Potentially habitable exoplanets are so incredibly common that astronomers have started to consider more unusual situations where life might arise.
Perhaps life can be found on the moon of a hot Jupiter or lingering in the warm ocean of a rogue planet.
Recently, there has even been the idea that habitable worlds might orbit white dwarfs.
We know some white dwarfs have planets, and despite lacking nuclear fusion, white dwarfs do emit enough light and heat to have a habitable zone.
But the question remains whether a planet could retain a water-rich environment through the red giant stage of a star before it becomes a white dwarf. This is the focus of a new study on the arXiv.
The study starts by stating the obvious. Any habitable world around a main-sequence star will likely be stripped of its atmosphere and water as the star swells to a red giant.
By the time the star becomes a white dwarf, any planet that was habitable will be barren, if not consumed by its star.
The work then goes on to consider more distant worlds in a system. Perhaps a cold and icy hycean world might become habitable in the white dwarf stage.
It turns out there are two critical stages. The first is that an ocean world would need to retain a large portion of its water during the dying stage of the main sequence star.
As you might expect, the more distant a planet is from its star, the more water it retains.
For a sunlike star, an ocean world would need to be more than three times Earth’s distance to retain water. To retain vast oceans similar to Earth, the planet would have to be about 10 AU away, or roughly the distance of Saturn.
The second critical stage is orbital migration.
Once the star becomes a white dwarf, an ocean world at Saturn’s orbit would be an ice planet far beyond the habitable zone. To become a living world, it would need to move inward to a close, warm orbit.
This is possible both through interaction with the nebula formed during the red giant stage, as well as through gravitational interactions between planets.
Our own solar system, for example, had a migration phase in its youth. As the study shows, however, the timing of this migration is critical. If the inward migration of a world happens too soon, then much of the water will boil off.
If it happens too late, then the system will have stabilized to the point that the world won’t be able to enter the habitable zone.
Overall, the study finds that most worlds around a white dwarf will either be 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 is *possible* for an outer hycean world to migrate at just the right time to retain water and become a warm Earth-like world. Not likely, but possible.
So finding a habitable planet around a white dwarf is a long shot. But given how easy it might be to study the atmospheres of these worlds, it’s certainly worth taking a look.
Written by Brian Koberlein/Universe Today.
