Can water-rich exoplanets survive orbiting white dwarf stars, the latter of which are remnants of Sun-like stars? This is what a recent study accepted to *The Astrophysical Journal* hopes to address as a team of researchers investigated the likelihood of small, rocky worlds with close orbits to white dwarfs could harbor life. This study has the potential to help scientists better understand the conditions for finding life as we know it, or don’t know it, and where to find it.
For the study, the researchers used a series of mathematical models to examine how first-generation exoplanets could retain water while migrating inward towards a white dwarf as the latter transitions from a Sun-like star to a white dwarf. First-generation planets are planets that initially formed with the host star, meaning these planets survived the entire main sequence of the initial Sun-like star, along with surviving when it became a reg giant before transitioning to a white dwarf.
To accomplish this, the researchers started off by calculating a planet’s effective temperature, which is used to estimate habitable zones, as the habitable zone could change as the star transitions from Sun-like to white dwarf. Additionally, the researchers modeled how much water would evaporate from the planet’s surface due to the change in solar radiation, along with examining how planetary scattering events—when two planets pass so close to each other they influence each other’s orbit—could influence water retention.
In the end, the researchers found that Earth-sized or Mars-sized planets would need a sufficiently larger amount of water than Earth or need to have orbits that originated much farther out than their final orbit. Additionally, they found that the amount of radiation the planet would experience while orbiting so close to a white dwarf would completely evaporate all water from the planet.
The study notes in its conclusions, “We found that planets starting with large initial reservoirs of water and located at considerable distances from their host stars are more likely to retain their oceans. Moreover, the timing of planetary migration and the onset of tidal heating plays a crucial role in determining the fate of these water reserves. If a scattering event that initiates planetary migration occurs after the white dwarf has sufficiently cooled, the feasibility of ocean retention increases due to reduced XUV radiation.”
As noted, white dwarfs are remnants of Sun-like stars after the latter exits the main sequence phase of their lifetime, becoming a reg giant as it sheds its outer layers. Finally, this transition culminates in a white dwarf that is approximately the size of Earth but contains approximately 60 percent of the original Sun-like star’s mass. This makes it so dense that it is estimated that one teaspoon of the white dwarf would weigh tons of pounds. While our Sun is approximately 4.6 billion years old, scientists estimate that it will not leave the main sequence phase of its lifetime, become a red giant, and finally a white dwarf for another 5-7 billion years from now.
This study comes as astronomers continue expanding their knowledge and understanding regarding where to search for habitable worlds and life. While astronomers have long searched for Sun-like stars for Earth-like worlds, this study demonstrates, despite its findings, that white dwarfs might be suitable future targets for astrobiologists and the search for life. If nothing else, future studies could further examine the potential for water retention in Earth-like worlds orbiting white dwarfs.
What new insights into potentially habitable exoplanets orbiting white dwarfs will researchers make in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!
