The main sequence life of a star like the Sun may not end in a supernova like the most massive stars out there, but it won’t be a quiet affair.
As the star runs out of fuel and becomes unstable, it swells to an absolutely massive size before blowing away its outer material, the core collapsing into a small, extremely dense white dwarf.
For the Sun, this inflated red giant stage could extend all the way to Mars, a process that could destabilize and destroy planets close enough.
We have seen white dwarf stars that have planets, suggesting that they can survive the process (or form after it). But more and more, scientists are discovering that many exoplanets are being eaten by the white dwarf.
We can tell because of “pollution” by planetary elements in the atmospheres of white dwarf stars, the study of which is known as necroplanetology.
And now, astronomers have discovered the oldest known example: An exoplanet swallowed by a white dwarf that formed 10.2 billion years ago.
The white dwarf is about 90 light-years from Earth, incredibly small and faint, an unusual shade redder than any other white dwarf star. A second white dwarf star, unusually blue, formed 9 billion years ago. Both stars, the team found, experience constant pollution from falling planetary debris.
But while the red star, called WD J2147-4035, represents the oldest polluted white dwarf discovered to date, the blue star, called WD J1922+0233, is potentially more interesting: Evidence found in its atmosphere suggests that the star is eating a planet very similar to Earth.
“We are finding the oldest stellar remnants in our Galaxy that have been contaminated by once-Earth-like planets,” says astrophysicist Abbigail Elms of the University of Warwick in the UK. “It’s amazing to think that this happened on a scale of 10 billion years, and that these planets died long before Earth even formed.”
We can dissect the chemical composition of a star’s atmosphere from the light produced by a star. Not all wavelengths are emitted equally: some are stronger, some are weaker. This is because the elements can absorb and re-emit light, changing the spectrum of light emitted by the star.
It’s not immediately obvious which elements are at play, but scientists are increasingly adept at identifying which absorption and emission features in a spectrum are associated with which elements.
When the European Space Agency’s Gaia space observatory identified the two unusually colored white dwarfs, Elms and her colleagues subjected the two strange balls to various studies.
Since white dwarf stars are no longer fueled by the fusion of elements in their cores, their temperatures slowly decrease at a known rate. by measuring the temperatures of the two stars, the researchers were able to gauge how long it had been since they formed from the death of a Sun-like star.
They then subjected the stars’ spectra to analyzes to determine their atmospheric composition. In the red star they found sodium, lithium, potassium and possibly carbon. In the blue star they found sodium, calcium and potassium.
Since white dwarfs are so gravitationally intense, heavy elements like these should disappear into the white dwarf’s interior, beyond detection, very quickly. This suggests that the material that produces these elements is still falling onto the stars from the debris clouds around them.
In the case of WD J2147-4035, the team found that the debris was likely the remnants of a planetary system that had orbited the star before it died, survived stellar death, and is now slowly, over billions of years, falling into the star.
Since the star turned into a white dwarf more than 10 billion years ago, this makes it the oldest known planetary system in our Galaxy (although it is decaying and disappearing).
Meanwhile, the debris littering WD J1922+0233 has a similar composition to Earth’s continental crust, suggesting an Earth-like planet orbiting a Sun-like star that lived and died billions of years before the Solar System formed.
It’s like a galactic fossil record that can tell us what the planetary systems in the Galaxy were like eons before we arrived here to marvel at its wonders.
“When these old stars formed more than 10 billion years ago, the universe was less rich in metals than it is now, as metals form in evolved stars and giant starbursts,” says astrophysicist Pier-Emmanuel Tremblay of the University of Warwick.
“The two observed white dwarfs provide a fascinating window into planet formation in a metal-poor, gas-rich environment that was different from the conditions when the Solar System formed.”
The research has been published in Monthly Notices of the Royal Astronomical Society.
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