Increased Surface Temperatures of Habitable White Dwarf Worlds Relative to Main-Sequence Exoplanets

Discoveries of giant planet candidates orbiting white dwarf stars and the demonstrated capabilities of the James Webb Space Telescope bring the possibility of detecting rocky planets in the habitable zones of white dwarfs into pertinent focus. We present simulations of an aqua planet with an Earth-like atmospheric composition and incident stellar insolation orbiting in the habitable zone of two different types of stars - a 5000 K white dwarf and main-sequence K-dwarf star Kepler-62 with a similar effective temperature - and identify the mechanisms responsible for the two differing planetary climates. The synchronously-rotating white dwarf planet's global mean surface temperature is 25 K higher than that of the synchronously-rotating planet orbiting Kepler-62, due to its much faster (10-hr) rotation and orbital period. This ultra-fast rotation generates strong zonal winds and meridional flux of zonal momentum, stretching out and homogenizing the scale of atmospheric circulation, and preventing an equivalent build-up of thick, liquid water clouds on the dayside of the planet compared to the synchronous planet orbiting Kepler-62, while also transporting heat equatorward from higher latitudes. White dwarfs may therefore present amenable environments for life on planets formed within or migrated to their habitable zones, generating warmer surface environments than those of planets with main-sequence hosts to compensate for an ever shrinking incident stellar flux.