 times smaller than Neptune.

So although Neptune is quite a bit bigger than Earth, it’s still the smallest of the four Jovian planets – it’s a very similar size to Uranus, but less than half the size of both Saturn and Jupiter, which is the largest planet of them all. This is just one of many cool facts about the planet Neptune. When we talk about how big something is, we’re referring to the actual size of the two objects, but we can compare them in more ways than that.

For example, we can also look at the mass of the two planets too, which gives us a more drastically different result. Neptune has a pretty large mass of more than 102000000000000000000000000 kg, or 1.02 x 1026 kg for short. This is more than 17 times that of the Earth, which is a noticeable difference.

We can also look at the volume of the two planets to compare them to one another too. Neptune has a volume of 6.3 x 1013 km3, which means that you could comfortably fit 57 Earth’s inside Neptune without having to worry. Sometimes, these kinds of measurements are better used to explain the difference in size between two planets, as they give an accurate representation of the difference.

A walk across Neptune

Now it’s not actually possible for us to ever walk on any of these Jovian planets like it is on Mars. Humans could potential take a stroll Mars, which is primarily surfaced with a soft rock composition which is likely similar to sand or soil. But the surface of Neptune is covered in water and ice, though this is still buried beneath a thick atmosphere made up of several different gases.

If we were to theoretically walk across the surface of the planet, it would be almost 100,000 miles all the way round. And actually, interestingly enough beneath this surface of gas and then water, Neptune does have a solid core at its center. And, this center alone is predicted to be a similar size to Earth.

So although there may be an Earth sized core at its heart, it is buried deep beneath all of this water, ice and gas. However, this is far more ice and less gas than previous thought – both Neptune and Uranus are only made up of 20% gas, as opposed to the likes of Jupiter which is more than 90% gas. This is the main reason why we call the two most distant planets ice giants nowadays, as opposed to gas giants.


In conclusion, Neptune is pretty big when you compare it to our planet, but it’s not made up of a solid material quite like Earth is, which is one of the reasons why the planet itself is so massive. Neptune is often referred to as an ice giant due to its large size and frosty exterior, but even so the truth is that just like Uranus, a good amount of its size is still made up of gas.

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