Other
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.0404d&link_type=abstract
American Astronomical Society, DPS meeting #38, #04.04; Bulletin of the American Astronomical Society, Vol. 38, p.487
Other
Scientific paper
The known extrasolar planets exhibit a wide range of orbital eccentricities e. This has a considerable influence on their rotations and climates. Because of tides in their interiors, Earth-like exoplanets are expected eventually to despin to synchronous rotation, where their spin periods equal their orbital periods. Such planets seem doomed to roast on one side and freeze on the other. However, synchronous planets rock back and forth by an angle of 2 e radians with respect to the sub-stellar point. For the Moon, with e = 0.055, this optical libration amounts to 7 degrees, but for a synchronous planet with e = 0.40, it would rise to 47 degrees! This greatly expands the temperate ``twilight zone'' near the terminator and considerably improves the planet's prospects for habitability. Furthermore, the synchronous state is not the only possible spin resonance; others occur when the orbital period is some half-integer times the rotation period. For example, Mercury (with e = 0.206) has a spin period exactly 2/3 of its orbital period. A terrestrial exoplanet with e = 0.40 is liable to have a rotation period of 2/3, 2/4, or 2/5 of its orbital period. The corresponding insolation patterns are all quite different from the synchronous state.
Finally, one of these spin resonances is the likely tidal end-state for mostly solid bodies (constant-Q tides). For example, we find that the synchronous state is the end-state for e < 0.2366, while the 2/3 resonance becomes the tidal equilibrium for 0.2366 < e < 0.3672. The 1/2 state dominates for 0.3672 < e < 0.4522, the 2/5 state for 0.4522 < e < 0.5132, the 1/3 state for 0.5132 < e < 0.5596, and so on. It is unlikely for a planet to spin slower than its tidal end-state.
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