The Birth of Planets and the Story of Carbon

Elemental carbon is the backbone of the chemistry of life and its presence on the planetary surface is essential for life’s origin. In this talk I will preset our current understanding of planet formation (for both gas-rich planets and terrestrial worlds) through the lens of following the supply of carbon. Three aspects of this story will be discussed that link planetary atmospheric composition to that of its natal disk at formation. The first topic relates to the gaseous elemental C/O ratio that is theorized to link the location of planet formation in the disk to its ultimate atmospheric composition. Here I will present the current state of the art that utilizes Atacama Large Millimeter Array (ALMA) observations of molecular emission in nearby disk systems to measure the bulk C/O ratio as a function of distance from the star. This will be compared to measurements of the C/O ratio in the atmospheres of planets found at large distances from their host star via direct detection. The second topic will explore the use of the ¹²C/¹³C isotopic ratio as an additional method that may link disk and planet composition. The recent detection by Zhang et al. (2021) of a ¹³CO-rich atmosphere in a young super-Jupiter requires the presence a ¹³C enrichment in some uncertain carrier in its natal disk. I will discuss how simple chemical reactions, measured in Earth laboratories, can produce this outcome. As the third and final topic, I will conclude with the story of carbon supply to terrestrial worlds, both Earth-like and super-Earth’s, and sub-Neptune’s in the inner few astronomical units of the disk. I will present a new model of planet formation that melds astrophysics and geophysics to predict that a population of these planets will form in particular locations in their protoplanetary disks such that they receive significant inventories of organics, with low water content. As a result of geochemical equilibrium, the mantle of such a planet could be rich in reduced carbon but have relatively low oxygen (water) content. Outgassing would naturally yield the ingredients for haze production, which is widely observed in super-Earth’s and sub-Neptune’s. Although this type of planet has no solar system counterpart, it should be common in the galaxy, and for terrestrial worlds, it will hold uncertain effects on the potential for habitability.