Earthed

Why does the Earth and other terestrial planets have a relatively low mass but a relatively high density?

Why does the Earth and other terestrial planets have a relatively low mass but a relatively high density? Is it because gravity is pulling the matter closer together making it denser. Or terestrial planets are more dense because they are made out of denser material?

Public Comments

1. Material.
2. Because past Mars there are the planets we call the Gas Giants made up mostly of gas so its not as dense, but from Mercury to Mars the planets are dense because they're mostly made up of heavy metals, such as iron, and rock.
3. Because God made it that way.
4. Conservation of Momentum suggests a constant reduction of local density if the universe is really expanding:
   
   
   
   http://www.a2dvoices.com/realityCheck/Coriolis/index.html
5. The last part of your question is right.