The emergence of life on a planet is a profound event that hinges on a complex interplay of chemical and physical processes. Central to this phenomenon is the availability of certain key chemical elements, including phosphorus and nitrogen, which are indispensable for the biochemical architecture that supports life. Phosphorus is a critical constituent of DNA and RNA, the molecules responsible for genetic information storage and transmission, as well as playing a pivotal role in cellular energy dynamics. Nitrogen, on the other hand, forms an essential part of amino acids and proteins, the building blocks of cellular structures and enzymatic functions. Without these elements in adequate quantities, the genesis of life from inert matter is fundamentally constrained.
Recent research led by Dr. Craig Walton, a postdoctoral fellow at ETH Zurich’s Centre for Origin and Prevalence of Life, together with ETH professor Maria Schönbächler, has unveiled a crucial chemical criterion for planetary habitability. Their study demonstrates that the formation of phosphorus and nitrogen reservoirs on a rocky planet is intricately linked to the conditions prevailing during the planet’s core formation phase. Specifically, the element abundances on a planet’s surface are highly sensitive to the oxygen availability when heavy metals separate to form the metallic core. This process, which occurred on Earth about 4.6 billion years ago, required an optimal balance of oxygen to ensure that sufficient phosphorus and nitrogen remained accessible in the planet’s mantle rather than being sequestered in the core or lost to the atmosphere.
The core formation phase resembles a cosmic sieve where planetary differentiation shapes the spatial distribution of elements. As molten rock cools, denser materials such as iron descend to form the core, while lighter elements contribute to the mantle and crust. Crucially, the chemical environment during this phase determines element partitioning. In scenarios with insufficient oxygen, phosphorus tends to alloy with iron and sinks into the core, effectively removing it from surface geochemical cycles. Conversely, in oxygen-rich conditions, phosphorus remains in the mantle but nitrogen, likely in gaseous form, is prone to escape into space, depleting the planet’s nitrogen inventory. This delicate chemical balance defines a narrow “Goldilocks zone” of oxygen partial pressure that allows both elements to coexist in surface-accessible reservoirs.
Through extensive geochemical modeling and simulation, Walton and his collaborators established that Earth’s core formation conditions were serendipitously within this Goldilocks window. This milieu favored the retention of phosphorus and nitrogen within the mantle and crustal domains, underpinning the planet’s capacity to support life’s molecular machinery. Minor deviations from this narrow oxygen range would have critically limited key elemental availability and may have precluded the development of Earth-like biospheres. This insight fundamentally reconfigures our understanding of what makes a planet chemically habitable.
The implications extend beyond Earth, providing an explanatory framework for why planets like Mars lack sufficient bioessential elements despite other potentially favorable conditions. Mars’s core formation likely occurred outside the Goldilocks oxygen range, resulting in inadequate phosphorus and nitrogen concentrations in its mantle and crust. This elemental scarcity may be a principal reason for the planet’s failure to evolve complex life, underscoring the integral role of early planetary geochemistry in habitability assessments.
Beyond local planetary conditions, the team’s findings also recalibrate the astronomical criteria for prioritizing biosignature searches in exoplanetary systems. Traditional astrobiological missions have heavily weighted the presence of liquid water as the primordial indicator of habitability. However, the new research reveals that the chemical environment during planetary formation, specifically the oxygen budget dictating elemental partitioning, imposes fundamental constraints on a planet’s life-supporting potential, regardless of water presence. This suggests a paradigm shift where the star’s elemental composition, which governs the primordial chemical inventory, becomes a critical vector for evaluating exoplanet habitability.
Since planets inherit their elemental baselines from the protoplanetary disk formed around their host stars, the stellar chemical signature becomes a proxy for planetary composition. Stars whose oxygen abundances and associated chemical ratios fall outside the Earth-like range are less likely to host planets amenable to life. Consequently, exoplanet surveys might achieve greater efficiency and focus by narrowing their targets to stellar systems with chemical profiles akin to the Sun’s. This approach could revolutionize the search for life in the cosmos by integrating stellar chemistry into habitability models.
The research underscores how planetary formation processes are a form of natural selection, with planetary cores acting as filters that determine elemental availability on planetary surfaces. This planetary geochemical filtering process sets fundamental limits on the emergence of biologically relevant environments. It broadens the notion of a “habitable zone” from a simplistic metric of orbital distance and surface temperature to include chemical and geophysical factors operative in the planet’s earliest history.
Moreover, the study’s multi-disciplinary approach, combining geochemical modeling with astrophysical observation, exemplifies the integrative science required to tackle the question of life’s origins beyond Earth. It invites further investigations into the precise oxygen levels and planetary differentiation mechanisms needed for sustaining life-essential chemical reservoirs. Future observations of exoplanet host stars and refined planetary formation simulations will likely enrich this framework, refining our ability to identify true life-bearing worlds.
This breakthrough in understanding planetary habitability encourages a recalibration of how we interpret data from current and upcoming space missions aimed at detecting biosignatures. Instruments exploring exoplanet atmospheres and surfaces must consider both the chemical heritage imparted during planetary accretion and subsequent geochemical cycling to assess true potential for life. Integrating these chemical habitability criteria with water presence and other environmental markers will enhance the robustness of life detection strategies.
The findings published in the esteemed journal Nature Astronomy promise to catalyze a transformative shift in astrobiology, planetary science, and astronomy. They represent a leap forward in comprehending the chemical prerequisites that nature enforces on habitable planet formation. This understanding not only illuminates why life emerged on Earth but also guides the future search for extraterrestrial life amid the vast expanse of the galaxy.
As humanity peers into the cosmos with ever more sensitive instruments, the recognition that chemical conditions during planetary forging are as crucial as environmental factors reshapes our cosmic outlook. The “chemical Goldilocks zone” described by Walton and Schönbächler refines the map for discovering life beyond our home, suggesting that Earth’s life-friendliness is a rare but decipherable outcome of precise chemical and geophysical choreography billions of years ago.
Subject of Research: Chemical prerequisites for the development of life on rocky planets, focusing on phosphorus and nitrogen retention during core formation.
Article Title: The chemical habitability of Earth and rocky planets prescribed by core formation
News Publication Date: 9-Feb-2026
Web References: 10.1038/s41550-026-02775-z
Keywords: planetary habitability, phosphorus, nitrogen, core formation, chemical Goldilocks zone, planetary differentiation, astrobiology, exoplanet chemistry, oxygen levels, mantle geochemistry, biosignature search, stellar composition
