A whole-planet model of the Earth without life for terrestrial exoplanet studies

As the only known habitable (and inhabited) planet in the universe, Earth informs our search for life elsewhere. Future telescopes like the Habitable Worlds Observatory (HWO) will soon look for life on rocky worlds around Sun-like stars, so it is critical that we understand how to distinguish habitable planets from inhabited planets. However, it remains unknown if life is necessary to maintain a habitable planet, or how all of the components of an evolving planet impact habitability over time. To address these open questions, we present a coupled interior-atmosphere evolution model of the Earth without life from 50 Myr to 5 Gyr that reproduces 19 key observations of the pre-industrial Earth within measurement uncertainties after 4.5 Gyr. We also produce a reflected light spectrum covering the possible wavelength range of HWO. Our findings support the view that life is not required to maintain habitable surface conditions. The model presented here is apt for predicting the long-term habitability of Earth-like exoplanets via evolving bulk properties. By generating realistic reflected light spectra from evolved atmospheric states, this model represents significant progress towards whole-planet modeling, which may ultimately provide a robust abiotic baseline for interpreting biosignature observations with HWO.

💡 Research Summary

The paper presents a comprehensive one‑dimensional “whole‑planet” model that simulates the Earth without any biological activity from 50 Myr after formation to 5 Gyr, integrating the core, mantle, crust, atmosphere, ocean, and climate systems. The authors’ primary motivation is to provide a robust abiotic baseline for interpreting future direct‑imaging observations of Earth‑like exoplanets, especially those to be obtained by the upcoming Habitable Worlds Observatory (HWO).

The model consists of four tightly coupled modules. The interior module tracks the thermal evolution of the core‑mantle system, incorporating radiogenic heating, core‑mantle boundary heat flow, mantle convection, viscosity, and melt fraction. The atmospheric‑climate module solves for radiative‑convective equilibrium, explicitly treating CO₂ and H₂O as greenhouse gases, including their phase changes, cloud effects, and surface albedo variations. The ocean chemistry module computes the carbonate system (DIC, aqueous CO₂, HCO₃⁻, CO₃²⁻) and pH under chemical equilibrium, linking atmospheric gas exchange with oceanic carbon storage. Finally, the external forcing module imposes the Sun’s luminosity and X‑ray/UV evolution, allowing the model to account for hydrogen escape and changing stellar irradiance over geological time.

Running the simulation from 50 Myr to 5 Gyr, the authors calibrate the model against 19 observed properties of the pre‑industrial Earth (PIE) at the 4.5 Gyr epoch. These constraints include atmospheric CO₂ (28.4 ± 0.4 Pa), atmospheric H₂O (900 ± 700 Pa), global mean surface temperature (286.9 ± 0.1 K), surface albedo (0.14 ± 0.02), ocean mass (1.4 × 10²¹ kg), dissolved inorganic carbon (2000 ± 200 µmol kg⁻¹), oceanic CO₂(aq) (8 ± 2 µmol kg⁻¹), ocean pH (8.18 ± 0.05), upper mantle temperature (1587 +164/‑34 K), core‑mantle boundary temperature (4000 ± 200 K), mantle heat flow (38 ± 3 TW), core heat flow (11 ± 6 TW), mantle viscosities, melt fractions, inner‑core radius, magnetic moment, and magnetopause radius. The model reproduces all of these within 1σ (or 2σ for ocean chemistry) without invoking any biological feedbacks.

A key scientific insight is that, even in the absence of life, the coupled interior‑atmosphere‑ocean system can maintain a temperate climate with liquid water for billions of years. Volcanic outgassing and mantle degassing supply enough CO₂ to sustain a modest greenhouse effect, while the water cycle remains vigorous enough to regulate surface temperature through latent heat transport. This result challenges the strong version of the Gaia hypothesis that posits life as a prerequisite for long‑term habitability, demonstrating instead that planetary physics alone can produce a stable, habitable state on an Earth‑mass planet with Earth‑like composition.

To connect the model with observational prospects, the authors generate reflected‑light spectra spanning 0.3–2.5 µm for the simulated atmospheric states at 4.5 Gyr. The spectra display prominent CO₂ absorption bands at 1.6 µm and 2.0 µm and H₂O bands near 0.94 µm and 1.13 µm, matching the wavelength range that HWO will target. Importantly, these absorption features are strong even in the abiotic scenario, indicating that detection of CO₂ or H₂O alone cannot be taken as a biosignature. The spectra thus provide a necessary “false‑positive” baseline against which genuine biosignature gases (e.g., O₂, CH₄ in disequilibrium) must be compared.

The paper also discusses limitations and future extensions. The current framework is 1‑D, assumes a fixed bulk composition (primarily N₂‑CO₂‑H₂O), and does not include dynamic plate tectonics, 3‑D atmospheric circulation, or explicit biogeochemical cycles. The authors propose adding 3‑D climate‑ocean coupling, exploring a wider range of planetary bulk compositions (e.g., higher iron or water fractions), and applying the model to stars of different spectral types and activity levels. Such upgrades would broaden the applicability of the model to the diverse population of terrestrial exoplanets that HWO and other future missions will observe.

In summary, this work delivers a physically grounded, validated model of an abiotic Earth that reproduces key present‑day observables, demonstrates that life is not a prerequisite for maintaining surface habitability, and supplies realistic reflected‑light spectra for use as an abiotic baseline in the interpretation of forthcoming exoplanet observations.