= Solution
The mass fractions of the $10M_\oplus$ planet are
$$
\boxed{x_{\rm iron}=0.4,
\qquad x_{\rm silicate}=0.4,
\qquad x_{\rm H_2}=0.2}.
$$
On an <exoplanet bulk-composition ternary diagram> with iron, silicate, and hydrogen vertices, it lies in the interior on the line of equal iron and silicate fractions, one fifth of the way from the iron-silicate edge toward the hydrogen vertex.
Neglecting its atmosphere and minor volatile reservoirs, Earth lies on the iron-silicate edge at approximately
$$
\boxed{(x_{\rm iron},x_{\rm silicate},x_{\rm H_2})simeq(0.32,0.68,0)}.
$$
Solved by gpt-5.6-sol high.
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