Obliquity variations of terrestrial planets in habitable zones

Abstract

We have investigated obliquity variations of hypothetical terrestrial planets in habitable zones (HZs) in extrasolar planetary systems. Using analytical expressions, we have shown that the terrestrial planets in a HZ tend to be in spin-orbit resonance when the giant planet is located at where they can keep the orbits of the terrestrial planets marginally stable. We found that the systems with both small obliquity variations and stable orbits in their HZs are only 1/5 of the known extrasolar planetary systems. However, retrograde spin, or a large and/or close satellite might be able to reduce the obliquity variation.