Long-Term Planetary Habitability and the Carbonate-Silicate Cycle

Area of Science:

• Planetary Science
• Astrobiology
• Geochemistry

Background:

• Exoplanet habitability is traditionally assessed using the circumstellar habitable zone, defined by surface liquid water potential.
• Current models predict surface temperatures using radiative-convective climate models but overlook biogeochemical plausibility of atmospheric gases.
• Carbon dioxide (CO2) is a critical greenhouse gas, regulating planetary temperature through a balance of volcanic outgassing and sequestration via weathering and sedimentation.

Purpose of the Study:

• To develop a model incorporating Earth's short- and long-term carbon cycle to assess CO2 greenhouse effects on exoplanets.
• To explore how variations in planet size and stellar insolation influence CO2 concentrations and surface temperatures.
• To investigate the impact of planetary topography, tectonics, and hydrology on habitability assessments.

Main Methods:

• Developed a model integrating key aspects of Earth's biogeochemical carbon cycle.
• Simulated CO2 greenhouse effects considering planetary size and stellar insolation.
• Analyzed the influence of geological factors like topography and tectonics on surface temperature.

Main Results:

• Larger planets (0.5-2 R⊕) with greater geological activity exhibit proportionally higher surface temperatures for a given incident flux.
• Changes in topography, tectonics, and hydrology on larger planets can lead to average global surface temperature deviations of up to 20 K.
• These findings highlight the necessity of incorporating biogeochemical processes into habitability models.

Conclusions:

• Traditional habitable zone calculations are insufficient without considering planetary biogeochemistry.
• Planet size and geological activity significantly influence atmospheric CO2 and surface temperatures, affecting habitability.
• Future exoplanet habitability studies must integrate detailed carbon cycle models for accurate assessments.