New research indicates that some white dwarfs, remnants of sun-like stars, may host planets capable of sustaining water and possibly life. This study explores the fate of planets during their host star's transition from the main sequence to a red giant and eventually to a white dwarf.
Scientists modeled changes in luminosity and mass of solar-type stars, planetary orbits, and water retention as the star evolves. It was found that planets within the habitable zone at the onset of this transition generally lose their oceans, either being engulfed by the expanding star or heated to the point where water evaporates.
However, planets located beyond two astronomical units (AU) from the star may retain some water through these changes. Specifically, those starting at distances of at least 10 AU—similar to Saturn's current orbit—could maintain a significant portion of their initial water, potentially up to three-quarters.
As the star becomes a white dwarf, distant planets may freeze, and their orbits could expand due to the star's reduced mass. This orbital expansion may lead to instability, increasing the chances of two planets passing close together, with one potentially being drawn closer to the white dwarf.
If a planet migrates into the habitable zone, it could experience conditions suitable for liquid water. Researchers noted that if this migration occurs shortly after the formation of the white dwarf, intense ultraviolet radiation could strip the planet of its atmosphere and oceans. Conversely, if migration happens three billion years or more after the white dwarf's formation, the weaker ultraviolet radiation may allow oceans to persist.
While this scenario necessitates a series of fortunate, coincidental events, it raises the possibility that habitable planets could exist around white dwarfs.