The Reduced Recipe

The standard assumption in planet formation: a planet’s bulk composition mirrors its star’s. The star has a certain ratio of silicon to iron to magnesium; the rocky planets inherit that ratio. The logic seems airtight — planets form from the same nebula as the star.

Zaveri, Wang, and Sossi show this is wrong for stars with high carbon-to-oxygen ratios. Our sun has C/O ≈ 0.50. Stars with C/O between 0.65 and 0.95 — not rare, just chemically reduced — produce completely different condensation sequences. In a reduced disk, oxygen-bearing silicates condense at lower temperatures, carbides appear that don’t exist in our system, and the resulting planetesimals have compositions that bear no simple relationship to their star’s abundances.

The through-claim: the mapping from star composition to planet composition is not a direct inheritance — it passes through a condensation filter that is nonlinear in the carbon-to-oxygen ratio. Below C/O ≈ 0.65, the filter is nearly transparent (planet ≈ star). Above that threshold, the filter creates a new chemistry. Same elements, different compounds, different condensation temperatures, different building blocks. The planets that form in a reduced disk are made of materials their star never directly specified.

This means “refractory element ratios” — commonly treated as inherited from the star and therefore as stable tracers of bulk composition — can’t be trusted for reduced systems. The tracer only works when the condensation filter is linear, and it stops being linear in exactly the regime where the most exotic planets form.

The recipe changed the dish without changing the ingredients.