The life of the main sequence of a star like the Sun may not end in a supernova like the most massive stars, but it won’t be a quiet affair.
As the star runs out of fuel and becomes unstable, it swells to absolutely enormous size before blowing away its outer material as the core collapses into a small, ultra-dense white dwarf.
For the Sun, this puffy red giant stage could extend to Mars, a process that could destabilize and destroy fairly nearby planets.
We’ve seen white dwarf stars that have planets, suggesting they may survive the process (or form afterwards). But increasingly, scientists are finding that many exoplanets are being devoured by the white dwarf.
We can tell by the “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 devoured by a white dwarf that formed 10.2 billion years ago.
The white dwarf is about 90 light-years from Earth, incredibly small and dim, with an unusual tint redder than any other white dwarf star. A second, exceptionally blue, white dwarf star formed 9 billion years ago. The team found that both stars experience continuous pollution from falling planetary debris.
However, while the red star, named WD J2147-4035, represents the oldest polluted white dwarf discovered to date, the blue star, named WD J1922+0233, is potentially more interesting: elements found in its atmosphere suggest that the star is eating a planet very similar to Earth.
“We find the oldest stellar remnants in the Milky Way that are polluted by once Earth-like planets,” says astrophysicist Abbigail Elms from the University of Warwick in the UK. “It’s amazing to think that this happened on the scale of 10 billion years and that these planets died long before Earth was even formed.”
We can dissect the chemical composition of a star’s atmosphere from the light produced by a star. All wavelengths are not emitted in the same way: some are stronger, others weaker. This is because the elements can absorb and re-emit light, altering the spectrum of light coming out of the star.
It’s not immediately clear which elements are at play, but scientists are increasingly adept at identifying which absorption and emission features on 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 his colleagues subjected the two oddballs to various studies.
Since white dwarf stars are no longer fueled by the fusion of elements in their core, their temperatures slowly decrease at a known rate; by taking the temperatures of the two stars, the researchers were able to assess how long ago they formed from the death of a Sun-like star.
Next, they subjected the spectra of the stars to analyzes to determine their atmospheric compositions. On the red star, they found sodium, lithium, potassium, and possibly carbon. On the blue star, they found sodium, calcium and potassium.
Since white dwarfs are so gravitationally intense, heavy elements like these should disappear inside the white dwarf, beyond detection, very quickly; this suggests that material producing these elements always falls on stars from the debris clouds that surround them.
In the case of WD J2147-4035, the team determined that the pollution was likely the remnants of a planetary system that had orbited the star before it died, survived the stellar agony, and is now slowly, on billion years, falling into the star.
Since the star transformed into a white dwarf over 10 billion years ago, this makes it the oldest known planetary system in the Milky Way (although it is disintegrating and disappearing).
Meanwhile, WD J1922+0233 polluting debris has a composition similar to Earth’s continental crust, suggesting an Earth-like planet orbiting a Sun-like star that lived and died billions of years before. the formation of the solar system.
It’s like a fossil record of the galaxy that can tell us what the planetary systems of the Milky Way looked like in the 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 today since metals form in evolved stars and gigantic explosions stellar,” says astrophysicist Pier-Emmanuel Tremblay of the University of Warwick.
“The two observed white dwarfs provide an exciting window into planetary formation in a metal-poor, gas-rich environment that was different from the conditions of solar system formation.”
The research was published in the Royal Astronomical Society Monthly Notices.