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If the photospheres of solar-type stars represent the composition ofcircumstellar disks from which any planets formed, spectroscopic determinationsof stellar elemental abundances offer information on the composition of thoseplanets, including smaller, rocky planets. In particular, the C/O ratio isproposed to be a key determinant of the composition of solids that condensefrom disk gas and are incorporated into planets. Also, planets may leavechemical signatures on the photospheres of their host stars by sequesteringheavy elements, or by being accreted by the stars. The presence, absence, andcomposition of planets could be revealed by small differences in the relativeabundances between stars. I critically examine these scenarios and show that(i) a model of Galactic chemical evolution predicts that the C/O ratio isexpected to be close to the solar value and vary little between dwarf stars inthe solar neighborhood; (ii) spectroscopic surveys of M dwarf stars limit theoccurrence of stars with C/O $\gtrsim 1$ to $<10^{-3}$; and (iii) planetesimalchemistry will be controlled by the composition of oxygen-rich dust inheritedfrom the molecular cloud and processed in a dust-rich environment, not a gaswith the stellar composition. A second generation of more reduced planetesimalscould be produced by re-equilibration of some material with dust-depleted gas.Finally, I discuss how minor differences in relative abundances between starsthat correlate with condensation temperature can be explained by dust-gassegregation, perhaps in circumstellar disks, rather than planet formation.

What are little worlds made of? Stellar abundances and the building blocks of planets