A Methane Extension to the Classical Habitable Zone

Abstract

Abstract The habitable zone (HZ) is the circumstellar region where standing bodies of liquid water could exist on the surface of a rocky planet. Conventional definitions assume that CO2 and H2O are the only greenhouse gases. The outer edge of this classical N2–CO2–H2O HZ extends out to nearly ∼1.7 au in our solar system, beyond which condensation and scattering by CO2 outstrip its greenhouse capacity. We use a single-column radiative-convective climate model to assess the greenhouse effect of CH4 (10–∼100,000 ppm) on the classical HZ (N2–CO2–H2O) for main-sequence stars with stellar temperatures between 2600 and 10,000 K (∼A3 to M8). Assuming N2–CO2–H2O atmospheres, previous studies have shown that cooler stars heat terrestrial planets more effectively. However, we find that the addition of CH4 produces net greenhouse warming (tens of degrees) in planets orbiting stars hotter than a mid-K (∼4500 K), whereas a prominent anti-greenhouse effect is noted for planets around cooler stars. We show that 10% CH4 can increase the outer edge distance of the hottest stars (T EFF = 10,000 K) by over 20%. In contrast, the CH4 anti-greenhouse can shrink the HZ for the coolest stars (T EFF = 2600 K) by a similar percentage. We find that dense CO2–CH4 atmospheres near the outer edge of hotter stars may suggest inhabitance, highlighting the importance of including secondary greenhouse gases in alternative definitions of the HZ. We parameterize the limits of this N2–CO2–H2O–CH4 HZ and discuss implications in the search for extraterrestrial life.