Researchers studying planets orbiting burnt-out stars say they may be promising targets for astrobiologists searching for signs of alien life.
The atmospheres of planets orbiting these stars, known as white dwarfs, would be much easier to characterize than those of planets orbiting brighter stars due to the limited amount of radiated light they emit. While such planets would have to survive the violent phases that accompany the star’s death, researchers involved with the new research believe that some may indeed “thread the celestial needle” needed to keep their water and protect anything living on them.
If true, this could potentially offer Earth’s telescopes an unprecedented opportunity to scour their atmospheres for signs of life.
“White dwarfs are so small and so featureless that if a terrestrial planet transited in front of them, you could actually do a much better job of characterizing its atmosphere,” explained University of Wisconsin–Madison astronomy professor Juliette Becker, the lead author of the study outlining the team’s research. “The planet’s atmosphere would have a much larger, clearer signal because a larger fraction of the light you’re seeing is passing through exactly what you want to study.”
Burnt-Out Stars Go Through a Particularly Violent Transition Before Death
The team’s study, which is currently under review at AAS Journals after being presented in Madison at the 244th meeting of the American Astronomical Society, outlines the different phases a dying star goes through and what would be required for a planet to survive.
“There are two pulses, basically, during which the star grows to 100 times its normal radius,” Becker explains. “While it does that — we can call this part Destruction Phase No. 1 — it will engulf any planets that are within that radius.”
For example, Earth is close enough to our Sun that it will likely be completely engulfed once it transitions from a main sequence G-type star to one of billions of burnt-out stars in our Milky Way galaxy. Even if a planet survives this dramatic expansion, the team says any water on the planet’s surface may still be impacted because the star’s rapid expansion would add significant heat energy to any planet it does not engulf.
If that heat energy is high enough, any water on the nearby planet could be partially or even completely burned off.
“The fact that the star gets so much brighter means that all planets in the system, even ones that used to be cold in the outer solar system, will suddenly have their surface temperatures increase drastically,” Becker says. “That can evaporate their oceans and cost them a lot of water.”
According to the team’s calculations, a planet hoping to retain its water while its host star dies would likely need to orbit at least 5 or 6 astronomical units away. Unfortunately, the researchers say that any planet at this distance that survives the star’s violent death may end up too far away once it cools. The result would be a planet that doesn’t receive enough starlight energy to maintain its H20 in water form.
“If you can be sufficiently far away during this dangerous time that you don’t lose your surface water, that’s good,” Becker says. “But the downside is you’re going to be so far away from the star that all the water is going to be ice, and that’s not great for life.”
Planets That Thread the Celestial Needle Could Be Perfect Places to Find Life
Fortunately for life hunters studying planets outside of our solar system, the researchers say that they believe a viable number of these space bodies will be just the right distance from their host to “thread the celestial needles necessary to await discovery and closer scrutiny” and avoid being engulfed while also keeping enough liquid water to support life as we know it. That’s because some planets undergo something called “tidal migration,” which could move a planet from a distance safe enough to survive the star’s expansion to around 1% of an AU. Such a move would allow enough heat energy to keep the water that remains on the planet in liquid form, increasing the chances for life.
“A planet’s orbit changing is pretty normal,” Becker says. “In tidal migration, some dynamical instability between planets in the system puts one of them into a high-eccentricity orbit, like a comet, where it swings in really close to the central body in the system and then far out again.”
If this assumption is correct, researchers would have a number of planets that not only have the potential for life. They would also be much easier to study thanks to the limited life from the burnt-out stars hosting them.
“If you put all these models together, what you see is that it is a perilous journey for the planet and difficult for oceans to survive this process, but it is possible,” says Becker, whose collaborators include Andrew Vanderburg, a Massachusetts Institute of Technology astrophysicist who was recently a UW–Madison professor, and UW–Madison graduate student Joseph Livesey.
Moving forward, the researchers say they hope to learn more about these burnt-out stars and their dynamics. Ideally, they believe this research can help guide astrobiologists to the most ideal white dwarf targets, a crucial advantage due to the severely limited availability of the world’s most advanced telescopes.
“If we find a lot of white dwarfs that are good candidates to host potentially habitable exoplanets, they could be worth the time,” says Becker. “And these theoretical techniques will help us separate the best targets, so we don’t spend too much time on the uninteresting ones.”
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.