[Paper Review] Chemical Habitability: Supply and Retention of Life's Essential Elements During Planet Formation

Sebastiaan Krijt, Mihkel Kama+6 more|arXiv (Cornell University)|Mar 18, 2022

Astro and Planetary Science13 citations

TL;DR

This paper introduces the concept of 'chemical habitability'—the capacity of a planet to supply and retain life's essential elements (CHNOPS: C, H, N, O, P, S) throughout planet formation. It synthesizes astrophysical, geochemical, and planetary science data to show how elemental distribution in protoplanetary disks, planetesimal formation, and planetary accretion critically shape a world's potential for life, with key outcomes depending on pebble drift, dynamical instabilities, and volatile retention during planetary assembly.

ABSTRACT

Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus and Sulfur (CHNOPS) play key roles in the origin and proliferation of life on Earth. Given the universality of physics and chemistry, not least the ubiquity of water as a solvent and carbon as a backbone of complex molecules, CHNOPS are likely crucial to most habitable worlds. To help guide and inform the search for potentially habitable and ultimately inhabited environments, we begin by summarizing the CHNOPS budget of various reservoirs on Earth, their role in shaping our biosphere, and their origins in the Solar Nebula. We then synthesize our current understanding of how these elements behave and are distributed in diverse astrophysical settings, tracing their journeys from synthesis in dying stars to molecular clouds, protoplanetary settings, and ultimately temperate rocky planets around main sequence stars. We end by identifying key branching points during this journey, highlighting instances where a forming planets' distribution of CHNOPS can be altered dramatically, and speculating about the consequences for the chemical habitability of these worlds.

Motivation & Objective

• To define and formalize the concept of 'chemical habitability' as a planetary property distinct from the classical habitable zone.
• To synthesize current understanding of the cosmic journey of CHNOPS elements from stellar nucleosynthesis through protoplanetary disks to terrestrial planets.
• To identify critical branching points in planet formation where CHNOPS distribution is altered, affecting long-term habitability.
• To assess how variations in elemental inventories influence prebiotic chemistry and the potential for abiogenesis.

Proposed method

• Synthesizes observational data from ALMA, Herschel, and exoplanet surveys to map CHNOPS reservoirs in protostellar and protoplanetary environments.
• Integrates geochemical data on Earth's CHNOPS reservoirs and their roles in biosphere formation.
• Models the behavior of CHNOPS-bearing molecules in protoplanetary disk midplanes, focusing on pebble drift and radial mixing.
• Analyzes the role of dynamical processes—planet migration, gravitational instabilities, and late-stage accretion—in redistributing volatile elements.
• Evaluates the impact of planetary tectonics and atmospheric evolution on long-term CHNOPS retention.
• Uses comparative planetology to assess how variations in disk chemistry and system architecture lead to diverse CHNOPS budgets in rocky exoplanets.

Experimental results

Research questions

• RQ1 How do the abundances and distributions of CHNOPS elements in protoplanetary disks influence the chemical composition of forming terrestrial planets?
• RQ2 What role do pebble drift and dynamical instabilities play in altering the CHNOPS inventory of planetary embryos and final planets?
• RQ3 How do planetary outgassing, atmospheric evolution, and crustal sequestration affect the long-term availability of bioessential elements?
• RQ4 To what extent can variations in inner disk chemistry lead to planetary systems with differing chemical habitability?
• RQ5 What are the implications of high nitrogen or sulfur inventories for atmospheric pressure, greenhouse effects, and prebiotic chemistry?

Key findings

• CHNOPS elements are synthesized in dying stars and distributed through molecular clouds before being incorporated into protoplanetary disks, where their spatial distribution is shaped by temperature gradients and radial transport.
• Pebble drift and planetesimal formation are critical in concentrating volatile elements like C, N, and P in the inner disk, directly influencing the CHNOPS budget of terrestrial planets.
• Planets forming in the habitable zone may inherit vastly different CHNOPS inventories due to variations in disk chemistry, migration history, and late-stage accretion of volatile-rich planetesimals.
• High nitrogen inventories can lead to thick N2-dominated atmospheres that suppress volcanic degassing and may influence greenhouse warming, with implications for surface conditions.
• Phosphorus, though scarce, is a key limiting factor for prebiotic chemistry; its availability depends on early planetary accretion processes and mineralogy.
• Sulfur's multiple reservoirs—gaseous, aqueous, mineral, and molten—create complex sequestration pathways that may reduce its bioavailability, especially in high-sulfur environments like Venus-like exoplanets.