Why are some planets made out of rock or gas?

Answer 1

The ones farther out from our sun acquired a lot of their mass through accretion, the gathering of large amounts of gasses. The fact that they are farther away from the sun and therefore colder also affects this. Gases in colder temperatures are not as active and therefore don't escape the planet's gravity easily. In general you will find heavier elements on the closest planets and lightest elements on the farthest ones. The metals were drawn closer to the sun as the solar system formed due to their greater mass. The gases like hydrogen and helium got farther away before settling into orbit. There are many other factors involved, the main one being that conditions on the initial state of the planet influenced the formation of other compounds, sometimes unique to that planet, like oxygen on Earth.

Answer 2

It all depends on where the planet was when the solar system was forming. The inner planets (Mercury, Venus, Earth and Mars) are rocky because of their close proximity to the sun. When the sun was formed (as with all stars) it released an explosion of gases and solids. The solids stayed closer to the sun due to its gravitational pull. The gases were flung further out into the solar system, due to having less mass then the rocky bits, which formed our giant planets (Jupiter, Saturn, Uranus and Neptune). Hope this helped.