What happens to a star's planets after it dies? Those planets are presently being ripped apart and consumed by that star, if it is a specific white dwarf 86 light-years distant, like some macabre cosmic performance of Kronos devouring his children.
This isn't unheard of among white dwarfs. G238-44, on the other hand, is a glutton: scientists have witnessed one of these stars ingest material from both the inner and outer limits of its planetary system at the same time for the first time, in the most far-reaching demonstration of stellar filial cannibalism seen to yet.
Astronomers have discovered signs of components in G238-atmosphere 44's that indicate the dead star had recently accreted metallic and rocky material, such as asteroids from the inner Solar System, as well as ice material, such as the frozen bodies seen in the Kuiper Belt.
"We have never seen both of these kinds of objects accreting onto a white dwarf at the same time," said University of California Los Angeles physicist and astronomer Ted Johnson. "By studying these white dwarfs, we hope to gain a better understanding of planetary systems that are still intact."
When an ordinary star with a mass of up to eight times that of the Sun approaches the end of its existence, it becomes a white dwarf. When a star runs out of material to fuse, it blows out to the size of a red giant before ejecting its outer material, and the stellar core compresses under gravity to create a dense structure that shines brilliantly with leftover heat light. That's the white dwarf you're looking at.
Although this process appears to be harsh for the planets circling the star – the Sun may blow out large enough to swallow Mars when it reaches red giant o'clock in a few billion years – scientists have lately discovered evidence that some portions of planetary systems may be able to withstand it.
White dwarfs have been discovered to have exoplanets around them. Then there's necroplanetology, which examines the remnants of white dwarf exoplanets for indications of heavy metals that "polluting" white dwarf atmospheres.
Because white dwarfs are so dense (imagine the mass of the Sun packed into a spherical the size of Earth), heavy elements should fall out of sight very quickly, implying that any heavy element pollution in the atmosphere of a white dwarf must have been deposited lately.
This is amazing news because it implies we now have an indirect probe into the innards of exoplanets. We know what Earth is composed of, and we're quite sure we know what other Solar System planets are made of to some extent, but exoplanets circling far away stars are hard to examine in the same way that Earth or even other Solar System planets are.
Probing the innards of exoplanets eaten by white dwarfs will help scientists decide whether exoplanetary interiors are different, too, because other planetary systems discovered so far appear to be extremely unlike the Solar System in many respects. This brings us full round to G238-44.
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| Diagram illustrating what scientists think is happening around G238-44 |
Johnson and his colleagues discovered that the contamination in this white dwarf's atmosphere is unlike anything they've ever seen before. Carbon, nitrogen, oxygen, magnesium, aluminum, silicon, phosphorus, sulfur, calcium, and iron were found to be heavier than helium.
The abundances of iron and nitrogen were notably high; the former implies a body with a distinct iron core, while the latter indicated the presence of frozen bodies, according to the study.
"The best fit for our data was a nearly two-to-one mix of Mercury-like material and comet-like material, which is made up of ice and dust," Johnson explained. "Iron metal and nitrogen ice each suggest wildly different conditions of planetary formation. There is no known solar system object with so much of both."
The findings also show that the components needed to create a livable world aren't that uncommon in the Milky Way galaxy. Earth is a rocky planet that is assumed to have been seeded with life-sustaining materials such as water by asteroid bombardment. The discovery of nitrogen-rich material might indicate the presence of frozen stores of these elements.
"Life as we know it requires a rocky planet covered with a variety of volatile elements like carbon, nitrogen and oxygen," said UCLA physicist and astronomer Benjamin Zuckerman.
"The abundances of the elements we see on this white dwarf appear to have come from both a rocky parent body and a volatile-rich parent body – the first example we've found among studies of hundreds of white dwarfs."
In fact, aliens looking at the Sun from afar would expect to observe something similar until it has developed into a white dwarf in roughly 5 billion years. Although the growing white dwarfs may melt the inner Solar System's objects, the asteroid belt between Mars and Jupiter may survive, only to be agitated by a destabilized Jupiter and shower down on the dead star.
