Initial conditions for tidal synchronisation of a planet by its moon
arXiv:2509.09858 · doi:10.3390/universe11090309
Abstract
Moons tidally interact with their host planets and stars. A close moon is quickly synchronised by the planet, or becomes captured in a higher spin-orbit resonance. However, the planet requires much more time to significantly alter its rotation rate under the influence of moon-generated tides. The situation becomes more complex for close-in planets, as star-generated tides come into play and compete with the moon-generated tides. Synchronisation of the planet by its moon changes the tidal dynamics of the entire star-planet-moon system and can lead to long-term stable configurations. In this paper, we demonstrate that a certain initial condition must be met for this to occur. Based on the angular-momentum conservation, the derived condition is universal and bears no dependence upon the planet's internal structure or tidal dissipation model. It is applicable to dwindling systems as well as tidally expanding orbits, and to the cases of initially retrograde motion. We present calculations for specific planet-moon systems (Earth and the Moon; Neptune and Triton; Venus and its hypothetical presently-extinct moon Neith; Mars, Phobos, and Deimos; Pluto and Charon), to constrain the dynamically plausible formation and evolution scenarios. Among other things, our analysis prompts the question of whether Pluto and Charon evolved into their current state from an initially more compact configuration (as is commonly assumed) or from a wider orbit -- a topic to be discussed at length elsewhere. Our results are equally applicable to exoplanets. For example, if asynchronous close-in exoplanets are detected, the possibility of tidal synchronisation by an exomoon should be considered.
References in corpus (14)
- Bodily tides near spin-orbit resonances
- HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I
- An Exomoon Survey of 70 Cool Giant Exoplanets and the New Candidate Kepler-1708 b-i
- Spin-orbit evolution of Mercury revisited
- No Evidence for Lunar Transit in New Analysis of Hubble Space Telescope Observations of the Kepler-1625 System
- Complete Tidal Evolution of Pluto-Charon
- Tidal dissipation in a homogeneous spherical body. II. Three examples: Mercury, Io, and Kepler-10 b
- Why is the Moon synchronously rotating?
- Pathways of Survival of Exomoons and Inner Exoplanets
- A dynamical context for the origin of Phobos and Deimos
- Extreme exomoons in WASP-49 Ab: dynamics and detectability
- Tidal evolution of the Pluto-Charon binary
- Chaotic Capture of a Retrograde Moon by Venus and the Reversal of Its Spin
- A synchronous moon as a possible cause of Mars' initial triaxiality