The eight planets of our solar system range from hot, rocky Mercury to the huge gas giants further out, but Earth is unique in that it is the densest of all the planets. The reasons behind that have to do with the way the planets formed in the first place. They coalesced from material spinning around the sun as it formed, all at different distances from the star that affected what they are made of.
If everything were based purely on the elements making them up, Mercury would be the densest planet. Mercury has a higher proportion of elements that are higher on the periodic table compared to any other known world in the Solar System. Even the asteroids that have had their volatile ices boiled off aren't as dense as Mercury is based on elements alone. Venus is #2, Earth is #3, followed by Mars, some asteroids, and then Jupiter's innermost moon: Io.
But it isn't just the raw material composition of a world that determines its density. There's also the issue of gravitational compression, which has a greater effect for worlds the larger their masses are. This is something we've learned a lot about by studying planets beyond our own Solar System, as they've taught us what the different categories of exoplanet are. That's allowed us to infer what physical processes are at play that lead to the worlds we observe.
The process of planet formation made Earth unique, and may go a long way toward explaining why it is habitable. The story is told in fascinating detail at Forbes. -via Damn Interesting
(Image credit: NASA)