LIMIT CYCLES CAN REDUCE THE WIDTH OF THE HABITABLE ZONE.

Area of Science:

• Planetary Science
• Climate Modeling
• Astrobiology

Background:

• The liquid water habitable zone (HZ) is defined by orbital distances allowing surface water, regulated by the carbonate-silicate cycle.
• Previous models suggested outer HZ planets might undergo climate oscillations ('limit cycles') between glaciation and warmth, unfavorable for complex life.
• Earth's biota influence atmospheric CO2, a factor potentially overlooked in abiotic climate models.

Purpose of the Study:

• To calculate the 'limit cycle' region within the HZ using an updated energy balance climate model.
• To assess the importance of limit cycles for planetary habitability, considering abiotic CO2 levels.
• To determine which stellar types and planetary conditions favor stable, warm climates.

Main Methods:

• Utilized an updated energy balance climate model to simulate planetary climate dynamics.
• Incorporated abiotic CO2 partial pressures, assuming greater levels than present-day Earth.
• Calculated the conditions under which limit cycles occur for planets around different star types (G, K, M).

Main Results:

• Limit cycles are less prevalent than previously estimated, particularly for planets with CO2 outgassing rates similar to or higher than modern Earth.
• For G-type stars, limit cycles occur only at lower CO2 outgassing rates.
• K- and M-star planets are unlikely to experience limit cycles, though M-star habitability may be limited by other factors.

Conclusions:

• Planets orbiting late G-type and early K-type stars offer the best potential for stable, warm climates conducive to life.
• Host star type, volcanic activity, and seafloor weathering are critical factors influencing planetary climate stability and habitability.
• The prevalence and impact of climate limit cycles on habitability require further investigation with refined models.