The center is only the center because that's where the most material coalesced to create a gravity well. Before that happened, the solar system was a pretty evenly distributed mass of gases and heavy elements.

Slight variations in the distribution of that cloud meant that parts of it condensed together due to gravity, and then drew in more elements, and so on.

Eventually, enough hydrogen and helium collected to ignite a fusion reaction, which made the sun. But it's sort of backwards to say that hydrogen and helium fell to the center. It's more accurate to say that the center is defined as wherever the most hydrogen and helium (and everything else) went.

You're also failing to grasp the scale of the sun. Yes, the sun is 99.9% hydrogen and helium, but the other 0.1% is heavier elements.

That 0.1% weighs more than Jupiter (which is also mostly gaseous). I haven't run the exact numbers, but it seems likely that there is more metal and other heavy elements in the core of the Sun than in the entire rest of the solar system combined.

Edit: did the math.

* Sun: 0.1% heavier materials (1.99E27 kg) *edit: this number might be as high as 2%*
* All inner planets and asteroid belt combined: 1.18E25 kg
* Gas planets: \~8% heavy elements (2.13 E26 kg)
* **Total heavy elements in the sun: 1.99E27 kg**
* **Total heavy elements in the rest of the solar system: 2.25E26kg**

So even using some pretty rough numbers, there's 10 times *(possibly up to 2000 times)* as much heavy material in the Sun as in the entire rest of the solar system.

The heavy elements *did* accumulate in the center.

The Sun is so huge and heavy that it's kinda hard to understand.

The rest of the solar system; all the planets, proto-planets, moons, asteroids, comets, dust and gas make up what amounts to a few flecks of dust compared to the Sun.

The Sun came first, and gobbled up *everything*. All that's left over are a few crumbs on the floor. That's *us!*

It’s important to remember how solar system formation happened. The protostellar disc (before the Sun formed) was a mixture mostly composed of hydrogen and helium with trace heavy elements. The chemical composition would have been roughly uniform.

Towards the center of the disc, the Sun eventually formed. When the Sun formed, the process of fusion created an outwards radiative pressure. This pressure drove light elements like hydrogen and helium away from the Sun, meaning that the material closest to the Sun contained a higher concentration of heavier elements. Some planets further away like Jupiter and Saturn, though, have a chemical composition similar to the Sun by virtue of having formed father away (meaning less radiative pressure; they also had higher gravity than Mercury etc. so they could retain lighter elements more easily). Meanwhile, the Sun of course keeps most of its hydrogen and helium.

It’s also important to note that most of the iron etc. in the solar system is indeed contained in the Sun. Really, the main difference is that the Sun retained most of its lighter elements, and meanwhile the abundance of lighter elements on the planets differ significantly (for various reasons). So the explanation really comes down to “what happened to the hydrogen and helium near each planet?”, and “why was the chemical composition of the protostellar disc uniform?”.


The reason why the composition in the protostellar disc was uniform is because it was well mixed, turbulent, and spinning. If the disc were static, then yes all the heavier metals would have sunk towards the center of the solar system over a long period of time. But, this isn’t the only dynamic process in a system like this, and competing forces etc. kept things mixed. In contrast, the conditions of protoplanetary discs allowed for heavier metals to sink towards the center (leading to an iron core in the Earth, for example).

The premise of your question is wrong. The heavier elements, and everything else, do accumulate at the centre of a solar system - there is simply a lot more helium and hydrogen around than anything else


The question I think you actually should be asking is "why don't rocky planets like earth have any hydrogen and helium"? To which the answer is simply that they aren't massive enough to keep them around


It's easy to get cognitively biased since we developed on earth among heavy elements, but the most common states of matter in the universe are those lighter elements - earth is the 'strange' one

During the early stages of the universe, the Big Bang primarily produced light elements like hydrogen and helium. These light elements were distributed throughout space. As the universe expanded and cooled, gravity started to play a crucial role in the formation of structures, including galaxies, stars, and planetary systems.

Inside massive stars, through nuclear fusion reactions, lighter elements like hydrogen and helium are converted into heavier elements such as carbon, oxygen, and iron. This process occurs during the stellar life cycle, culminating in the star's explosive death as a supernova. The supernova explosion disperses enriched material, including heavier elements, into space.

This ejected material, enriched with heavy elements, then becomes part of interstellar clouds, which are the birthplaces of new stars and planetary systems. As these clouds collapse under the influence of gravity, they form a protoplanetary disk—a rotating disk of gas and dust. Over time, the disk begins to accrete matter and undergoes further condensation and fragmentation, eventually leading to the formation of planets.

In the process of planet formation, lighter elements such as hydrogen and helium tend to accumulate in the outer regions of the disk due to their lower boiling points and higher volatility. Closer to the central star, where it is hotter, the lighter elements are more likely to be in a gaseous state. As a result, the inner planets, like Earth, are primarily composed of heavier elements like rock and metal.

In summary, the distribution of elements in the solar system is a consequence of the nucleosynthesis processes that occurred in stars and the subsequent formation of planets from interstellar material. The lighter elements, like hydrogen and helium, were more abundant in the early universe and became enriched with heavier elements through stellar processes and supernova explosions. The specific distribution of elements in the solar system is influenced by the dynamics and conditions of the protoplanetary disk during planet formation.

Initially, there was hydrogen and helium in Earth's orbit, but Earth's gravity wasn't strong enough to hold on to most of it. Earth ended up with heavy elements, because lighter elements got blown away. The Sun's gravity was strong enough to hold onto its hydrogen and helium.

All the material in the solar system wants to be at the centre, because of the gravitational attraction. But angular momentum means that some fraction of the initial material (which was a big mix of elements) ended up rotating around the centre instead. But at the start the mix of elements in the Sun and the mix of elements in this disc had identical compositions.

Within this rotating material, the parts further from the hot protostar cooled down. Now, Hydrogen & Helium effectively remain as gasses even in the coldest parts of space. But the other elements can condense into solid ice & rock. Solids (unlike liquids and gasses) are quite good at sticking together, so the solid parts orbiting within that protoplanetary disc coalesced into larger and larger balls, growing like snowballs until they became planets. In the case of the gas giants, they grew enough to have enough gravity to pull some of the hydrogen-helium that was also swirling around the young sun onto their planets. Not so in the case of the terrestrial planets, so it's no wonder they are formed of solids. And then at some point, all the loose gas which wasn't either accreted onto the star or planets got blown away by solar radiation.

So TLDR: Both the Sun and the planet-forming disc started with the same mix of elements, but rocky planets only formed from heavy elements because it's solid (not gaseous) material which sticks together during planet formation.

The heavier elements are made within stars from the hydrogen & helium, then dispersed into outer space through supernovae I've always heard. Way more simple elements like hydrogen & helium available to create those heavier elements within stars due to the effects of heat and gravity upon hydrogen & helium.

https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/background-elements.html

I believe it was because the heavier elements were formed after the lighter elements, with hydrogen and helium being created in the Big Bang, while the heavier elements were caused by supernova, which take a long time to happen. This is a basic answer, and if anyone more scientifically literate than me wants to expand on this or correct me, please do