Insolation on exoplanets with eccentricity and obliquity
Abstract
The pattern of insolation on an extrasolar planet has profound implications for its climate and habitability. A planet’s insolation regime depends on its orbital eccentricity, the obliquity of its spin axis, its rotation rate, and its longitude of vernal equinox. For example, although a planet receives the same timeaveraged insolation at both poles, the peak insolation at its poles can differ by a factor up to 27, depending on its eccentricity and equinox. This is of particular interest for planets with polar icecaps (or lakes and seas), like Mercury, Earth, and Mars (or Titan). The nearly 600 exoplanets now with known eccentricities span a wide range of eccentricity from essentially zero up to near unity; but their obliquities are still unknown, and also may range widely.
Including both nonzero eccentricity and obliquity together vastly broadens the variety of global insolation patterns on extrasolar planets. This applies especially to planets in synchronous rotation, or in other spinorbit resonances (like Mercury), which can exhibit quite complicated and unusual insolation patterns. For example, regions of eternal daylight and endless night occur only on synchronous exoplanets, whose rotation periods equal their orbital periods; but the peak and timeaveraged insolation can vary by factors of at least 32 and 88, respectively, over a planet with a rotation period of half its orbital period, an eccentricity of 0.20, and an obliquity of 60°.
Patterns of both mean and peak insolation display various symmetries with respect to latitude and longitude on the planet’s surface. Most of these are relatively simple and easily understood; for example, a resonant planet whose orbital period is half of an odd multiple of its rotation period (as in Mercury’s 3/2 resonance) experiences identical insolation patterns at longitudes 180° apart. However, such halfodd resonances also exhibit a totally unexpected symmetry of the timeaveraged insolation with respect to the planet’s equator, not shared by the peak insolation, or by any wholenumber resonances. This emergent symmetry can be understood by Fourier analysis of the timevarying insolation.
 Publication:

Icarus
 Pub Date:
 September 2013
 DOI:
 10.1016/j.icarus.2013.06.026
 Bibcode:
 2013Icar..226..760D