A new study by Manuel Barrientos and colleagues from the University of Oklahoma reveals that between 0.6% and 2.5% of white dwarfs in our solar neighbourhood undergo dramatic cooling delays that could extend habitable zones for billions of additional years.
The secret lies in an element known as neon-22, which after carbon and oxygen, is the most abundant element inside white dwarfs.
When white dwarfs contain at least 2.5% neon-22 by mass, they undergo a process called “distillation” as their cores crystallise.
The researcher team discovered this occurs because the solid crystals become depleted in neon-22 compared to the surrounding liquid, making them lighter and causes them to float upward where they melt.
This astronomical equivalent of a lava lamp releases enormous amounts of gravitational energy, effectively putting the white dwarf’s cooling on pause for up to 10 billion years.
The neon-22 forms during the star’s lifetime through a well understood process. During the helium burning stage, nitrogen-14 (produced by the CNO cycle) transforms into neon-22.
This means stars with higher initial abundances of carbon, nitrogen, and oxygen (collectively called “metallicity”) produce more neon-22 in their white dwarf descendants.
To test this theory, the research team analysed approximately 4,000 stars from the Hypatia catalog, which contains high resolution spectroscopic measurements of nearby stars within 500 parsecs of the Sun.
Using the MESA stellar evolution code, they modelled how much neon-22 each star would produce in its white dwarf remnant.
The team’s predictions align remarkably with observations from the European Space Agency’s Gaia satellite, which detected an unusual clustering of white dwarfs on the “Q-branch” in stellar brightness diagrams.
About 6% of massive white dwarfs appear to have paused their cooling for up to 10 billion years, creating what the researchers describe as a “cosmic traffic jam.”
Additional evidence comes from stellar velocities. Stars in the Q-branch move faster than expected for their apparent age, indicating they’re actually much older than their brightness suggests, a telltale sign of the cooling delay.
Perhaps most intriguingly, the research reveals a clear galactic pattern. Using a comprehensive model of Milky Way stellar populations, the team found that distilled white dwarfs are most common near the galactic centre (7.6% within 2 kiloparsecs) and decline steadily toward the outer disk (1.0% at 8-10 kiloparsecs from the centre).
This gradient reflects the underlying chemistry of our Galaxy; metal rich stars that can produce neon enhanced white dwarfs are more abundant in the inner regions. The finding suggests that long lived habitable zones around white dwarfs should be most common in the inner Milky Way.
The discovery has profound implications for astrobiology. White dwarfs undergoing neon distillation can maintain habitable zones for dramatically longer periods than previously thought, and these zones are located farther from the star, reducing the destructive effects of tidal forces on any orbiting planets.
Some surveys have found suspiciously high numbers of massive white dwarfs in the solar neighbourhood, which rather goes against the new model.
The team suggests this apparent overabundance may actually be an observational bias with distilled white dwarfs remaining more luminous for extended periods and are therefore preferentially detected in magnitude limited surveys, leading to their over representation.
This research changes our understanding of where life might exist in the universe. While white dwarfs were once considered astronomical dead ends, they may actually represent some of the most stable, long term habitable environments.
With billions of white dwarfs in our Galaxy alone, and a significant fraction potentially harbouring these extended habitable zones, the universe may offer far more opportunities for life than we ever imagined, hidden in plain sight among the Galaxy’s most common stellar remnants.
Written by Mark Thompson/Universe Today.
