Potentially habitable exoplanets are so incredibly common that astronomers have started thinking about more unusual situations in which life could arise. Maybe life can be found on the moon of a hot Jupiter or in the warm ocean of an alien planet. Recently there has even been the idea that habitable worlds could orbit white dwarfs. We know that some white dwarfs have planets, and although they lack nuclear fusion, white dwarfs emit enough light and heat to form a habitable zone. But the question remains whether a planet could maintain a water-rich environment beyond a star's red giant stage before becoming a white dwarf. This is the focus of a new study on the arXiv.
The study begins by stating the obvious. Any habitable world around a main sequence star is likely to be stripped of its atmosphere and water as the star swells into a red giant. If the star becomes a white dwarf, any previously habitable planet will be barren unless it is swallowed up by its star. The next step is to take more distant worlds into account in a system. Perhaps a cold and icy Hycean world could become habitable in the white dwarf stage.
It turns out that there are two critical phases. The first is that an ocean world would have to retain much of its water during the main sequence star's dying stage. As you might expect, the further a planet is from its star, the more water it stores. For a sun-like star, an ocean world would have to be more than three times as far away from Earth to store water. To obtain vast Earth-like oceans, the planet would have to be about 10 AU away, which is about the same distance as Saturn.
Water retention for planets at different distances. Photo credit: Becker et al
The second critical phase is orbital migration. Once the star becomes a white dwarf, an ocean world in Saturn's orbit would be an ice planet far beyond the habitable zone. To become a living world, it would have to move inward into a tight, warm orbit. This is possible both through interaction with the nebula formed in the red giant stage and through gravitational interactions between planets. Our own solar system, for example, had a migration phase in its youth. However, as the study shows, the timing of this migration is crucial. If a world's immigration occurs too early, much of the water will evaporate. If this happens too late, the system will have stabilized to such an extent that the world can no longer advance into the habitable zone.
Overall, the study finds that most worlds around a white dwarf are either dry before entering the habitable zone or retain water and remain at the outer edge of the system. But as the authors point out, it's *possible* that an outer Hycea world migrated at just the right time to store water and become a warm, Earth-like world. Not likely, but possible.
So it's a long shot to find a habitable planet around a white dwarf. But considering how easy it may be to study the atmospheres of these worlds, it's definitely worth taking a look.
Reference: Becker, Juliette, Andrew Vanderburg and Joseph Livesey. “The fate of the oceans on first-generation planets orbiting white dwarfs.” arXiv preprint arXiv:2412.12056 (2024).
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