Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p13c1334d&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P13C-1334
Physics
5450 Orbital And Rotational Dynamics (1221), 5744 Orbital And Rotational Dynamics (1221)
Scientific paper
A previous paper (Dobrovolskis 2007; Icarus 192, 1-23) showed that eccentricity can have profound effects on the climate, habitability, and detectability of extrasolar planets. This complementary study shows that obliquity can have comparable effects. The known exoplanets exhibit a wide range of orbital eccentricities, but those within several million km of their suns are generally in near-circular orbits. This fact is widely attributed to the dissipation of tides in the planets, which is particularly effective for solid/liquid bodies like "Super-Earths". Along with friction between a solid mantle and a liquid core, tides also are expected to despin a planet until it is captured in the synchronous resonance, so that its rotation period is identical to its orbital period. The canonical example of synchronous spin is the way that our Moon always keeps nearly the same hemisphere facing the Earth. Tides also tend to reduce the planet's obliquity (the angle between its spin and orbital angular velocities). However, orbit precession can cause the rotation to become locked in a "Cassini state", where it retains a nearly constant non-zero obliquity. For example, our Moon maintains an obliquity of about 6.7° with respect to its orbit about the Earth. For comparison, stable Cassini states can exist for practically any obliquity up to 180° for planets of binary stars, or in multi-planet systems with high mutual inclinations, such as are produced by scattering or by the Kozai mechanism. This work considers planets in synchronous rotation with circular orbits. For obliquities greater than 90°, the ground track of the sub-solar point wraps around all longitudes on the surface of such a planet. For smaller obliquities, the sub-solar track takes the figure-8 shape of an analemma. This can be visualized as the intersection of the planet's spherical surface with a right circular cylinder, parallel to the spin axis and tangent to the equator from the inside. The excursion of the sub-solar point in latitude is equal to the obliquity β, while the corresponding libration in longitude is smaller (±arcsin(tan2(β/2))). Obliquity thus affects the distribution of insolation over the planet's surface, particularly near its poles. For β = 0, one hemisphere bakes in permanent sunshine, while the opposite hemisphere experiences eternal darkness. As β increases, the region of permanent daylight and the antipodal realm of endless night both shrink, while a more temperate area of alternating day and night spreads in longitude, and especially in latitude. The regions of permanent day or night disappear at β = 90°. The insolation regime passes through several more transitions as β continues to increase toward 180°, but the surface distribution of insolation remains non-uniform in both latitude and longitude.
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