The Fate of Oceans on First-Generation Planets Orbiting White Dwarfs

TL;DR

The paper tackles whether oceans can endure on first-generation planets that migrate to the white dwarf’s habitable zone, confronting rapid post-main-sequence brightening, XUV-driven photoevaporation, and tidal heating during inward migration. It proposes a coupled model that combines RG/AGB irradiation, energy-limited atmospheric escape, and tidal evolution with full eccentricity dynamics to track ocean mass loss across a range of initial conditions. The results indicate that substantial initial water or larger initial orbital distances favor ocean retention, and delaying the planet’s migration to later WD cooling ages further enhances survival by reducing XUV exposure. These findings delineate a narrow but plausible regime for WD-hosted ocean-bearing worlds and inform strategies for detecting habitable planets or moons around white dwarfs.

Abstract

Several groups have recently suggested that small planets orbiting very closely around white dwarf stars could be promising locations for life to arise, even after stellar death. There are still many uncertainties, however, regarding the existence and habitability of these worlds. Here, we consider the retention of water during post-main-sequence evolution of a Sun-like star, and during the subsequent migration of planets to the white dwarf's habitable zone. This inward migration is driven by dynamical mechanisms such as planet-planet interactions in packed systems, which can excite planets to high eccentricities, setting the initial conditions for tidal migration into short-period orbits. In order for water to persist on the surfaces of planets orbiting white dwarfs, the water must first survive the AGB phase of stellar evolution, then avoid being lost due to photoevaporation due to X-ray and extreme ultraviolet (XUV) radiation from the newly-formed white dwarf, and then finally survive the tidal migration of the planet inwards to the habitable zone. We find that while this journey will likely desiccate large swaths of post-main-sequence planetary systems, planets with substantial reservoirs of water may retain some surface water, especially if their migration occurs at later white dwarf cooling ages. Therefore, although stellar evolution may pose a challenge for the retention of water on exoplanet surfaces, it is possible for planets to retain surface oceans even as their host stars die and their orbits evolve.

Figures (6)

• Figure 1: A model of the evolving properties of a Sun-like star as it evolves off the main sequence and eventually becomes a white dwarf, along with the resultant evolution of affected planet properties for a selection of initial planetary orbits. Top panel: The luminosity evolution of the central body, constructed by combining the models of Hidalgo2018, Bertelli2008, and Salaris2022. Second panel: The mass of the central body Bertelli2008. Third panel: The semi-major axis of three planets with starting locations 3, 6, and 10 AU. Fourth panel: The effective temperature of each planet based on its orbital location and the luminosity of the central body. Bottom panel: The fraction of surface water remaining for an Earth-like planet with an initial 1 Earth ocean's mass worth of water.
• Figure 2: The fractional ocean remaining after 23 Gyr of main sequence and post-main-sequence evolution for planets with a variety of staring semi-major axes and initial ocean masses due to photoevaporative mass loss only. The calculation was performed as in the bottom panel of Figure \ref{['fig:lum_func']}, but only the final value of total surface water is reported. Surface water is not expected to change significantly past this time as the WD has cooled sufficiently to no longer drive outflows. Planets with Earth-like oceans are totally desiccated due to the radiative evolution of the host stars to fairly large initial orbital radii ($\sim5$ AU), but planets with larger initial oceans may retain significant amounts of water.
• Figure 3: (Top panel) Molar fractions of different constituents of water vapor at a pressure of 1 atm, showcasing the relative abundances of each species 197959. (Middle panel) Jeans escape rates, given in particles/m$^2$/s, for each constituent of water vapor on an Earth-like planet at varying temperatures. The harsh left edges in $H$ and $H_2$ particle escape rates are due to the lack of dissociated water vapor products at lower temperatures. (Bottom panel) The ocean mass loss rate in grams per second for an Earth-like planet at various temperatures. The calculation assumes that the planetary atmosphere is made entirely of water vapor and its dissociation products, and that no other sources of hydrogen or oxygen are available. The ocean mass loss rate only counts particles lost from the planet; otherwise, it is possible they could re-condense later at lower temperatures.
• Figure 4: Evolution of planetary parameters and heating over time. The top two panels show the change in semi-major axis and eccentricity from initial values ($a_i$ = 5.00 AU, $e_i$ = 0.997) to final values ($a_f$ = 0.02 AU, $e_f$ = 0.00). The third panel presents the total heating (TW) from solar radiation and tidal forces. The bottom panel illustrates the fraction of oceans lost over time (in terrestrial oceans, TO) due to the combined effects of radiation and tidal heating. For this migration process, which begins with a scattering event that occurs shortly after the formation of the white dwarf, no surface water remains.
• Figure 5: Evolution of planetary parameters following a scattering event that excites a planet to large eccentricity. The top row shows the semi-major axis evolution of a planet after the scattering event, which occurs when the white dwarf's temperature is (first column) 10000 K, (second column) 8500 K, and (third column) 6000 K. The second row shows eccentricity evolution computed. The third row displays the heating rates from tides and radiation over time, along with the total heating experienced by the planet. The bottom panel illustrates the fraction of oceans remaining as a function of time since the scattering event, highlighting the significant impact of these events on the long-term habitability of the planet. The initial and final semi-major axes ($a_f$ and $a_i$) and eccentricities ($e_f$ and $e_i$) are labeled for each case. For scattering events that occur later (once the white dwarf has already cooled), ocean retention is easier as both the XUV luminosity fraction and the total white dwarf luminosity are lower.