Obliquity-driven sculpting of exoplanetary systems

Abstract

NASA’s Kepler mission revealed that ~30% of Solar-type stars harbour planets with sizes between that of Earth and Neptune on nearly circular and coplanar orbits with periods less than 100\u2009days1–4. Such short-period compact systems are rarely found with planet pairs in mean-motion resonances (MMRs)—configurations in which the planetary orbital periods exhibit a simple integer ratio—but there is a significant overabundance of planet pairs lying just wide of the first-order resonances5. Previous work suggests that tides raised on the planets by the host star may be responsible for forcing systems into these configurations by draining orbital energy to heat6–8. Such tides, however, are insufficient unless there exists a substantial and as-yet-unidentified source of extra dissipation9,10. Here we show that this cryptic heat source may be linked to ‘obliquity tides’ generated when a large axial tilt (obliquity) is maintained by secular resonance-driven spin–orbit coupling. We present evidence that typical compact, nearly coplanar systems frequently experience this mechanism, and we highlight additional features in the planetary orbital period and radius distributions that may be its signatures. Extrasolar planets that maintain large obliquities will exhibit infrared light curve features that are detectable with forthcoming space missions. The observed period ratio distribution can be explained if typical tidal quality factors for super-Earths and sub-Neptunes are similar to those of Uranus and Neptune.Compact exoplanetary systems frequently experience spin–orbit coupling driven by secular resonances, which can shape their architecture, allowing the planet to maintain a large obliquity and inducing the piling up of planets just wide of the first-order resonance.