Surprising Spin-orbit Resonances of Rocky Planets

Recent works suggest that, in multiplanetary systems, a close-in exoplanet can sometimes avoid becoming tidally locked to its host star if it is captured into a secular spin-orbit resonance with a companion planet.

In such a resonance, the planet remains at a sub-synchronous spin rate and an appreciable obliquity (the planet’s spin-orbit misalignment angle). However, many of these works have only considered planets with fluid-like rheologies. Recent observations suggest that planets up to a few Earth masses may be rocky and thus may have an appreciable rigidity.

In this work, we study the spin-orbit dynamics of such rigid planets using a linear dissipative tidal model and not enforcing principal axis rotation about the body’s shortest principal axis.

We identify a new class of spin-orbit resonances when the planet spins at twice its orbital frequency. These resonances exist at nonzero obliquity and spontaneously excite non-principal-axis rotation upon resonance capture.

While these resonances eventually disappear as tidal dissipation damps the obliquity to zero (and the body returns to principal-axis rotation), they still modify the spin evolutionary history of the planet. Such resonances may enhance the prevalence of secular spin-orbit resonances in exoplanetary systems.

Henry Yuan, Yubo Su, Jeremy Goodman

Comments: 20 pages, 4 figures, submitted to ApJ. Comments welcome!
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2410.05373 [astro-ph.EP] (or arXiv:2410.05373v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2410.05373
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Submission history
From: Henry Yuan
[v1] Mon, 7 Oct 2024 18:00:00 UTC (2,586 KB)
https://arxiv.org/abs/2410.05373

Astrobiology, Astrogeology,