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Abstract EANA2025-11


Extending the Concepts of Habitability and the Habitable Zone beyond Surface Liquid Water: Proposal for an Ecozone (EZ)

Amri Wandel (1) and Dirk Schulze-Makuch (2,3,4)
(1) Racah Inst. of Physics, The Hebrew University of Jerusalem, Israel (2) Astrobiology Group, ZAA, Technische Universität Berlin, Hardenbergstr. 36A, 10623 Berlin, Germany (3) German Research Centre for Geosciences (GFZ), Section Geomicrobiology, 14473 Potsdam, Germany (4) Department of Plankton and Microbial Ecology, Leibniz Institute of Freshwater Ecology and Inland Fisheries, 16775 Stechlin, Germany


We propose to extend the terms "Habitable Zone" (HZ) and "habitability" in their conservative interpretation.
While the conservative HZ is defined as the circumstellar region where a planet can sustain liquid water
on its surface, given an adequate atmosphere, this definition may be extended to include planets
inwards of the inner edge, and outside of the outer edge of the HZ. Examples are potential habitats with
subglacial water on the night side of tidally locked planets, icy moons orbiting giant planets on which
geothermal heat is produced by tidal forces or radioactivity, and hydrogen-rich Hycean worlds. Another
type of extension may be Dune-like arid worlds, with an Earth-analog in places like the Atacama Desert
(Schulze-Makuch 2024). The objective is to determine an EcoZone (EZ), in which environmental
conditions consistent with water-based life may exist, which would include
(1) an extension of the HZ with surface liquid water on locked planets (Wandel 2018), Dune worlds,
Hycean worlds with hydrogen atmospheres or insulating ice layers,
(2) an extension of the HZ enhanced to include subglacial liquid water (Ojha et al. 2022; Wandel 2023),
and
(3) planetary bodies which are habitable due to geothermal heat, which is independent of a host star
(e.g., tidal heating like in Europa and Enceladus, or radiogenic elements). This category could also apply
to habitable rogue planets.
Even this region may be further extended considering a different biochemistry, e.g., usage of a solvent
other than water, which would lead to very different limitations.

References

Ojha, L., Troncone, B., Buffo, J. et al. (2022) Liquid water on cold exo-Earths via basal melting of ice sheets. Nature Comm. 13, 7521.
Schulze-Makuch, D. (2024) We may be looking for Martian life in the wrong place. Nature Astronomy 8: 1208-1210.
Wandel A. (2018) On the biohabitability of M-dwarf planets Astrophys. J. 858, 165W.
Wandel A. (2023) Habitability and subglacial liquid water on planets of M-dwarf stars. Nature Comm. 14, 2125.