White dwarfs are 'heavy metal' zombie stars endlessly cannibalizing their dead planetary systems

Planetesimal orbits around a white dwarf. Initially, every planetesimal has a circular, prograde orbit. The kick forms an eccentric debris disk which with prograde (blue) and retrograde orbits (orange).
An illustration showing a planetesimal orbiting a white dwarf. (Image credit: Steven Burrows/Madigan group)

Astronomers may have finally solved the mystery of how superdense dead stars called white dwarfs keep their heavy metal shells fresh — by cannibalizing what remains of their planetary systems. 

Our sun is destined to turn into a white dwarf in around five billion years, after it runs out of the hydrogen that fuels nuclear fusion at its core. The new research therefore may give us a hint about what could happen to the rest of the solar system after this transformation has taken place.

White dwarfs form when stars like the sun die, creating stellar remnants with masses around that of the sun and widths about that of Earth. They are the most common stars in the Milky Way, accounting for 97% of stellar bodies. Yet, despite their commonality in our galaxy, the chemical composition of white dwarfs is a conundrum. That's because the surfaces of these stellar remnants are adorned with elements heavier than helium, which astronomers call "metals." 

Now, a team of scientists has discovered that heavier metals like silicon, magnesium and calcium find their way to the surface of white dwarfs when these zombie stars devour small rocky bodies that orbit them, like comets and asteroids. The researchers also pinpointed the mechanism that white dwarfs use to feed themselves by accreting planetesimals.

Related: Zombie star earns metal scar while chewing its own planets: 'Nothing like this has been seen before'

"The vast majority of planets in the universe will end up orbiting a white dwarf," study team member Ann-Marie Madigan, a professor of astrophysical and planetary sciences at the University of Colorado Boulder, said in a statement.  "It could be that 50% of these systems get eaten by their star, including our own solar system. Now, we have a mechanism to explain why this would happen."

How do white dwarfs stay heavy metal

The initial discovery of heavy metals on the surface of white dwarfs was a perplexing one. That's because, through the collapse of these stars, when they transform from main sequence stars to white dwarfs, heavy metals should sink into the interiors of these stellar remnants. 

"We know that if these heavy metals are present on the surface of the white dwarf, the white dwarf is dense enough that these heavy metals should very quickly sink toward the core," said study team member Tatsuya Akiba, a PhD candidate at the University of Colorado. "So, you shouldn't see any metals on the surface of a white dwarf unless the white dwarf is actively eating something."

This leads to a question: How are these zombie stars snacking on their surroundings in such a way that heavy metals are constantly replenished at their surface? 

Illustration of the magnetic white dwarf WD 0816-310, where astronomers have found a scar imprinted on its surface as a result of having ingested planetary debris. (Image credit: ESO/L. Calçada)

To investigate this, the team created computer simulations that recreated a white dwarf getting the "natal-kick" these remnants receive during their formation as the result of a loss of material in a preferred direction. This alters the motion of the white dwarf and the dynamics of the material surrounding it.

"Simulations help us understand the dynamics of different astrophysical objects,” Akiba said. "So, in this simulation, we throw a bunch of asteroids and comets around the white dwarf, which is significantly bigger, and see how the simulation evolves and which of these asteroids and comets the white dwarf eats."

In 80% of the team's tests, the white dwarf prenatal kick changed the orbits of asteroids and comets out to a distance of 240 times the distance between Earth and the sun. These altered orbits became more elongated and aligned with each other. They also found that 40% of planetesimals that were eaten by a white dwarf had retrograde orbits, meaning they circled around the stellar remnant in a direction opposing its rotation.

The team allowed the simulation to run for 100 million years, finding that the planetesimals close to the white dwarf, at distances around 30 times the distance between Earth and the sun — roughly Neptune's orbital distance — stayed on elongated orbits and began moving as a unit.

"This is something I think is unique about our theory: we can explain why the accretion events are so long-lasting," Madigan said. "While other mechanisms may explain an original accretion event, our simulations with the kick show why it still happens hundreds of millions of years later."

The findings suggest that heavy metals are found on the surfaces of white dwarfs because these zombie stars, like undead creatures from a George Romero movie, are mindless moving forward and continuously consuming whatever is in their path. 

In the future, the team hopes to increase the scale of their simulation to see what happens when objects larger than comets and asteroids, like planets, interact with white dwarfs. 

Until then, these findings reveal what is going on around the most common stars in the Milky Way and serve as a crystal ball to peer at the future of the solar system.

"Planetesimals can give us insight into other solar systems and planetary compositions beyond where we live in our solar region," McIntyre concluded. "White dwarfs aren't just a lens into the past. They're also kind of a lens into the future."

The team's research was published last month in The Astrophysical Journal Letters.

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Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.