Catching a planet: A tidal capture origin for the exomoon candidate Kepler 1625b I

Abstract

The (yet-to-be confirmed) discovery of a Neptune-sized moon around the ~3.2 Jupiter-mass planet in Kepler 1625 puts interesting constraints on the formation of the system. In particular, the relatively wide orbit of the moon around the planet, at ~40 planetary radii, is hard to reconcile with planet formation theories. We demonstrate that the observed characteristics of the system can be explained from the tidal capture of a secondary planet in the young system. After a quick phase of tidal circularization, the lunar orbit, initially much tighter than 40 planetary radii, subsequently gradually widened due to tidal synchronization of the spin of the planet with the orbit, resulting in a synchronous planet-moon system. Interestingly, in our scenario the captured object was originally a Neptune-like planet, turned into a moon by its capture.

arXiv commentAccepted for publication in ApJL. 7 pages, 5 figures

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• Can close-in giant exoplanets preserve detectable moons?
• Exploring formation scenarios for the exomoon candidate Kepler 1625b I
• Transit Origami: A Method to Coherently Fold Exomoon Transits in Time Series Photometry
• The ominous fate of exomoons around hot Jupiters in the high-eccentricity migration scenario
• The Exomoon Corridor for Multiple Moon Systems
• Exomoons in Systems with a Strong Perturber: Applications to $\alpha$ Cen AB
• Collisional formation of massive exomoons of super-terrestrial exoplanets
• Consequences of dynamically unstable moons in extrasolar systems
• Probing Planets with Exomoons: The Cases of Kepler-1708 b and Kepler-1625 b
• On the Stability of Additional Moons Orbiting Kepler-1625 b
• Updated Studies on Exomoons in the HD 23079 System
• Transit Duration and Timing Variations from Binary Planets
• The Dynamics of Co-orbital Giant Exomoons -- Applications for the Kepler-1625 b and Kepler-1708 b Satellite Systems
• Exploring Exoplanet Dynamics with JWST: Tides, Rotation, Rings, and Moons
• GENGA II: GPU planetary N-body simulations with non-Newtonian forces and high number of particles