reduced planets

the Earth is oxidized, not just because there is free oxygen in the atmosphere, but also because the rocks are predominantly silicon oxides
there has been some speculation about "carbon planets", but not much detailed modeling

now we're getting there.
There were several presentations on carbon chemistry and formation of rocky planets with reduced surface chemistry.
Results suggest for C/O > 0.8 reduced carbon can dominate the chemistry of the terrestrial planets with some interesting implications.

It is possible to have autocatalytic chemistry on micro grains, with carbon chemistry on the surface sustaining and encouraging conditions for further reactions;
there is model evidence for a "soot line", where temperatures are cool enough that polyaromatic hydrocarbons are not oxidized and hydrogenated carbon rather than oxidized carbon dominates molecular chemistry, and ultimately this implies the possibility of silicon carbide rather than silicon oxide rocks.

Lots more work to do on this, not the least of which is the phase diagrams for SiC at the relevant pressure and temperatures, along with what happens when other metals join the mix.

Planets like this are likely rare, H₂O and oxygen are dominant in most astronomical situations, but there are carbon rich environments and the planets could be interesting with very rich prebiotic chemistry and interesting metabolic options for any life that evolves.

Fun stuff