  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 to about 100,000 ppm) on the classical
habitable zone (N2-CO2-H2O) for main-sequence stars with stellar temperatures
between 2,600 to 10,000 K (about A3 to M8). Assuming N2-CO2-H2O atmospheres,
previous studies have shown that cooler stars more effectively heat terrestrial
planets. However, we find that the addition of CH4 produces net greenhouse
warming (tens of degrees) in planets orbiting stars hotter than a mid-K (about
4500K), whereas a prominent anti-greenhouse effect is noted for planets around
cooler stars. We show that 10% CH4 can increase the width of the classical HZ
of the hottest stars (TEFF = 10,000 K) by over 20%. In contrast, the CH4
anti-greenhouse can shrink the HZ for the coolest stars (TEFF = 2,600 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 habitable zone and discuss
implications in the search for extraterrestrial life.
