How a close-in planet protects its white dwarf host from pollution

Approximately 25–50% of the white dwarfs (WDs) exhibit metal absorption lines in their photospheres. The lines are attributed to accretion from their remnant planetary systems. Although WDs with detected planetary systems are more likely to show photospheric pollution, one notable exception, WD 1856+534, hosts a close-in giant planet, but exhibits no detectable photospheric metal pollution. Previous studies have proposed that massive close-in planets can block the inward transport of small particles driven by radiative forces (e.g., Poynting–Robertson drag and the Yarkovsky effect). It remains unclear, however, whether the close-in planet can similarly prevent the delivery of larger bodies via dynamical interactions. We aim to quantify the protective effect of close-in planets on WD pollution by asteroids that approach on near-parabolic orbits and to explore the planetary masses and orbital separations required to provide an effective protection. We performed ensembles of short-term N-body integrations that sampled a range of planet masses and orbital separations and initialized asteroids on highly eccentric orbits with periapses near the WD Roche radius in order to measure scattering, capture, and ejection outcomes and to quantify the planet’s shielding efficiency. For WD1856+534b-like configurations (a_p=0.02 au), giant planets with masses greater than 0.5 Jupiter masses are sufficient to clear over 80% of the highly eccentric small-body contaminants. The effectiveness of the protective effect diminishes with decreasing planetary mass and increasing semimajor axis. These findings help us to explain why some WDs that host close-in giant planets do not show the photospheric metal pollution commonly observed in other systems.