The white dwarf planet WD J0914+1914 b: barricading potential rocky pollutants?

Abstract

An ice giant planet was recently reported orbiting white dwarf WD J0914+1914 at an approximate distance of 0.07 au. The striking non-detection of rocky pollutants in this white dwarf's photosphere contrasts with the observations of nearly every other known white dwarf planetary system. Here, I analyze the prospects for exterior extant rocky asteroids, boulders, cobbles and pebbles to radiatively drift inward past the planet due to the relatively high luminosity (0.1L ⊙ ) of this particularly young (13 Myr) white dwarf. Pebbles and cobbles drift too slowly from Poynting-Robertson drag to bypass the planet, but boulders and asteroids are subject to the much stronger Yarkovsky effect. In this paper, I (i) place lower limits on the timescales for these objects to reach the planet's orbit, (ii) establish 3 m as the approximate limiting radius above which a boulder drifts too slowly to avoid colliding with the planet, and (iii) compute bounds on the fraction of boulders which succeed in traversing mean motion resonances and the planet's Hill sphere to eventually pollute the star. Overall, I find that the planet acts as a barrier against rather than a facilitator for radiatively-driven rocky pollution, suggesting that future rocky pollutants would most likely originate from distant scattering events.

Short-term stability of particles in the WD J0914+1914 white dwarf planetary system

“…Instead, rocky particles external to the planet could have any orbital eccentricity or inclination. These values may or may not have been radiatively damped through the relatively high luminosity (≈ 0.1L ) of this white dwarf (Veras 2020) and could be at any stage of damping.…”

“…Both the extent of the gas disc as well as the lack of rocky debris motivate dynamical questions about the role of the planet, denoted as WD J0914+1914 b, in shaping the system. Veras (2020) found that WD J0914+1914 b acts as an effective barricade for pebbles and most boulders which are radiatively dragged towards the white dwarf. However, he only sampled parameter space which was relevant to those bodies and considered only radiative drag and one type of planet.…”

The entry geometry and velocity of planetary debris into the Roche sphere of a white dwarf

“…According to various arguments in Section 2.2 of Malamud et al (2021), the initial size of tidal fragments falls exactly within that range. However, better theoretical understanding of the seasonal Yarkovsky orbital shrinking effect in highly eccentric orbits is still required (Veras et al 2015a(Veras et al , 2019Veras 2020).…”

Planetary Systems Around White Dwarfs

“…Instead, the planet's current location my be explained by a previous gravitational scattering event within the system, or from a dissipative capture from an external companion or cluster. In contrast, the close separations of WD J0914+1914 b and WD 1856+534 b require either (i) a gravitational scattering event during the white dwarf phase, plus some form of tidal shrinkage (Muñoz & Petrovich 2020;O'Connor, Liu & Lai 2021;Veras & Fuller 2020;Veras 2020;Zotos et al 2020;Stephan, Naoz & Gaudi 2021), or (ii) survival of the planet within a common envelope of the white dwarf precursor (Chamandy et al 2021;Lagos et al 2021).…”