We assume planets reflect their stars. A metal-rich star should host metal-rich planets. A carbon-rich star should host carbon-rich planets. The host composition sets the menu; the planet picks from what’s available.

In reduced (carbon-rich) stellar systems, thermochemical modeling shows this assumption fails. The planet’s composition diverges dramatically from the star’s. Carbon-rich disks produce iron-nickel alloys, silicon carbide, and graphite as the primary condensates — materials that bear little resemblance to the solar-composition silicates we’re used to. The resulting planets can be “super-Mercuries”: metal-enriched worlds with thin mantles, whose bulk composition is decoupled from the photospheric abundance ratios of their host star.

The decoupling happens because condensation is nonlinear. Small changes in the C/O ratio of the disk gas shift the condensation sequence — which minerals form first, which remain in the gas, which get incorporated into planetary cores. The star’s atmosphere, well-mixed and homogeneous, gives no warning of these phase boundaries. Two stars with similar spectra can produce radically different planetary systems.

This breaks one of exoplanet science’s favorite shortcuts: inferring planetary composition from stellar spectroscopy. The inference requires a mapping from star to planet, and that mapping is not one-to-one. The same star can produce very different worlds depending on where in the disk the planet forms and what condensation sequence operates there.