Eli5 Why was the sun made up of mainly just hydrogen when it was formed compared to inner planets that have so many more elements, especially higher ones?

Question

All,

I have a basic understanding that a lot of elements were brought to planets by comets, supernova explosions and so on.

Why didn’t the sun attract most of these comets and fragments from celestial explosions instead of the planets? And why was / is the sun mainly just hydrogen?

In: 105

Answer 1

All of the material in the planets began life with the same composition as the Sun, but they’ve all evolved away from it. We know this from examining of a type of meteorite known as a carbonaceous chondrite, believed to have formed very early in the history of the Solar System. With the exception of gases – these strange black rocks have an elemental composition almost exactly the same as the Sun.

The planets smaller than Jupiter and Saturn don’t have strong enough gravity to hold on to the lightest elements like hydrogen and helium which make up most of the Sun. Closer in to the Sun, the heat and solar wind strip the elements with the lowest vaporisation points and blow them into space, so the inner planets are rocky with high concentrations of metals.

Later, as planets continue to evolve, factors like their internal heat become important. Neither Venus or Mars seem to have a churning metal core that generates a magnetic field, so in both cases, the solar wind has been busy stripping their atmospheres of light elements and blowing them into space.