The Impact of Classical and General Relativistic Obliquity Precessions on the Habitability of Circumstellar Neutron Stars’ Planets

Abstract

Abstract Recently, it has been shown that rocky planets orbiting neutron stars can be habitable under plausible circumstances. If a distant, point-like source of visible light, such as a Sun-like main-sequence star or the gravitationally-lensed accretion disk of a supermassive black hole is present, possible temporal variations Δ ε p t of the planet’s axial tilt ε p to the ecliptic plane should be included in the overall habitability budget since the obliquity determines the insolation at a given latitude on a body’ s surface. I point out that, for rather generic initial spin–orbit initial configurations, general relativistic and classical spin variations induced by the post-Newtonian de Sitter and Lense–Thirring components of the field of the host neutron star and by its pull to the planetary oblateness J 2 p may induce huge and very fast variations of ε p that would likely have an impact on the habitability of such worlds. In particular, for a planet’s distance of, say, 0.005 au from a 1.4 M ⊙ neutron star corresponding to an orbital period P b = 0.109 day, obliquity shifts Δε p as large as ε p max − ε p min ≃ 50 ° – 100 ° over characteristic timescales as short as 10 days ( J 2 p ) to 3 Myr (Lense–Thirring) may occur for arbitrary orientations of the orbital and spin angular momenta L , S ns, S p of the planet-neutron star system. In view of this feature of their spins, I dub such hypothetical planets as “nethotrons.”