What worlds are made of

Every exoplanet with both a measured mass and radius, plotted against the lines theory predicts for pure rock, Earth-like rock-and-iron, and water-rich worlds. Where a planet lands tells you what it's likely built from.

Composition curves are theoretical models for solid planets up to ~20 Earth masses: pure rock and Earth-like rock+iron from Zeng et al. (2016), the 50%-water ocean-world line from Brugger et al. (2017) / Sotin et al. (2007). Below the rock lines a world must be iron-enriched (pure iron is the hard density floor); well above the water line it almost certainly carries a thick hydrogen/helium envelope — a sub-Neptune or gas giant. Mass and radius each carry real measurement uncertainty not shown here. Data: NASA Exoplanet Archive.