Establish whether slow rotation can be attained by hot Neptunes

Determine whether hot Neptunes can attain the sufficiently slow rotation required for indefinite tidal stalling at low eccentricity, accounting for spin-up by f-modes, magnetized photoevaporative mass loss, and possible scattering from initially synchronized orbits.

Background

The calculations show that low-eccentricity hot Neptunes can avoid continued tidal damping if their initial rotation periods are sufficiently long. The physical origin of such sluggish rotation is not established: f-mode absorption may spin planets up, whereas magnetized mass loss or scattering from synchronized configurations could produce slow rotation. The authors explicitly identify whether the required spin state can actually be achieved as unresolved.

References

Whether such sluggish rotation can be achieved is an open question; planets are likely spun-up after runaway inflation as their $f$-modes absorb orbital angular momentum \citep{ivapap04,yuweiarr22}, but planetary spin-down could be affected by magnetized photoevaporative mass loss \citep[][]{webdav67,oweada14}.

— The Neptunian Final Parsec Problem: How Inertial Waves Enable Neptunes and Gas Giants to Survive Tidal Migration  (2610.12968 - Hallatt et al., 9 Oct 2026) in Section 6.2, subsection “Population-Level Expectations”

We end the integration once the orbit integral $|K_{lm}|{>}10{2}$, as we are unable to calculate the non-linear evolution during this phase of $f$-mode excitation with the present linear approach.

— The Neptunian Final Parsec Problem: How Inertial Waves Enable Neptunes and Gas Giants to Survive Tidal Migration  (2610.12968 - Hallatt et al., 9 Oct 2026) in Section 5.1, subsection “Evolution Under Inertial Waves & Runaway Inflation”