Almost 100 light-years from Earth, researchers have found the oldest star known to have formed and maintained orbiting planets.
It shouldn’t be much of a surprise to anyone to hear that stars have been around for billions and billions of years. Despite this, some stars are older than the rest. Among these, one can find some of the first stars in our galaxy to have ever harbored planets. New research reports that the corpse of one such star, christened WDJ2147-4035, lies just 90 light years away from Earth.
WDJ2147-4035 began its life as a regular star some 10.7 billion years ago, a short 3 billion years after the Big Bang. Since then, however, it has expended all its hydrogen fuel and extinguished into a cool white dwarf.
While all that is very sad for WDJ2147-4035, researchers are less interested in the star itself, and more in what used to orbit around it. According to a new paper, the white dwarf is one of two bodies of its kind discovered by the European Space Agency’s Gaia galaxy-mapping mission that are heavily polluted by planetary debris.
“The red star WDJ2147-4035 is a mystery as the accreted planetary debris are very lithium- and potassium-rich and unlike anything known in our own solar system,” said Abigail Elms, a PhD student at the University of Warwick in the U.K. and lead author of the paper. “It’s amazing to think that this happened on the scale of 10 billion years, and that those planets died way before Earth was even formed.”
Although this isn’t the first white dwarf found to be accreting rubble from former planets nearby, it is the oldest one, and, as such, it can teach us a lot about the chemical composition of planets that formed 11 billion years ago, some of the earliest in our galaxy and the universe at large.
The progenitor of WDJ2147-4035 was a star more massive than our Sun, but not big enough that it would explode into a supernova upon using up its hydrogen. Instead, it shone for half a million years until about 10.2 billion years ago. It expanded into a red giant close to the end of its life, then vented its outer layers to expose an inert, helium-rich core — it became a white dwarf.
Disruptions in its gravitational field as the star moved through these phases caused some of the orbiting planets to be completely destroyed or disrupted, although some managed to survive the ordeal. This produced large quantities of planetary debris around the star, material that has since been falling into the white dwarf.
The team used measurements of light spectra emissions captured by Gaia, alongside data from the Dark Energy Camera on the Victor M. Blanco Telescope in Chile, and the X-Shooter instrument on the Very Large Telescope also in Chile, to analyze the chemical composition of the red-colored WDJ2147-4035 and WDJ1922+0233, the second white dwarf, which appears blue.
According to the results, there was a surprising level of chemical diversity in the planetary remains swallowed up by these white dwarfs. The Blue-tinted one is polluted with material similar to that of Earth’s continental crust. The red-tinted one is more puzzling, showing high amounts of lithium, potassium, sodium, and some quantity of carbon.
“These metal-polluted stars show that Earth isn’t unique, [that] there are other planetary systems out there with planetary bodies similar to Earth,” said Elms.
For now, the data is still being crunched. But preliminary interpretations show that rocky planets such as Earth were able to form in the Universe’s distant past, despite the fact that heavy elements were less common at the time. It is likely, however, that as those elements continued to be synthesized inside every subsequent generation of stars, rocky planets became more and more common with time.
The paper “Spectral analysis of ultra-cool white dwarfs polluted by planetary debris” has been published in the journal Monthly Notices of the Royal Astronomical Society.