Madison Reporter

Watery planets orbiting dead stars may be good candidates for studying life — if they can survive long enough

June 13, 2024 By Chris Barncard For news media More information

The small footprint and dim light of white dwarfs, remnants of stars that have burned through their fuel, may make excellent backdrops for studying planets with enough water to harbor life. The challenge lies in spotting the shadow of a planet against a former star that has shrunk significantly and ensuring it has retained its water oceans for billions of years after surviving the star’s explosive final stages. A new study on the dynamics of white dwarf systems suggests that some watery planets might indeed survive these conditions, making them viable candidates for discovery and further examination.

Astronomers searching for signs of life on exoplanets gather data during the planets' transit across their star. They analyze the light from the star passing through the planet's atmosphere to determine which elements and molecules are present. Observing a planet orbiting a smaller, cooler white dwarf simplifies this process.

“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,” says University of Wisconsin–Madison astronomy professor Juliette Becker, lead author of the study under review at AAS Journals. “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.”

The first major challenge for such a planet is surviving the final days of its host star. When stars like our sun exhaust their core’s fusion fuel, they expand dramatically.

“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.”

Even if a water-harboring planet avoids being swallowed by its expanding star, it faces further threats from increased brightness and mass loss.

“The fact that the star gets so much brighter means that all planets in the system...will suddenly have their surface temperatures increase drastically,” Becker says. “That can evaporate their oceans and cost them a lot of water.”

To retain an appreciable amount of water through these phases, an Earth-like planet needs to be situated at least roughly 5 to 6 astronomical units away from its dying star.

However, as the once-rampaging star cools over billions of years into a white dwarf, another issue arises: maintaining liquid water.

“If you can be sufficiently far away during this dangerous time...that’s good,” Becker notes. “But...you’re going to be so far away from the star that all the water is going to be ice.”

Eventually, as the white dwarf becomes very small and cold, maintaining liquid water requires closer proximity — around 1% of 1 astronomical unit away from the white dwarf.

One potential solution involves tidal migration.

“A planet’s orbit changing is pretty normal,” Becker states. “In tidal migration...it swings in really close to [the central body]...and then far out again.”

Such orbits may stabilize close enough to support liquid water near a white dwarf.

“If you put all these models together...it is possible,” says Becker. Her collaborators include Andrew Vanderburg from MIT and UW–Madison graduate student Joseph Livesey.

Further research on potential white dwarf-planet pairings could refine these models and optimize telescope resource allocation for finding habitable exoplanets.

“If we find a lot of white dwarfs...they could be worth [the search],” concludes Becker. “And these theoretical techniques will help us separate [promising] targets.”