Inside the Eirenesphere: The Interplay of Porosity, Heat Flux and Mineralogy in Exoplanetary Aquable and Habitable Layers
Abstract
The classical circumstellar habitable zone restricts the search for life to planetary surfaces where stellar irradiation sustains liquid water, overlooking vast subsurface environments. Here, we present a geophysical model integrating internal radial structure, mineralogy, radiogenic heat, and pressure-dependent porosity to quantify the three-dimensional habitable volume-the eirenesphere-of rocky exoplanets. We distinguish between aquability (thermodynamic water stability) and habitability, which requires temperatures and pressures within extremophile biological limits, plus sufficient porosity for fluid circulation. To compare planetary capacities, we introduce the Eirenesphere Volumetric Index (EVI), measuring average subsurface biosphere volume. Applying this framework, we find that Earth's current state (EVI≈ 0.06 terrestrial oceans) represents only a moderate regime. Instead, mature super-Earths with high geothermal activity provide the most extensive environments for deep biospheres. Crustal mineralogy exerts a first-order control: thermally insulating felsic crusts sustain significantly larger eirenespheres than primary mafic lithologies. Tracking secular cooling reveals internal habitability is an evolutionary property; young planets host confined biospheres due to steep thermal gradients, whereas mature worlds maximize habitable volumes over billions of years. Finally, we demonstrate that around solar-like stars, subsurface habitability persists out to 5−7 au, effectively decoupling life's potential from surface radiative balance. Extrapolating to the Milky Way reveals a staggering galactic capacity for subsurface life, on the order of billions of terrestrial oceans. Ultimately, this framework shifts the astrobiological paradigm from a surface-dependent phenomenon to an intrinsic planetary property, offering a metric to prioritize targets.
Related
- Improved Understanding of Lunar Surface Hydration Signatures through Comparison of Thermally Corrected Spectra from the Moon Mineralogy Mapper (M 3)
- MicroRaman Spectroscopy of Carbonaceous Chondrites to Constrain Peak Metamorphic Temperature and Mineralogy
- Stellar Abundances as Probes of Rocky Exoplanet Interiors: The Mantle Composition and Mineralogy of GJ 486b
- The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems VIII: patchy forsterite and enstatite clouds in the atmosphere of VHS 1256 b, retrieval lessons learned and outlook to the future
- TOI-1408: the limb darkening models, photometric and Doppler noises, and the exoplanetary system
BibTeX
@misc{orjuela2026inside,
title = {{Inside the Eirenesphere: The Interplay of Porosity, Heat Flux and Mineralogy in Exoplanetary Aquable and Habitable Layers}},
author = {Santiago A. Orjuela and Jorge I. Zuluaga},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.25280},
eprint = {2609.25280},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.25280},
}
RIS
TY - UNPB
TI - Inside the Eirenesphere: The Interplay of Porosity, Heat Flux and Mineralogy in Exoplanetary Aquable and Habitable Layers
AU - Santiago A. Orjuela
AU - Jorge I. Zuluaga
PY - 2026
DA - 2026/09/21
PB - arXiv
DO - 10.48550/arxiv.2609.25280
UR - https://arxiv.org/abs/2609.25280
AB - The classical circumstellar habitable zone restricts the search for life to planetary surfaces where stellar irradiation sustains liquid water, overlooking vast subsurface environments. Here, we present a geophysical model integrating internal radial structure, mineralogy, radiogenic heat, and pressure-dependent porosity to quantify the three-dimensional habitable volume-the eirenesphere-of rocky exoplanets. We distinguish between aquability (thermodynamic water stability) and habitability, which requires temperatures and pressures within extremophile biological limits, plus sufficient porosity for fluid circulation. To compare planetary capacities, we introduce the Eirenesphere Volumetric Index (EVI), measuring average subsurface biosphere volume. Applying this framework, we find that Earth's current state (EVI≈ 0.06 terrestrial oceans) represents only a moderate regime. Instead, mature super-Earths with high geothermal activity provide the most extensive environments for deep biospheres. Crustal mineralogy exerts a first-order control: thermally insulating felsic crusts sustain significantly larger eirenespheres than primary mafic lithologies. Tracking secular cooling reveals internal habitability is an evolutionary property; young planets host confined biospheres due to steep thermal gradients, whereas mature worlds maximize habitable volumes over billions of years. Finally, we demonstrate that around solar-like stars, subsurface habitability persists out to 5−7 au, effectively decoupling life's potential from surface radiative balance. Extrapolating to the Milky Way reveals a staggering galactic capacity for subsurface life, on the order of billions of terrestrial oceans. Ultimately, this framework shifts the astrobiological paradigm from a surface-dependent phenomenon to an intrinsic planetary property, offering a metric to prioritize targets.
ER -