Titan under a red dwarf star and as a rogue planet: requirements for liquid methane

Titan has a surface temperature of 94 K and a surface pressure of 1.4 atmospheres. These conditions make it possible for liquid methane solutions to be present on the surface. Here, we consider how Titan could have liquid methane while orbiting around an M4 red dwarf star, and a special case of Titan orbiting the red dwarf star Gliese 581. Because light from a red dwarf star has a higher fraction of infrared than the Sun, more of the starlight will reach the surface of Titan because its atmospheric haze is more transparent to infrared wavelengths. If Titan was placed at a distance from a red dwarf star such that it received the same average flux as it receives from the Sun, we calculate the increased infrared fraction, which will warm surface temperatures by an additional ∼10 K. Compared to the Sun, red dwarf stars have less blackbody ultraviolet light but can have more Lyman α and particle radiation associated with flares. Thus depending on the details, the haze production may be much higher or much lower than for the current Titan. With the haze reduced by a factor of 100, Titan would have a surface temperature of 94 K at a distance of 0.23 AU from an M4 star and at a distance of 1.66 AU, for Gliese 581. If the haze is increased by a factor of 100 the distances become 0.08 and 0.6 AU for the M4-star and Gliese 581, respectively. As a rogue planet, with no incident stellar flux, Titan would need 1.6 W/m2 of geothermal heat to maintain its current surface temperature, or an atmospheric opacity of 20× its present amount with 0.1 W/m2 of geothermal heat. Thus Titan-like worlds beyond our solar system may provide environment supporting surface liquid methane.