Planetary Rhythms: Synchronous Circulation on Variably Irradiated Asynchronous Planets

Abstract

Tidal locking of planets to their host stars results in an atmospheric circulation with a hotspot fixed to the frame of reference of the planet. On the other hand, asynchronously rotating planets feature moving hotspots either lagging or leading the corresponding substellar point as it translates along the surface. We show that a planet falling in the latter category could mimic the circulation of tidally synchronous planets under the influence of time-varying instellation, possibly provided by pulsating or multiple star systems. This happens when the planets diurnal period is in resonance with the period of instellation variation, leading to a planet-frame-fixed hotspot. Slight differences in the above periods lead to East-West or West-East creeping hotspots with a period significantly longer than both. The rate of hotspot motion is given by the difference between the diurnal and instellation variation rates, similar to the lower envelope frequency of beat patterns formed by two superposed waves in linear wave theory. We call this phenomenon beating. A combination of the radiative, rotational, wave propagation, and drag timescales establishes dynamical constraints on beating. Based on this we classify a set of Kepler and TESS circumbinary planets with two candidates exhibiting climatic departures from the no-variation scenario. In general, hotter and faster-spinning planets are more susceptible to climatic departures. Beating, if it occurs, may additionally create optimistic extensions of habitable zones for corresponding systems.