Evolution of magnetic protection in potentially habitable terrestrial planets (1204.0275v1)

Abstract: We present a model for the evolution of the magnetic properties of habitable terrestrial planets and their effects on the protection of planetary atmosphere against the erosive action of stellar wind. Using up-to-date thermal evolution models and dynamo scaling laws we predict the evolution of the planetary dipole moment as a function of planetary mass and rotation rate. Combining these results with models for the evolution of the stellar wind, stellar XUV fluxes and planetary exosphere characteristics, we determine the properties of the magnetosphere and the exobase radius in order to estimate the level of atmospheric mass losses. We use this model to evaluate the magnetic protection of the potentially habitable super-Earths GJ 667Cc, Gl 581d and HD 85512b. We confirm that Earth-like planets, even under the highest attainable magnetic field strengths, will lose a significant fraction of their atmospheric volatiles if they are tidally locked in the habitable zone of dM stars, or even if having N/O-rich atmospheres they are in habitable zones closer than $\sim$ 0.8 AU. Similar mass-dependent inner limits have been found for super-Earths $M_p\gtrsim 3 M_\oplus$ that in any case seem to have better chances of preserving their atmospheres even if they are tidally locked. We predict that the atmosphere of GJ 667Cc has probably already been obliterated and it is presently uninhabitable. On the other hand, our model predicts that the atmospheres of Gl 581d and HD 85512b would be well protected by intrinsic magnetic fields, even under the worst expected conditions of stellar aggression. (abrigded abstract).

• The paper develops a comprehensive model to predict the evolution of planetary magnetic fields and atmospheric loss due to stellar winds on potentially habitable planets.
• Findings suggest that super-Earths above approximately 3 Earth masses may retain atmospheres better around M-dwarfs due to a mass-dependent protective threshold.
• The research provides benchmarks for astrobiological missions and emphasizes that magnetic fields alone do not guarantee atmospheric retention in harsh stellar environments.

Overview of "Evolution of Magnetic Protection in Potentially Habitable Terrestrial Planets"

The paper by Zuluaga, Cuartas-Restrepo, and Hoyos presents a detailed investigation into the evolution of magnetic fields in terrestrial planets and their implications for atmospheric protection in potentially habitable environments. The authors explore the role of planetary magnetic fields in protecting atmospheres from erosion due to stellar winds, a factor critical for maintaining habitable conditions over geological time scales.

Key Contributions

The paper develops a comprehensive model incorporating multiple aspects of planetary evolution:

1. Thermal Evolution Models: By integrating state-of-the-art thermal evolution models, the paper predicts the evolution of the planetary dipole moment as a function of planetary mass and rotation rate. This approach involves utilizing dynamo scaling laws, which relate the magnetic field strength generated in a planet's core to its convective energy budget.
2. Stellar Effects and Atmospheric Dynamics: The authors extend the model to include the effect of evolving stellar winds and extreme ultraviolet (XUV) fluxes on atmospheric erosion. This aspect is crucial for modeling the atmospheric loss processes, especially for planets close to their host stars, such as those orbiting M-dwarf stars.
3. Planetary Magnetosphere: Utilizing models for planetary magnetospheres, the paper estimates the scale height of the atmosphere and the resultant thermal and non-thermal atmospheric mass losses. The model outcomes are applied to evaluate the early magnetic protection of Earth and known potentially habitable super-Earths including GJ 667Cc, Gl 581d, and HD 85512b.

Conclusions and Implications

The findings suggest that Earth-like planets, even when possessing the strongest plausible intrinsic magnetic fields, may still suffer significant atmospheric loss if they are tidally locked within the habitable zones (HZ) of M-dwarf stars. This is due to the sustained exposure to harsh stellar conditions, which can exceed the protective capacity of a planet's magnetic field.

• Mass-dependent Habitable Zones: The paper reveals that for super-Earths, there appears to be a mass-dependent protective threshold. Planets with masses above approximately 3 Earth masses ($M_p \gtrsim 3 M_\oplus$) show better prospects for retaining substantial atmospheres, even if tidally locked around M-dwarfs. This finding is pertinent when assessing the potential for habitability and further underscores the complex interplay between planetary mass, distance to the star, and magnetic properties.
• Practical and Theoretical Speculations: From a practical standpoint, the research provides critical benchmarks for astro-biological missions seeking habitable exoplanets, suggesting that magnetic field presence is not an unequivocal indicator of atmospheric retention capabilities. The results encourage further theoretical developments aimed at improving our understanding of the interaction between stellar activity and planetary magnetic fields, also highlighting the necessity for comprehensive models that integrate these factors with evolving stellar dynamics.

Future Directions

The authors emphasize the need for ongoing studies that can unravel the compositional and structural diversity of exoplanets, which could influence magnetic field generation and atmospheric retention. Additionally, advancements in observational techniques may refine the assumptions about planetary magnetic fields and their interaction with stellar environments.

In conclusion, the paper significantly enhances our understanding of the sustainability of life-supporting atmospheres on exoplanets, contributing valuable insights for astro-biological research and mission design aimed at habitable exoplanet detection.