What type of material is thought to explain the larger size of the outer planets relative to the inner planets?

Question

Quizplus · Accepted Answer

The outer planets are thought to be composed largely of ice (such as water, methane, and ammonia) and other volatile materials, which have lower densities than the rock and metal that make up the inner planets. This means that the outer planets can have larger sizes without having much greater masses than the inner planets.

What Type of Material Is Thought to Explain the Larger