A primary atmosphere is an atmosphere of a planet that forms by accretion of gaseous matter from the accretion disc of the planet's sun. Planets such as Jupiter and Saturn have primary atmospheres. Primary atmospheres are very thick compared to secondary atmospheres like the one found on Earth. The primary atmosphere was lost on the terrestrial planets due to a combination of surface temperature, mass of the atoms and escape velocity of the planet.^[1]

Primary atmospheres begin to form during the early stages of a solar system's development. As a star forms from a collapsing cloud of gas and dust, the remaining material flattens into a rotating disk around the star, known as the protoplanetary disk. This disk is rich in gases like hydrogen and helium, which are the most abundant elements in the universe.^[2]

Planets start to form within this disk through the process of accretion. As dust and solid materials coalesce to form planetesimals and eventually protoplanets, these bodies begin to exert gravitational forces. The gravity of these growing protoplanets attracts surrounding gases from the protoplanetary disk. Larger planets, particularly those forming in the colder outer regions of the disk, are capable of attracting more substantial envelopes of gas, leading to the formation of thick primary atmospheres.^[2]

The composition of a primary atmosphere is primarily hydrogen and helium, with minor amounts of other volatiles like water vapor, methane, and ammonia, depending on the temperature and region of the protoplanetary disk. These atmopsheres are generally thick and extended, enveloping the young planet in a dense layer of gas.^[2]

Recent observations by the James Webb Space Telescope (JWST) have provided groundbreaking insights into the atmospheric composition of exoplanets, which are vital for understanding both primary and secondary atmospheres. A notable example is the detection of carbon dioxide (CO2) in the atmosphere of the exoplanet WASP-39b. This detection, achieved through transmission spectroscopy in the 3.0-5.5 micrometre wavelength range, has revealed a significant CO2 absorption feature at 4.3 micrometres, with a 26-sigma significance.^[3] These findings are critical as CO2 is a key indicator of metal enrichment in planetary atmospheres, influence the formation process of primary atmospheres of gas giants.

The JWST data suggests that the atmospheric composition of WASP-39b includes not only CO2 but also water, carbon monoxide, and hydrogen sulfide, with moderate cloud opacity and little methane. This composition aligns with one-dimensional models assuming a ten-times solar metallicity and radiative-convective-thermochemical equilibrium.^[3] The presence of these gases in the atmospheres of hot gas giants like WASP-39b provides insights into the processes that may have shaped the primary atmospheres of similar massive planets in our solar system.