Wide Dust Gaps in Protoplanetary Disks Induced by Eccentric Planets: A Mass-Eccentricity Degeneracy

TL;DR

This paper investigates how eccentric planets can create dust gaps in protoplanetary disks that mimic those caused by more massive planets on circular orbits, potentially explaining observed continuous gap distributions.

Contribution

It demonstrates that dust gap features caused by eccentric Neptune-mass planets can resemble those from Saturn-mass planets, resolving the apparent contradiction in planet mass distributions.

Findings

Eccentric Neptune-mass planets can produce gap-ring separations similar to Saturn-mass planets.

Small eccentricities can sustain in disks, affecting dust distribution symmetry.

Eccentricity-induced gaps are axisymmetric, consistent with ALMA observations.

Abstract

The tidal perturbation of embedded protoplanets on their natal disks has been widely attributed to be the cause of gap-ring structures in sub-mm images of protoplanetary disks around T Tauri stars. Numerical simulations of this process have been used to propose scalings of characteristic dust gap width/gap-ring distance with respect to planet mass. Applying such scalings to analyze observed gap samples yields a continuous mass distribution for a rich population of hypothetical planets in the range of several Earth to Jupiter masses. In contrast, the conventional core-accretion scenario of planet formation predicts a bi-modal mass function due to 1) the onset of runaway gas accretion above \sim20 Earth masses and 2) suppression of accretion induced by gap opening. Here we examine the dust disk response to the tidal perturbation of eccentric planets as a possible resolution of this paradox. Based on simulated gas and dust distributions, we show the gap-ring separation of Neptune-mass planets with small eccentricities might become comparable to that induced by Saturn-mass planets on circular orbits. This degeneracy may obliterate the discrepancy between the theoretical bi-modal mass distribution and the observed continuous gap width distribution. Despite damping due to planet-disk interaction, modest eccentricity may be sustained either in the outer regions of relatively thick disks or through resonant excitation among multiple super Earths. Moreover, the ring-like dust distribution induced by planets with small eccentricities is axisymmetric even in low viscosity environments, consistent with the paucity of vortices in ALMA images.