Journal
ASTRONOMICAL JOURNAL
Volume 155, Issue 6, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/1538-3881/aabfd1
Keywords
methods: analytical; methods: numerical; planets and satellites: dynamical evolution and stability; planets and satellites: terrestrial planets
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Funding
- Natural Science and Engineering Research Council (NSERC) of Canada
- Harvard William F. Milton Award
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Obliquity variability could play an important role in the climate and habitability of a planet. Orbital modulations caused by planetary companions and the planet's spin axis precession due to the torque from the host star may lead to resonant interactions and cause large-amplitude obliquity variability. Here we consider the spin axis dynamics of Kepler-62f and Kepler-186f, both of which reside in the habitable zone around their host stars. Using N-body simulations and secular numerical integrations, we describe their obliquity evolution for particular realizations of the planetary systems. We then use a generalized analytic framework to characterize regions in parameter space where the obliquity is variable with large amplitude. We find that the locations of variability are fine-tuned over the planetary properties and system architecture in the lower-obliquity regimes (less than or similar to 40 degrees). As an example, assuming a rotation period of 24 hr, the obliquities of both Kepler-62f and Kepler-186f are stable below similar to 40 degrees, whereas the high-obliquity regions (60 degrees-90 degrees) allow moderate variabilities. However, for some other rotation periods of Kepler-62f or Kepler-186f, the lower-obliquity regions could become more variable owing to resonant interactions. Even small deviations from coplanarity (e.g., mutual inclinations similar to 3 degrees) could stir peak-to-peak obliquity variations up to similar to 20 degrees. Undetected planetary companions and/or the existence of a satellite could also destabilize the low-obliquity regions. In all cases, the high-obliquity region allows for moderate variations, and all obliquities corresponding to retrograde motion (i.e., >90 degrees) are stable.
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