Carbon from Interstellar Clouds to Habitable Worlds

Abstract: Carbon is an essential element for a habitable world. Inner (r < 3 au) disk planetary carbon compositions are strongly influenced by supply and survival of carbonaceous solids. Here we trace the journey of carbon from the interstellar medium to the processes leading to planet formation. The review highlights the following central aspects: -Organics forming in evolved star envelopes are supplemented by aromatic molecules forming in the dense ISM to represent the seeds of (hydro)carbon supply through pervasive pebble drift to rocky planets and sub-Neptune cores. -Within the protoplanetary disk the sharp gradient in the C/Si content of Solar System bodies and mineral geochemistry outlines a tale of carbon loss from pebbles to within planetesimals and planets, and from planetary atmospheres. -Within two planet formation paradigms (pebble and planetesimal accretion) a range of planetary carbon content is possible that is strongly influenced by early (< 0.5 Myr) formation of a pressure bump that titrates drift. Overall, it is unlikely that the carbon architecture of our Solar System applies to all systems. In the absence of giant planets, carbon-rich rocky worlds and sub-Neptunes may be common. We outline observations that support their presence and discuss habitability of terrestrial worlds.