White dwarfs are “heavy metal” zombie stars that cannibalize their dead planetary systems

Astronomers may have finally solved the mystery of how super-dense dead stars called white dwarfs keep their heavy metal shells fresh – by cannibalizing the remnants of their planetary systems.

Our sun will turn into a white dwarf in about five billion years when it runs out of hydrogen, which powers nuclear fusion in its core. The new research could therefore give us a clue as to what might happen to the rest of the solar system after this transformation takes place.

White dwarfs form when stars like the Sun die and stellar remnants are formed with masses on the order of the Sun and widths on the order of the Earth. They are the most common stars in the Milky Way, making up 97% of stellar bodies. But despite their commonality in our galaxy, the chemical composition of white dwarfs remains a mystery. That’s because the surfaces of these stellar remnants are decorated with elements heavier than helium, which astronomers call “metals.”

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

Related: Zombie star gets metal scar while chewing his own planet: ‘You’ve never seen anything like it’

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

The first discovery of heavy metals on the surface of white dwarfs was confusing. Because of the collapse of these stars, when they transform from main sequence stars into white dwarfs, heavy metals should sink into the interior of these stellar remnants.

“We know that if these heavy metals are present on the surface of the white dwarf, the white dwarf is so dense that these heavy metals should sink toward the core very quickly,” said Tatsuya Akiba, a member of the study team and a graduate student at the University of Colorado. “So you shouldn’t see metals on the surface of a white dwarf unless the white dwarf is actively eating something.”

This leads to a question: How do these zombie stars eat their environment in such a way that their surface is constantly replenished with heavy metals?

Illustration of a white dwarf star with magnetic field lines represented by purple-white loops

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

To investigate this, the team created computer simulations that recreated a white dwarf receiving the “birth kick” that these remnants received during their formation due to a loss of material in a preferred direction. This changes the motion of the white dwarf and the dynamics of the matter surrounding it.

“Simulations help us understand the dynamics of various astrophysical objects,” Akiba said. “So in this simulation we throw a series of asteroids and comets around the white dwarf, which is significantly larger, and see how the simulation develops and which ones the white dwarf eats.”

In 80% of the team’s tests, the white dwarf’s prenatal kick changed the orbits of asteroids and comets to a distance equal to 240 times the distance between Earth and the Sun. These changed orbits became longer and aligned with each other. They also found that 40% of the planetesimals eaten by a white dwarf had retrograde orbits, meaning they orbited the stellar remnant in a direction opposite to its rotation.

The team ran the simulation for 100 million years and found that the planetesimals remained in extended orbits near the white dwarf, at distances of about 30 times the distance between Earth and the Sun – roughly the orbital distance of Neptune – and began to move as a unit.

“That’s what I think is unique about our theory: we can explain why the accretion events last so long,” Madigan said. “While other mechanisms could explain a primordial accretion event, our simulations have the kicker to show why it is still happening hundreds of millions of years later.”

The results suggest that heavy metals are found on the surfaces of white dwarfs because, like undead creatures from a George Romero film, these zombie stars advance mindlessly, constantly consuming everything in their path.

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

Until then, these findings reveal what’s going on around the Milky Way’s most common stars and serve as a crystal ball for a glimpse into the solar system’s future.

“Planetesimals can give us insights into other solar systems and planetary compositions beyond where we live in our solar region,” McIntyre concluded. “White dwarfs are not just a lens into the past. They are also a kind of lens into the future.”

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

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