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When we call a planet rocky, we usually picture a solid surface covered in craters, deserts, or mountains. But in planetary science, the term refers not to a world's exterior, but to what lies beneath. A rocky world is defined by its bulk composition and internal structure, specifically its separation into a dense metallic core and a silicate mantle. This episode examines the physical foundation of terrestrial planets, exploring how Mercury, Venus, Earth, and Mars all share a fundamental building material of rock and metal, yet evolved into radically different environments.

The discussion also extends beyond our solar system to the challenges of identifying rocky exoplanets. Astronomers can measure the radius and mass of distant worlds to calculate their bulk density, but this data often cannot uniquely determine what lies inside. A bare rock and a smaller core wrapped in a thick atmosphere can yield identical measurements, a fundamental limitation known as degeneracy. We look at how the radius valley helps statistically separate dense super-Earths from volatile-rich sub-Neptunes, even as individual worlds remain ambiguous.

I wanted to understand why a term as familiar as "rocky" becomes so complicated when applied across the universe. Some questions sound simple until you try to answer them, and tracing the definition of a terrestrial world from the molten differentiation of our inner planets to the statistical gaps in distant star systems reveals just how much hidden structure dictates a planet's fate. It is a starting point for understanding how solid worlds form, cool, and change over billions of years.