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D.M. Williams (Penn State Erie, The Behrend College), D. Pollard (Penn State University)
Planets on extremely eccentric orbits or with tilted spin axes will have climates that are very different from Earth's, even if they spend most of their time within the habitable zones of their stars. The inclination, or obliquity, of the spin axis sets the climatic zonation of a planet, and both orbital eccentricity and obliquity control the amplitude of seasonal temperature variations, which can strongly affect whether a planet is suitable for water-dependent life. Earth owes its presently hospitable climate in part to its approximately circular orbit and a small (23.5o) and stable obliquity. The obliquity of Earth is stabilized by the size and proximity of the Moon. Extrasolar earths lacking large moons will have obliquities that episodically approach 90o as a consequence of spin-orbit resonances. While no terrestrial-sized planets have been discovered to date around Sun-like stars, the large orbital eccentricities of the growing class of newly discovered extrasolar Jupiters suggest that the orbits of smaller extrasolar planets may also be very eccentric. A natural question is whether extreme values of orbital eccentricity or obliquity could render an Earth-like planet inhospitable to terrestrial-type life.
To test this possibility, we have begun a three-year study* to simulate the climates of Earth-like planets using a 3-D general-circulation climate model. Variable parameters for each of the runs include orbital eccentricity, obliquity, land-sea fraction and geography, which affect the rate of seasonal heating and cooling, and the atmospheric concentration of the greenhouse gas CO2. Initial results demonstrate that surface temperatures on Earth would seasonally exceed 90oC over high-latitude continents were its obliquity as high 85o. Also, an Earth-like planet on a highly eccentric orbit that seasonally leaves the habitable zone near periastron would have its climate buffered by cooling from the smaller stellar fluxes received near apoastron. Finally, Earth itself could have avoided complete glacial coverage during the Precambrian if its obliquity were >50o and CO2 concentration was significantly lower than today.
*Project funded by the NSF, Life In Extreme Environments (LExEn) Program