the melt compared to the parent rock. We find those planetary compositions with lower alkali and silica abundance, such as those crustal materials that form on Earth with basaltic composition, are most likely to sink, while those with greater alkaline and silicic compositions are less likely (Figure \[fig:rocktype\]; Supplementary Figure \[fig:nonEarth\]). This is consistent with Earth’s tectonics, in which low-density and alkali-rich, andesitic continental crust does not subduct [@Cloo93].

![Alkali abundance as a function of silica of the exocrust compositions for our sample of *Kepler* planet-host (diamonds) and @Adib12 stars (circles). Color is a function of the chemical buoyancy as calculated from a thermally equilibrated, 5% melt layer. Points outlined are those with known planets (exoplanets.org). The green box represents the Earth Range of average Earth basaltic compositions of @Gale13. The composition of Mercury’s Northern Volcanic Plains and Intercrater Plains and Heavily Cratered Terrains [diamonds; @Namu16] and Mars’ type 1 and type 2 crusts [squares, including K$_2$O; @McSw03] are included. Venus’ alkali composition has yet to be determined [@Trei13]. Each crustal composition of Mercury and Mars fall within the positively buoyant andesitic field, despite 3 of the 4 measurements containing Na$_2$O abundances within the Earth-like range.[]{data-label="fig:rocktype"}](RockType.pdf){width=".6\linewidth"}

The magnitude of chemical 