While a planet’s position in the habitable zone is thought critical for the development of life like ourselves, new work out of Rice University suggests an equally significant factor in planetary growth. Working at a high-pressure laboratory at the university, Damanveer Grewal and Rajdeep Dasgupta have explored how planets capture and retain key volatiles like nitrogen, carbon and water as they form The team used nitrogen as a proxy for volatile distribution in a range of simulated protoplanets.

Two processes are under study here, the first being the accretion of material in the circumstellar disk into a protoplanet, and the rate at which it proceeds. The second is differentiation, as the protoplanet separates into layers ranging from a metallic core to a silicate shell and, finally, an atmospheric envelope. The interplay between these processes is found to determine which volatiles the subsequent planet retains.

Most of the nitrogen is found to escape into the atmosphere during differentiation and is then lost to space as the protoplanet cools or, perhaps, collides with other protoplanets during the turbulent era of planet formation. The data, however, demonstrate the likelihood of nitrogen remaining in the metallic core. Says Grewal:

“We simulated high pressure-temperature conditions by subjecting a mixture of nitrogen-bearing metal and silicate powders to nearly 30,000 times the atmospheric pressure and heating them beyond their melting points. Small metallic blobs embedded in the silicate glasses of the recovered samples were the respective analogs of protoplanetary cores and mantles.”

Nitrogen, the researchers learned, is distributed in different ways between the core, the molten silicate shell and the atmosphere, with the extent of this fractionation being governed by the size of the body. The takeaway: If the rate of differentiation is faster than the rate of accretion for planetary embryos of Moon or Mars-size, then the planets that form from them will not have accreted enough volatiles to support later life.

Earth’s path would have been different. The scientists believe that the building blocks of Earth grew quickly into planetary embryos before they finished differentiating, forming within one to two million years at the beginning of the Solar System. The slower rate of differentiation allowed nitrogen, and other volatiles, to be accreted. Adds Dasgupta:

“Our calculations show that forming an Earth-size planet via planetary embryos that grew extremely quickly before undergoing metal-silicate differentiation sets a unique pathway to satisfy Earth’s nitrogen budget. This work shows there’s much greater affinity of nitrogen toward core-forming metallic liquid than previously thought.”

Image: Nitrogen-bearing, Earth-like planets can be formed if their feedstock material grows quickly to around moon- and Mars-sized planetary embryos before separating into core-mantle-crust-atmosphere, according to Rice University scientists. If metal-silicate differentiation is faster than the growth of planetary embryo-sized bodies, then solid reservoirs fail to retain much nitrogen and planets growing from such feedstock become extremely nitrogen-poor. Credit: Illustration by Amrita P. Vyas/Rice University.

This work takes the emphasis off the stellar nebula and places volatile depletion in the context of processes within the rocky body in formation, especially the affinity of nitrogen toward metallic cores. Here’s how the paper sums it up:

…we show that protoplanetary differentiation can explain the widespread depletion of N in the bulk silicate reservoirs of rocky bodies ranging from asteroids to planetary embryos. Parent body processes rather than nebular processes were responsible for N (and possibly C) depleted character of the bulk silicate reservoirs of rocky bodies in the inner Solar System. A competition between rates of accretion versus rates of differentiation defines the N inventory of bulk planetary embryos, and consequently, larger planets. N budget of larger planets with protracted growth history can be satisfied if they accreted planetary embryos that grew via instantaneous accretion.

And the nebular conclusion:

Because most of the N in those planetary embryos resides in their metallic portions, the cores were the predominant delivery reservoirs for N and other siderophile volatiles like C. Establishing the N budget of the BSE [bulk silicate Earth] chiefly via the cores of differentiated planetary embryos from inner and outer Solar System reservoirs obviates the need of late accretion of chondritic materials as the mode of N delivery to Earth.

Rajdeep Dasgupta, by the way, is principal investigator for the NASA-funded CLEVER Planets project (one of the teams in the Nexus of Exoplanetary Systems Science — NExSS — research network). CLEVER Planets, according to its website, is “working to unravel the conditions of planetary habitability in the Solar System and other exoplanetary systems. The overarching theme of our research is to investigate the origin and cycles of life-essential elements (carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus – COHNSP) in young rocky planets.”

All of which reminds us that the essential elements for life must be present no matter where a given planet exists in its star’s habitable zone.

The paper is Grewal et al., “Rates of protoplanetary accretion and differentiation set nitrogen budget of rocky planets,” Nature Geoscience 10 May 2021 (abstract / preprint).