# Exomoonlight Sonata: On the Fate of Exomoons Around Migrating White Dwarf Planets **Authors:** Ritvik Sai Narayan, Joseph R. Livesey, Juliette Becker **arXiv:** [2610.27484](https://arxiv.org/abs/2610.27484) **Date:** 2026-10-20 **Summary page:** https://www.summarizepaper.com/en/arxiv-id/2610.27484v1/ ## Key points - The study finds that primordial moons are unlikely to survive giant planets’ high-eccentricity migration into close orbits around white dwarfs under the adopted assumptions. - An analytic, orbit-averaged equilibrium-tide model separately examines tidal orbital evolution, evection resonance, and stellar perturbations during planetary pericenter passages. - For pseudosynchronously rotating planets, the corotation radius lies outside the pericenter Hill radius, causing both circular prograde moons within the Hill sphere and retrograde moons to migrate inward. - Compact moons generally spiral inward faster than their planets migrate; wider satellite orbits and more massive planets provide longer tidal lifetimes. - Wider moons face stronger stellar perturbations: at 0.8–0.9 pericenter Hill radii, estimated perturbation work per passage reaches 56–73 percent of their binding energy, potentially causing collisions or ejection. - Evection resonance may sweep across moon orbits during planetary migration and excite destructive eccentricities, but capture and disruption probabilities remain undetermined. - A moon discovered around a close-orbiting white dwarf planet could constrain its migration history, although capture after migration would complicate interpretation. - Survival predictions depend on uncertain tidal dissipation, sustained pseudosynchronous planetary rotation, and the separate treatment of loss mechanisms; faster migration through dynamical or chaotic tides might improve survival prospects. - Fully coupled, long-term three-body simulations are needed to determine whether any moon-survival window remains. ## Summary This paper investigates whether moons can survive around giant planets migrating into close orbits around white dwarfs. Such moons could potentially occupy temperate environments as the white dwarfs cool, but they must first survive their host planets’ migration. The authors focus on high-eccentricity migration, in which a planet is driven onto a highly elongated orbit that subsequently circularizes through tidal dissipation. Specifically, they ask whether a moon that survives stellar evolution and the initial excitation of planetary eccentricity can also survive the subsequent migration phase. The study uses an analytic, orbit-averaged equilibrium-tide model of a hierarchical white dwarf–planet–moon system. It separately examines tidal orbital evolution, potential encounters with evection resonance, and gravitational perturbations during planetary pericenter passages. The reference system consists of a white dwarf with 0.5 solar masses, a planet with 10 Jupiter masses and 1.2 Jupiter radii, and a Ganymede-like moon with 0.025 Earth masses and 0.413 Earth radii. The planet initially has a semimajor axis of 4.5 astronomical units and an eccentricity of 0.995. If orbital angular momentum is approximately conserved, these parameters imply a final circular orbit near 0.045 astronomical units. Satellite separations are expressed as fractions of the planet’s Hill radius at pericenter. The reference tidal quality factors are one million for the planet and 100 for the moon. A central result concerns the direction of satellite migration. For a planet rotating pseudosynchronously with its stellar orbit, the corotation radius always lies outside the pericenter Hill radius. Under the assumed mass hierarchy, the ratio of these radii depends only on planetary eccentricity, decreasing from approximately 1.44 for a circular orbit to about 1.30 at extreme eccentricity. Consequently, circular prograde moons within the Hill sphere orbit faster than the planet rotates and migrate inward through tidal interactions. Retrograde moons also migrate inward. Moons on compact orbits generally spiral inward faster than their host planets migrate. In the initial configuration, a moon at half the pericenter Hill radius has a planet-to-moon migration-timescale ratio of approximately 150 for a one-Jupiter-mass planet and approximately three for the reference ten-Jupiter-mass planet. Ratios greater than one indicate faster fractional contraction of the satellite orbit. For the ten-Jupiter-mass planet, the migration timescales are equal near 0.59 Hill radii. Wider satellite orbits and more massive planetary companions therefore provide longer tidal lifetimes. Under the adopted prescription, the satellite migration timescale depends steeply on orbital separation, scaling approximately as separation to the power of 6.5. However, wider moons are more vulnerable to stellar perturbations. At separations of 0.8–0.9 Hill radii, the estimated work done by these perturbations during a single planetary pericenter passage reaches approximately 56–73 percent of the satellite’s binding energy. Such perturbations could lead to collisions or ejection. Evection resonance provides another potential loss mechanism. As planetary migration changes the relationship between the satellite’s apsidal precession rate and the planet’s orbital frequency, the resonance boundary can sweep through satellite orbits and potentially excite destructive eccentricities. The study identifies candidate resonance configurations but does not establish capture or disruption probabilities. The authors conclude that primordial moons are unlikely to survive high-eccentricity planetary migration under the adopted assumptions. Discovering a moon around a close-orbiting white dwarf planet could therefore constrain the planet’s migration history, although subsequent capture would complicate that interpretation. Key limitations include uncertain tidal dissipation, the assumption of sustained pseudosynchronous planetary rotation, and the separate rather than fully coupled treatment of the three loss mechanisms. Dynamical or chaotic tides could accelerate planetary migration, reducing the time available for satellite inspiral. Coupled, long-term three-body simulations are needed to determine whether any window for moon survival remains.