Ringed versus Ringless Worlds: How Poynting-Robertson Drag Shapes Rings across the Solar System

Abstract

Planetary rings are not only ubiquitous around the giant planets in the outer Solar System, but have also been discovered around several small distant bodies. In contrast, no rings have been observed around any inner Solar System objects. To constrain the dynamical origin of this ringed-versus-ringless dichotomy, we employ a numerically cross-checked analytical model of gigayear-scale Poynting-Robertson (PR) drag due to the solar flux acting on an isolated particle, expressed as a function of the host body's heliocentric distance \(\,a_{\mathrm{pla}}\) and the particle radius \(\,r_{\mathrm{par}}\). Here we show that, in the absence of additional perturbations, PR drag alone can explain the observed ring architecture of the Solar System: outer planets and Centaurs/TNOs are able to retain rings for the age of the Solar System, whereas any rings around the inner planets are removed on much shorter timescales. Because the PR-drag lifetime scales steeply with heliocentric distance \(\bigl(\tau_{\mathrm{decay}}\propto a_{\mathrm{pla}}^{2} \,r_{\mathrm{par}}\bigr)\), we predict that forthcoming surveys will reveal an ever-growing population of ring-bearing bodies in the distant Solar System.