Planetary Rhythms: Synchronous Circulation On Variably Irradiated Asynchronous Planets

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.

a. Schematic of a circumbinary habitable zone (green) with regions of therminator habitability inside it (red shade) and, possible optimistic extensions within (blue) and beyond (yellow) the inner and outer edges, respectively, in the case of beating planets. b. Schematic of the shallow water model used in the simulations with different relevant heights serving as a proxy for atmospheric temperature affected by a periodically varying instellation source. The red line corresponds to the radiatively forced profile, while the blue represents the steady-state equilibrium height. — astro-ph.EP
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.

Top. Tidally synchronous circulation pattern for an Earth-like planet with a globally constant climate and the corresponding locations of the substellar point, the hotspot, and the latter’s mean over 40 diurnal cycles. Bottom. The usual asynchronous circulation pattern for the same configuration with translating substellar point and hotspot. Note that the hotspot mean is the same in both cases i.e. at 180◦ . For the asynchronous case, it is unintuitive to think of a mean longitude for the hotspot as it moves across the equatorial latitude, but we show it for comparison with the former and subsequent cases. — astro-ph.EP
Deepayan Banik
Comments: 14 pages, 6 figures, accepted for publication in Astrophysical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2501.09644 [astro-ph.EP] (or arXiv:2501.09644v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2501.09644
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Submission history
From: Deepayan Banik
[v1] Thu, 16 Jan 2025 16:34:41 UTC (1,622 KB)
https://arxiv.org/abs/2501.09644
Astrobiology