* Jovian Planets: Jupiter, Saturn, Uranus, and Neptune are gas giants. They are primarily composed of hydrogen and helium, with smaller amounts of other elements like methane and ammonia. Their vast size and low-density atmospheres result in lower overall densities.

* Terrestrial Planets: Mercury, Venus, Earth, and Mars are rocky planets with solid surfaces. They have a higher proportion of heavier elements like iron and nickel in their cores, leading to a higher overall density.

Here's a simple analogy: Imagine a balloon filled with air. It's large but lightweight, similar to a Jovian planet. Now imagine a rock. It's much smaller but heavier, like a terrestrial planet.

Key points:

* Mass: Jovian planets are much more massive than terrestrial planets.

* Density: Jovian planets have lower density than terrestrial planets. This is due to their composition and the vastness of their atmospheres.

* Composition: Jovian planets are primarily composed of hydrogen and helium, while terrestrial planets are composed of rock and metal.