Question

Explain why the terrestrial planets are so different from the gas giants.

Notice that I did not ask you to describe what the differences are but to explain why the differences exist.

Answer 1

In the solar system when the sun formed from nebula with 98% of mass the remaining matter started to spin at a faster rate to conserve angular momentum.

After that the remaining matter rotated in the form of a disc.

The innermost part of the rotating disc has comparatively higher temperatures than the outermost disc.

So more refractory elements start to accrete near to sun and more volatile elements move to outer disc part.

The inner part started colliding with each other to form small planetesimals and these planetesimals again collided to form the planets.

These inner planets are mostly composed of rock forming minerals like olivine, pyroxene like silicates. Hence the inner planets are mostly devoid of volatiles like H, He and are mostly dense planets .

But the outer planets are formed far away from sun , hence having less refractory and more volatile materials like gas clouds and ice bodies. These planets are rich in H, He because they could condense at lower temperature to form the thick atmosphere of gases.

The most of mass of disc was accumulated in Jovian planets and only 2-3% mass is inner terrestrial planets. So the Jovian planets are massive because their density is less . But the inner planets are more dense and low mass hence small volume.