Obliquity-driven sculpting of exoplanetary systems

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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 days1,2,3,4. 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 heat6,7,8. 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.

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Fig. 1: Schematic of the excitation of planetary obliquities through secular spin–orbit resonant interaction.
Fig. 2: Intrinsic frequency commensurability for typical compact extrasolar systems.
Fig. 3: Capture into simultaneous mean-motion and spin–orbit resonances.
Fig. 4: Domain for the inner planet’s spin–orbit resonant capture during convergent inward migration.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Change history

  • 24 April 2019

    In the version of this Letter originally published, the following ‘Journal peer review information’ was missing: “Nature Astronomy thanks Jianghui Ji and the other anonymous reviewer(s) for their contribution to the peer review of this work.” This statement has now been added.

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Acknowledgements

We thank K. Batygin, D. Fabrycky and Y. Wu for inspiring conversations. S.M. is supported by the National Science Foundation Graduate Research Fellowship Program under grant number DGE-1122492. This material is also based on work supported by the National Aeronautics and Space Administration through the NASA Astrobiology Institute under Cooperative Agreement Notice NNH13ZDA017C issued through the Science Mission Directorate. We acknowledge support from the NASA Astrobiology Institute through a cooperative agreement between NASA Ames Research Center and Yale University.

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S.M. performed the simulations, calculated the resonant proximity diagnostics and made the resonant capture parameter space map. G.L. conceived of and derived the constraints on the tidal quality factors, the radius distribution observations and the predictions regarding satellites. Both authors wrote the paper and made figures.

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Correspondence to Sarah Millholland.

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The authors declare no competing interests.

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Journal peer review information: Nature Astronomy thanks Jianghui Ji and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Millholland, S., Laughlin, G. Obliquity-driven sculpting of exoplanetary systems. Nat Astron 3, 424–433 (2019). https://doi.org/10.1038/s41550-019-0701-7

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