Thermally Regulated Viscoelastic Tidal Migration of Eccentric Planets

Abstract

A growing population of short-period Neptune-like planets has nonzero eccentricities and, in some cases, large stellar obliquities, suggestive of high-eccentricity migration. Modeling this evolution requires a prescription for tidal dissipation, which in Neptune-like planets may be dominated by rocky or icy cores rather than by extended gaseous envelopes as often assumed for gas giants. We study the coupled orbital and thermal evolution of eccentric Neptune-like planets whose tidal dissipation is controlled by a viscoelastic Maxwell core. We compute the full harmonic tidal response, follow the equilibrium spin state, and evolve the core temperature; the changing thermal state feeds back on the orbit by modifying the viscosity and hence the frequency-dependent tidal response. We find that the coupled evolution is self-regulated: tidal heating initially drives cold, weakly dissipative cores toward efficient dissipation, but the resulting temperature increase lowers the viscosity, shifts the tidal response toward the fluid-like side of the Maxwell peak, and slows the subsequent migration. This feedback drives the system toward a quasi-steady heating-cooling balance, produces long-lived eccentric phases, and weakens the otherwise steep dependence of circularization time on final orbital distance. Thermally regulated tides therefore offer a natural explanation for how observed hot Neptunes can remain eccentric across a broad range of orbital distances, where a single fixed tidal efficiency would either erase eccentricity too efficiently close in or fail to damp it farther out.