Tidal Disruption of Blanets in Kerr Spacetime

Abstract

Blanets are planetary-mass bodies ($20$--$3000\,\Me$) that may orbit supermassive black holes (SMBHs) in the circumnuclear disks of active galactic nuclei (AGN). We examine tidal disruption events produced by blanet--SMBH encounters, from the test-particle limit to massive planetary bodies in Kerr spacetime. Using the geodesic deviation equation and the Kerr tidal tensor, we derive disruption criteria, tidal radii, and Hills masses for planetary-mass objects, and show that blanet TDEs can remain observable for SMBHs up to $\sim10^{10}\,\Msun$, well above the stellar Hills mass of $\sim10^8\,\Msun$. The fallback rate retains the usual $t^{-5/3}$ form, but the peak timescales are shorter -- from hours to months -- with lower peak accretion rates and multi-wavelength signatures that differ from those of stellar TDEs. We also examine orbital stability, including Keplerian precession, Lense--Thirring nodal precession, migration in the circumnuclear disk, and the Kozai--Lidov resonance, and identify the region where blanets can survive before disruption. We derive relativistic corrections to the tidal radius, spin-dependent disruption thresholds, and the effect of Kerr spin on the disruption geometry. We also discuss gravitational-wave emission from blanet debris EMRIs and the prospects for LISA detection, which may help in interpreting unusual TDE-like transients in AGN environments.