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Potential long-term habitable conditions on planets with primordial H-He atmospheres

Space Sciences

Potential long-term habitable conditions on planets with primordial H-He atmospheres

M. M. Lous, R. Helled, et al.

Dive into groundbreaking research by Marit Mol Lous, Ravit Helled, and Christoph Mordasini, which uncovers the long-term habitability potential of super-Earths with primordial H-He atmospheres. Their simulations reveal how certain planets can maintain life-friendly conditions for billions of years, challenging what we thought we knew about planetary habitability.

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Playback language: English
Introduction
The search for extraterrestrial life typically focuses on conditions resembling Earth: liquid water, an energy source, and nutrients. However, the discovery of exoplanets, particularly super-Earths, suggests that our solar system may not be representative. Super-Earths, absent in our system, are common and could retain substantial primordial H-He atmospheres. While such atmospheres lack the greenhouse gases crucial for Earth's warmth, at sufficient pressures, collision-induced absorption by hydrogen could create a significant greenhouse effect, potentially leading to liquid water oceans. Previous studies have focused on the static conditions required for liquid water. This research aims to explore the long-term stability of these conditions, considering factors like atmospheric escape and stellar evolution.
Literature Review
Existing literature highlights the three key components for life as we know it: liquid water, energy source, and nutrients. Studies have also explored subsurface life on Earth, adapted to high pressures and non-photosynthetic metabolisms. The detection of hot Jupiters and the prevalence of super-Earths have reshaped our understanding of planetary systems. The possibility of H-He dominated atmospheres on super-Earths, particularly those further from their star, has been discussed. Previous analytical work and simulations have investigated the mass-radius relationship of 'Hycean worlds' showing the potential persistence of liquid water over a wider range of equilibrium temperatures than Earth-like planets. However, the long-term duration of such habitable conditions on these cold planets remains an open question and forms the focus of this paper.
Methodology
The researchers employed an evolutionary model to simulate the long-term habitability of planets with primordial H-He atmospheres. The model considered core mass, envelope mass, and semi-major axis as key parameters. Stellar evolution was modeled using a Sun-like star's luminosity evolution. The planet's intrinsic luminosity was calculated, accounting for the cooling of the envelope and core, as well as radiogenic decay. Atmospheric loss was considered using two escape models: Jeans escape and hydrodynamic escape. The model solved the structure equations of a planet's interior, assuming hydrostatic equilibrium. The temperature gradient was calculated using the adiabatic or radiative gradient, depending on convective stability. A double-grey atmosphere model was used to determine the temperature profile. The model included collision-induced absorption effects in hydrogen, a non-ideal equation of state for H-He, and the AQUA equation of state for water. The core structure was modeled using a modified polytropic equation of state for silicates. The intrinsic luminosity was estimated using an analytical fit and considered radiogenic heating. The duration of liquid water conditions was determined by checking pressure and temperature at the atmosphere's bottom against water's phase diagram. Sensitivity analyses were performed by varying parameters such as ice fraction in the core, intrinsic luminosity, and atmospheric composition. The model results were validated by comparing time-independent calculations to previously published models. The study used a large parameter grid, covering a wide range of core masses, envelope masses, and semi-major axes, to thoroughly investigate the conditions for long-term liquid water.
Key Findings
The simulations revealed that terrestrial and super-Earth planets (masses ~1–10 Earth masses) can sustain temperate surface conditions for 5–8 billion years at distances greater than ~2 AU. The necessary envelope masses are roughly 10⁻⁴ Earth masses, but can vary depending on orbital distance. Planets closer to their star require less massive envelopes, while those further out require more massive ones. Atmospheric loss, particularly hydrodynamic escape, significantly impacts close-in planets, making long-term habitability unlikely within 2 AU. Unbound planets, ejected from their star systems, can also maintain habitable conditions for extremely long timescales, potentially exceeding 50 billion years, largely driven by internal heat sources. Core mass plays a significant role in the duration of habitability for these unbound planets. The study also considered the constraint that the surface temperature remain below 400 K, a potential upper limit for life as we know it. The sensitivity analysis showed that the duration of liquid water conditions is affected by several factors. Ice in the core reduces radiogenic heating; the intrinsic luminosity, particularly the radiogenic component, has a major impact on habitability, especially at larger distances; and altered atmospheric compositions (via changes to infrared opacities and greenhouse parameters) influence habitability, especially at shorter distances.
Discussion
The findings significantly broaden the potential range of habitable environments. Planets with primordial H-He atmospheres, previously considered uninhabitable, can sustain liquid water for extended periods under specific conditions. The concept of a habitable zone needs to be reassessed to incorporate this novel type of habitability. The study highlights the importance of considering long-term temporal evolution rather than simply focusing on static conditions. The results also show the importance of internal heat sources in sustaining habitability, particularly for unbound planets. The relatively large scale height of H-He atmospheres could make detecting biomarkers easier, though the absence of photosynthesis presents challenges in detecting chemical disequilibrium.
Conclusion
This research demonstrates the potential for long-lasting habitable conditions on planets with primordial H-He atmospheres, significantly expanding the scope of habitable environments. Future research should focus on the likelihood of forming planets with the necessary initial conditions, particularly around M-dwarf stars. Investigating the effects of varying compositions, the presence of magnetic fields, and the possibility of life emerging and thriving under these conditions is also crucial. The study's findings provide targets for future telescope missions aimed at detecting and characterizing cold exoplanets and unbound planets.
Limitations
The model relies on several assumptions, such as a constant core mass, envelope mass, and semi-major axis; a Sun-like star's evolution; and simplified atmospheric models. Atmospheric composition and the presence of other greenhouse gases beyond hydrogen were simplified. The model's accuracy depends on the validity of these assumptions, and future work could refine these aspects. The absence of a precise estimate for the occurrence rate of planets with suitable initial conditions is a limitation.
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