Chemistry During the Gas-Rich Stage of Planet Formation

Abstract

In this chapter we outline some of the basic understanding of the chemistry that accompanies planet formation. We discuss the basic physical environment which dictates the dominant chemical kinetic pathways for molecule formation. We focus on three zones from both observational and theoretical perspectives: (1) the planet forming midplane and ice/vapor transition zones (snow-lines), (2) the warm disk surface that is shielded from radiation, which can be readily accessed by todays observational facilities, and (3) the surface photodissociation layers where stellar radiation dominates. We end with a discussion of how chemistry influences planet formation along with how to probe the link between formation and ultimate atmospheric composition for gas giants and terrestrial worlds.

Earth’s carbon deficit caused by early loss through irreversible sublimation

“…Refractory organics are likely pre-solar (5), although some could be created through reactions in the solar nebula (44)(45). They are not products of condensation from the solar nebula (1,(3)(4). Upon heating (red trace) the fraction of carbon remaining in dust decreases as ices sublimate at temperatures below 120 K and most refractory carbon carriers (thick vertical bars) in the inherited ISM grains sublimate near a characteristic temperature of ~500 K in 100 years.…”

“…However, equilibrium condensation cannot explain the quantities and forms of carbon found in primitive chondrites and comets. At 10 −3 bar pressure, major carbon carriers (CO, CO 2 , and CH 4 ) in the nebular gas do not condense at temperatures above 80 K and, therefore, remain gaseous within tens of astronomical units (AU) from the forming Sun (1,3,4). Consequently, chondrites and terrestrial worlds would have received no carbon.…”

Astrochemistry associated with planet formation

“…UV radiation fields from the central star impinging on the surface layers can be as high as 10 5 times the interstellar radiation field at 10 au from the star, thereby ionizing atoms and dissociating molecules. Thus, different types of chemistry are important in different parts of the disks (Henning & Semenov 2013;Bergin & Cleeves 2018;Öberg & Bergin 2020) (Fig. 3).…”

The trail of water and the delivery of volatiles to habitable planets

“…Since water represents the bulk volatile reservoir, this deficit will restrict our understanding of the delivery of volatiles to planets. ALMA is showing evidence for a rich and active chemistry (e.g., Bergin & Cleeves 2018) in the carbon and nitrogen elemental pools. This work is ground-breaking but cannot directly constrain the O in the C/O ratio for either the solids or the gas, as a direct, and robust, tracer of cold water and ice is lacking.…”