Question: what causes planets to be different sizes and colors ?

Kevin Motherway answered on 19 Nov 2014:

Different types of matter will settle at different distances from the Sun. Earth is one of the rocky planets composed of silicate rocks and an iron core and with a very thin veneer of gas (atmosphere). Crucially water can be liquid on earth surface. With so much water we’re know as the “blue dot” in the solar system.

So the really dense planets are close to the sun. Out past the asteroid belt the planets aren’t rocky any more but are Gas Giants….Out this far from the sun these gas planets haven’t had their gases boiled off by being closer to the sun and gases like methane can even fall as rain. the colour of the planet has to do with what colours the planets upper surface or atmosphere absorbs and what it reflects. The upper atmosphere of Neptune is composed of methane and methane absorbs red light but reflects blue and so its another blue planet due to how methane the dominant gas behaves. Jupiter and Saturn both look stripy as there are different gases and they separate into distinct bands. Jupiter great red spot is one big storm the size of the planet Earth! Venus is permanently covered in cloud and so it appears yelowish/white.

The gas giants are also bigger as its way easier to pull gas from surrounding space than additional rock and dust for a rocky planet. The bigger it gets the more gravity it has and the more gas it pulls in. Closer to the sun the gas boils gas off the inner planets or blasts it away by the solar wind (we have a magnetic field to stop this) and then sucks all that matter back to itself due to its huuuge gravity.