Why is the composition of the sun so distinct from that of earth?

Question

Given that the sun is – in astronomical distances – quite close to the earth, why are the two composed of such distinctly different substances?

Sun Composition

• Hydrogen 74.9%
• Helium 23.8%
• Oxygen ~1%
• Carbon 0.3%
• Others <1%

Earth Composition

• Iron 32.1%
• Oxygen 30.1%
• Silicon 15.1%
• Magnesium 13.9%
• Sulfur 2.9%
• Nickel 1.8%
• Calcium 1.5%
• Aluminum 1.4%
• Other 1.2%

A couple things I notice. The sun is quite homogenous compared to earth! It is mostly composed of just two elements whereas on earth no single element makes up more than 32% of the planet's mass.

Also, there is extremely little overlap in the elements: hydrogen and helium are the only game in town on the sun, but are nearly nonexistent on earth.

This makes me very curious! What aspect of the process of the formation of the solar system was responsible for essentially segregating these elements? Is it simply that the heavier elements were "burned" away in the hotter environment of the sun, or is there some other explanation?

Answer 1

The inner planets like Earth formed initially from the coalescence of solids in the protoplanetary disc. The disc itself would have a similar overall composition to the Sun itself, but close to the protosun, the composition of the solids would be quite different.

The high condensation temperatures of solids containing iron, silicon and oxygen mean that they predominate. Hydrogen and helium bearing substances (like water and methane) tend to be volatile and cannot condense at high temperatures.

Thus the inner planets form from solid material that is depleted of hydrogen and helium compared to the Sun and protosolar nebula. Further, after formation, the masses of the inner planets (or the planetesimals they formed from) are too low and their temperatures too high to capture or retain volatile materials or hydrogen and helium gas in their atmospheres.

Answer 2

The composition of the Sun is close to the composition of the universe as a whole. It's the Earth that's the outlier. If you look up the elemental composition of the universe as a whole, you'll see numbers for hydrogen and helium almost identical with the ones for the Sun. Theory can predict the elemental ratios. The universe started out as entirely hydrogen, and helium and a few light elements like lithium were created in the big bang. Heavier elements like oxygen and iron were made in stars, and elements heavier than iron are largely from supernovae. But enough background, let's answer your question. Earth, along with the other planets, formed from the same cloud of dust and gas as the Sun. The cloud started out with the same elemental composition as the Sun. The cloud collapsed under the force of gravity, and somehow chunks of material (called planetesimals) started to coalesce into planets (nobody is really certain how this process worked). The proto-Sun started to emit light and warm up the surroundings. The regions closer to the Sun, where the Earth was forming, got hot enough that light elements like hydrogen evaporated from the planetesimals. Left behind were heavier elements like oxygen, silicon (which make up most rocks) and iron. The lighter elements ended up further out, which is why Jupiter has a hydrogen-rich atmosphere.

Answer 3

Star formation requires a continuous energy loss of the material in the proto-stellar cloud. This can only happen through inelastic collisions of the atoms/ions (with the kinetic energy lost radiated away). Now, inelastic (as well as elastic) collisions are most effective with regard to energy loss for particles of equal or similar mass. So hydrogen and helium (which make up the bulk of the proto-stellar cloud) lose energy much more quickly colliding with each other (and thus collapse faster) than the heavier elements colliding with them. The density of the latter in the proto-stellar cloud is too small and the mass of those atoms too high for any significant energy loss to occur, so they stay 'in orbit' whilst hydrogen (and to some extent helium) collapse further. The few heavier elements you find in the sun are just contaminations in this sense.