Substellar Landmass Fraction and Atmospheric Circulation Regimes of Ultrashort-period Rocky Planets around White Dwarfs

Abstract White-dwarf (WD) systems are attractive candidates for planet detection and follow-up transmission spectroscopy campaigns. However, the climate and habitability of terrestrial planets around WDs still remain largely elusive, and a central question lies in the width and extent of their habitable zones (HZs) and how their climates may be shaped by their stellar environments. A recently published study, using a 3D general circulation model (GCM), found a novel atmospheric circulation pattern for ultrarapid rotators around WDs. This “bat-rotation” regime features mean equatorial subrotation and a displacement of the surface’s hottest regions from the equator toward the midlatitudes. Here, we used a GCM of reduced complexity to simulate a large range of landmass fraction (LMF) assumptions on these planets and examine the regimes in which these planets would exhibit the new dynamical regime. We find that as the dayside LMF approaches 80%, the bat-rotator features begin to diminish and the circulation behavior becomes more akin to the thermally driven direction circulation patterns observed around slowly rotating planets. Our results suggest that the introduction of different surface characteristics would drastically modify previous predictions of the atmospheres of ultrashort-period planets in the HZ of WD stars.