Exoplanet bulk-composition ternary diagram

An exoplanet bulk-composition ternary diagram represents three component mass fractions by one point inside a triangle. If the components have fractions $x_1,x_2,x_3$, then $x_i\ge 0$ and $x_1+x_2+x_3=1$. Each vertex is a pure component, an edge excludes the opposite component, and the centre corresponds to equal thirds. The fractions are barycentric coordinates relative to the vertices.

For an illustrative iron-rock-water model, measured mass and radius restrict the allowed part of the triangle through an exoplanet interior calculation. They need not select a unique point, making exoplanet interior-composition degeneracy visible. The diagram is conditional on the chosen three components: an unmodelled hydrogen-helium envelope cannot automatically be interpreted as more of one of those solid components.