The planets closest to the Sun, like Mercury and Venus, lost most of their lighter elements, such as hydrogen and helium, due to the intense heat. Here's why:

* Solar wind: The Sun constantly emits a stream of charged particles called solar wind. This wind is much stronger close to the Sun.

* High temperatures: The intense heat near the Sun gives these elements a high kinetic energy.

* Escape velocity: The escape velocity of a planet is the minimum speed an object needs to escape its gravitational pull.

* Combination of factors: The combination of high temperatures, strong solar wind, and relatively low escape velocity for lighter elements caused them to be stripped away from these inner planets over time.

In simpler terms: Imagine you're trying to hold onto a balloon filled with helium. If you're standing still, it's easy. But if there's a strong wind blowing, the balloon is more likely to escape your grip. The same principle applies to lighter elements near the Sun.

It's important to note that this process happened early in the solar system's history when the Sun was much hotter and the solar wind was much stronger.