That's a great question! It's not entirely true that outer planets are lighter than inner planets. It depends on how you're comparing them.

Here's why:

* Density: Outer planets (Jupiter, Saturn, Uranus, Neptune) are less dense than inner planets (Mercury, Venus, Earth, Mars). This means they have a lower amount of mass packed into the same volume. This is due to their composition, which is primarily hydrogen and helium, elements that are much lighter than the rock and metal that make up the inner planets.

* Mass: While outer planets have a lower density, they are significantly more massive than inner planets. For example, Jupiter is over 300 times more massive than Earth.

So, while outer planets are less dense, they are actually much heavier. They are essentially giant balls of gas, which explains their lower density but large mass.