Spin Dynamics of Rocky Bodies in Compact Multiplanetary Systems

Traditionally, it is thought that a planet sufficiently close to its host star will experience tidal dissipation, driving its spin period towards equality with its orbital period and its spin axis towards alignment with its orbit normal. Recently, works have pointed out that a planet can retain large spin-orbit misalignment even in the presence of tidal dissipation if it has a companion planet via a “spin-orbit resonance”. Since many of the known close-in exoplanets have companions, it has been suggested that a substantial fraction of these planets may exhibit spin-orbit misalignment. However, in systems with at least three planets, there are multiple such resonances. When these resonances overlap, chaotic evolution emerges, which generally results in spin-orbit alignment due to tidal dissipation. We present tentative results for the resonance dynamics of a few different classes of planetary architectures, beginning with the often-quoted “peas in a pod”. Separately, previous works have generally assumed approximately fluid planets, whose only deviation from sphericity comes from their rotational deformation. However, it is increasingly clear that planets with masses up to a few times that of the Earth can be predominantly rocky, retaining a permanent triaxial figure. We present studies of the spin-orbit dynamics of such rocky bodies undergoing tidal dissipation and compare them to previous studies of these dynamics for Solar System satellites.