Why do the sun & the gas giant planets have similar compositions? Why are the terrestrial planets different?

Answer 1

The sun and gas giants are mostly hydrogen and helium because that's mostly what the nebula was made of that collapsed to form the solar system. Before the planets formed, the sun ignited and light pressure blew most of the H, He, and H20 vapor out of the inner solar system, leaving behind solid particles, which had already begun to stick to each other. Solid particles a have much greater mass to surface area ratio than individual atoms and molecules, so they are more resistant to such transport. The particles kept sticking to each other until they were massive enough for gravity to cause them to clump even faster, forming the inner planets. As for water (H20), it could condense into particle far from the sun and form comets. Comet out there would frequently get bumped into the inner solar system and supply the earth would water. Meanwhile, the gas giants formed by gravitational collapse directly or onto rocky cores (which is controversial).

Answer 2

Great question.
The other answers are essentially correct, but to put it more simply....
The answer is that their distance from the sun is the determining factor.
The inner planets are so close the the sun that the heat and solar wind cleared the inner areas of most of its hydrogen cloud. The stronge solar wind blew the dust and hydrogen out into the outer solar system and what the solar wind didn't blow away, the heat from the sun boiled away until all that was left was a few rocky planets.

The outer planets are sometimes referred to as the "gas giants" and the "ice giants". The "gas giants" are Jupiter and Saturn and are mostly hydrogen. They are far enough from the sun that the solar wind is too weak to blow particles caught in their gravity away and they are too far for the sun to boil away the hydrogen.
The "ice giants" are even further away, and are cool enough that much of the hydrogen bonds with other elements to form ammonia, and methane.
That is why the planet groups are different. As for why the gas giants and the sun are similar, it is primarily a matter of gravity. Hydrogen is the most abundant element in the univierse and any object that has a strong gravitational force will form similarly as Jupiter and Saturn. If it accumulates enough mass, its core will start fusion, just like the sun did.

Answer 3

A good question. The two main differences between the terrestrial and outer planets are that the latter are bigger and colder. So they've retained dense atmospheres of hydrogen, methane and ammonia that boiled off into space here.