HABITABLE PLANETS WITH HIGH OBLIQUITIES

Earth's obliquity would vary chaotically from 0 degrees to 85 degrees were it not for the presence of the Moon (J. Laskar, F. Joutel, and P. Robutel, 1993, Nature 361, 615-617). The Moon itself is thought to be an accident of accretion, formed by a glancing blow from a Mars-sized planetesimal. Hence, planets with similar moons and stable obliquitie s may be extremely rare. This has lead Laskar and colleagues to sugges t that the number of Earth-like planets with high obliquities and temp erate, life-supporting climates may be small. To test this proposition , we have used an energy-balance climate model to simulate Earth's cli mate at obliquities up to 90 degrees. We show that Earth's climate wou ld become regionally severe in such circumstances, with large seasonal cycles and accompanying temperature extremes on middle-and high-latit ude continents which might be damaging to many forms of life. The resp onse of other, hypothetical, Earth-like planets to large obliquity flu ctuations depends on their land-sea distribution and on their position within the habitable zone (HZ) around their star. Planets with severa l modest-sized continents or equatorial supercontinents are more clima tically stable than those with polar supercontinents. Planets farther out in the HZ are less affected by high obliquities because their atmo spheres should accumulate CO2 in response to the carbonate-silicate cy cle. Dense, CO2-rich atmospheres transport heat very effectively and t herefore limit the magnitude of both seasonal cycles and latitudinal t emperature gradients. We conclude that a significant fraction of extra solar Earth-like planets may still be habitable, even if they are subj ect to large obliquity fluctuations. (C) 1997 Academic Press.