The Susceptibility of Extrasolar Earths to Large-scale Variations in Obliquity
D. M. Williams (Penn State University)
Earth owes its stable obliquity (Laskar et al. 1993, Nature 361, 615) and possibly its stable climate (Williams and Kasting 1997, Icarus, in press) to the presence of the Moon, which causes its spin axis to precess much faster than its orbit normal does in response to the motions of the other planets--if these rates were similar, Earth's obliquity would vary unpredictably and by large amounts. Terrestrial planets in other systems may precess more slowly than Earth if their moons are smaller or if they lack moons altogether, which might cause their obliquities to vary considerably over time. Earth's own obliquity would vary chaotically and possibly up to if the Moon were less than half its present mass. Earth-like planets in rapid precession may demonstrate similar problems if their orbits are made to precess at comparable rates. The rapidity of orbital precession grows with increasing mass and proximity of neighboring planets. Thus, in systems (e.g., 47 Ursae Majoris) containing a giant planet located near or within the habitable zone (Williams and Kasting 1997, Nature 385, 234), rapid spin precession may, by itself, be unable to safeguard potential life-supporting planets from large obliquity fluctuations.