How Often Do ‘Zombie’ Stars Eat Their Own Planets? A Spooky Green Star Provides a Clue.
Planet-eating is surprisingly common among stars, even after death.
Just in time for Halloween, scientists have announced the discovery of a decidedly spooky green star, the first ever found. Spookier still, the object is a white dwarf, the remnant of a star that ran out of fuel and perished—essentially a stellar ghost.
This object isn’t alone in its vibrance. In a new study published in October in the journal RAS Techniques and Instruments, scientists have identified a range of colorful white dwarfs which, despite the name, can come in shades of amber, rose, lime green, glacial blue, and more.
The colors, in this case, often represent the churned-up chemical remains of rocky worlds. “The most exotic colors revealed in this work—such as the green or bright-yellow white dwarfs—are the direct visual aftermath of planetary destruction,” says Lorenzo Spina, an astrophysicist at the Astronomical Observatory of Padua who was not involved with the new study. The discovery of technicolor white dwarfs can also help astronomers answer a macabre question: How often do stars eat their own planets?
Answering that question begins with understanding how a white dwarf forms. When many stars come to the end of their lives, they blast their outer shell off into space to become a resplendent planetary nebula. (Our own sun will meet a similar end.) The leftover core becomes a white dwarf, a roughly Earth-sized object that gradually loses heat until its embers are extinguished.
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When astronomers got their first good look at white dwarfs in the late-1800s and early-1900s, they all appeared to be pallid and frosty-white, hence the name.
“We normally associate a star's color with its temperature,” says Stephen Kane, a planetary astrophysicist at the University of California, Riverside who was not involved with the new study. For example, our sun appears white; hotter stars take on a bluer tincture, while cooler ones appear orange.
Normally, the chemical composition of a star can also subtly alter its color. But white dwarfs aren’t typical stars.
All stars have atmospheres made of charged gas called plasma. But “a white dwarf has an immense surface gravity, so their atmospheres are compressed to extraordinary, liquid-like densities,” says Spina. Some of the coolest white dwarfs have atmospheres as dense as liquid water, which is millions of times denser than the sun’s own.
In these extreme environments, if there’s anything other than helium present in this torrent of stellar plasma—carbon or calcium, for example—light is absorbed more comprehensively. That means more colors are soaked up and removed from the mix, making those thar remain shine more strongly. “This effect can be large enough to alter the overall mixture as our eyes perceive them,” says Mark Hollands, an astronomer at the University of Warwick and one of the authors of the new study.
Unfortunately, white dwarfs are small, faint objects many light-years from Earth, making these color-shifting features difficult to resolve using telescopes alone. Most appear white or off-blue to us, as the hottest white dwarfs are brighter, and therefore easier to spy from Earth.
But Hollands noticed several white dwarfs giving off suspicious mixtures of electromagnetic radiation. He wondered if some of these spectral features meant that some white dwarfs are actually glowing in different hues.
To find out if they were hiding their true colors, Hollands and his team looked at a wide range of white dwarfs. The team then painstakingly converted the electromagnetic spectra of all these white dwarfs into the palette a human eye would see—and, to their delight, found some of them bursting with color.
“There was always a suspicion,” says Hollands. “And the answer to that question was a resounding yes.”
Based on their findings, the team thinks that about one in every 30 white dwarfs in the universe shimmer in unusual tones. Among these stars, one hummed in a stark, salmon-like pink, while several others shone in varying shades of green. But one of the green dwarfs stood out from the pack, erupting with an intense chartreuse light, much brighter than the others.
Those colorful coats owe their existence to some interesting chemistry—encoded in each white dwarf’s electromagnetic radiation signature. “We can perform a kind of geological autopsy on objects we could never otherwise examine directly,” says Kane.
The pink star gets its shine from its own homegrown carbon, which scientists think is welling up from its deep heart. Conversely, the vividly green star had calcium, sodium, and lithium in its atmosphere. That sort of stuff is normally found in asteroids or rocky planets—meaning that, at some stage, this stellar corpse had gotten hungry and turned zombie.
White dwarfs aren’t alone in their planet-munching proclivities. A study in 2024, which looked at pairs of stars born at the same time, concluded that stars of all kinds gobble up their worlds more frequently than previously thought.
Stars can eat planets at any stage of their life, but they may be more prone to it as they start running out of nuclear fuel and reach old age. “Before becoming a white dwarf, a star expands through its giant phases and can engulf nearby planets,” says Kane.
The colorful atmospheres of white dwarfs, like the newly-discovered green one, also indicate something a little disturbing: “Surviving the death of your star apparently does not guarantee long-term survival,” says Kane.
As stars transform into white dwarfs, a huge amount of mass is lost to space, which upsets the gravitational balance of the system. Some planets may fly out into interstellar space. Others—along with lots of asteroids—can be drawn toward the dying but voracious star, whose gravity rips these space rocks to shreds before eating them. Signs of this annihilative cycle can be found all over the galaxy.
“We now think that around a quarter or more white dwarfs contain traces of planetary debris within their atmospheres,” says Hollands.
Ironically, studying these acts of worldly annihilation have helped scientists better understand how planets are made in the first place. It’s difficult to determine the exact compositions of intact planets orbiting alien stars. But that’s not such a problem when white dwarfs consume them before spitting out their remains, allowing astronomers to see their elemental compositions.
“Astronomers use those planetary traces to understand the compositions of the building blocks of planets,” says Hollands. In other words, we are getting clues to the grandest acts of creation, all thanks to incidences of cosmic indigestion.
