A team of astronomers led by the University of Warwick has identified the oldest star in the galaxy that is accumulating debris from surrounding planets.
The discovery, announced on November 5 in the monthly notice of the Royal Astronomical Society, concludes that a faint white dwarf star located 90 light-years from Earth, along with the remnants of its planetary system, is over 10 billion years old. This is one of the oldest rocky and icy planetary systems discovered in the Milky Way.
Most stars, including those similar to the Sun, end their lives as white dwarfs. A white dwarf is a star that has exhausted its fuel, undergoing a process of contraction and cooling. During this process, any orbiting planets will be disrupted and, in some cases, destroyed, with debris accumulating on the surface of the white dwarf.
The white dwarfs WDJ2147-4035 and WDJ1922 + 0233 are surrounded by orbiting planetary debris.
In this new study, the team of astronomers modeled two unusual white dwarfs discovered by the European Space Agency’s GAIA space observatory. Both stars are contaminated by planetary debris. One of them was found to have an unusual blue color, while the other star is faint and red.
Using spectroscopic and astrometric data from GAIA to determine the cooling ages of the stars, the astronomers discovered that the “red” star WDJ2147-4035 is about 10.7 billion years old, with 10.2 billion years spent cooling as a white dwarf. By analyzing the spectrum from WDJ2147-4035, the research team found sodium, lithium, potassium, and carbon metals accumulating on the star, making it the oldest metal-polluted white dwarf discovered to date.
The second “blue” star WDJ1922 + 0233 is only slightly younger than WDJ2147-4035 and is contaminated by planetary debris with a composition similar to Earth’s continental crust. The scientific team concluded that the blue color of WDJ1922 + 0233 is due to its unusual helium-hydrogen mixed atmosphere.
The debris found in the nearly pure atmosphere of star WDJ2147-4035 comes from an ancient planetary system that existed after this star evolved into a white dwarf. Astronomers concluded that this is the oldest star with a planetary system orbiting a white dwarf discovered in the Milky Way.
The lead author, Abbigail Elms from the Department of Physics at the University of Warwick, stated: “These metal-polluted stars show that Earth is not unique; there are other planetary systems out there with planetary bodies similar to Earth. 97% of all stars will become white dwarfs, and they are found throughout the universe, making it crucial to study them. They provide information about the formation and evolution of planetary systems around the oldest stars in the Milky Way.”
Astronomers can also use the spectrum of the star to determine how quickly those metals sank into the star’s core. This allows them to look back in time to ascertain the abundance of each metal in the original planetary body.
“By comparing that abundance with the bodies and materials found in the solar system, we can infer what those planets might have been like before the star died and became a white dwarf. It’s astonishing to think that this occurred over a scale of 10 billion years, and those planets died before Earth was formed,” Abbigail Elms remarked.
According to Abbigail, the red star WDJ2147-4035 is a mystery because the accumulated planetary debris is rich in lithium and potassium—unlike anything known in the solar system. This is a very intriguing white dwarf due to its extremely cool surface temperature. Additionally, the list of contaminating metals, its age, and the fact that WDJ2147-4035 is magnetic have made it extremely rare.
Professor Pier-Emmanuel Tremblay from the Department of Physics at the University of Warwick stated: “When these stars formed over 10 billion years ago, the universe was less metal-rich than it is now because metals are formed in evolving stars and supernova explosions. The two observed white dwarfs provide an exciting opportunity to study planet formation in a metal-poor environment rich in gas, different from the conditions when the solar system formed.”
