Astronomers led by the University of Warwick have identified the oldest star in our galaxy that is accreting debris from orbiting planetesimals, making it one of the Milky Way’s oldest rocky and icy planetary systems discovered. Their findings, published in the Monthly Notices of the Royal Astronomical Society, conclude that a faint white dwarf 90 light years from Earth, as well as the remnants of its orbiting planetary system, is more than ten billion years old.
The fate of most stars, including our Sun, is to become white dwarfs. A white dwarf is a star that has used up all of its fuel and shed its outer layers, and is now shrinking and cooling. During this process, any orbiting planets will be disrupted and in some cases destroyed, with their debris left to accrete onto the surface of the white dwarf.
For this study, an astronomical team led by the University of Warwick modeled two unusual white dwarfs discovered by the European Space Agency’s space observatory GAIA. Both stars have been polluted by planetary debris, with one being found to be unusually blue and the other being the faintest and reddest found to date in the local galactic neighborhood – the team has subjected both to further analysis.
Using spectroscopic and photometric data from GAIA, the Dark Energy Survey, and the European Southern Observatory’s X-Shooter instrument to calculate how long it has been cooling, the astronomers discovered that the ‘red’ star WDJ2147-4035 is around 10.7 billion years old, with 10.2 billion years spent cooling as a white dwarf.
We’re finding the oldest stellar remnants in the Milky Way that are polluted by once Earth-like planets. It’s amazing to think that this happened on a scale of ten billion years and that those planets died way before the Earth was even formed.
Spectroscopy involves analyzing the light from the star at different wavelengths, which can detect when elements in the star’s atmosphere are absorbing light at different colors and helps determine what elements those are and how much is present. By analyzing the spectrum from WDJ2147-4035, the team found the presence of the metals sodium, lithium, potassium and tentatively detected carbon accreting onto the star-making this the oldest metal-polluted white dwarf discovered so far.
The second ‘blue’ star, WDJ1922+0233, is only slightly younger than WDJ2147-4035 and has been contaminated by planetary debris of similar composition to the Earth’s continental crust. The scientists concluded that, despite its cool surface temperature, WDJ1922+blue 0233’s color is caused by its unusual mixed helium-hydrogen atmosphere.
The debris found in the otherwise nearly pure-helium and high-gravity atmosphere of the red star WDJ2147-4035 is from an old planetary system that survived the star’s evolution into a white dwarf, leading astronomers to conclude that this is the Milky Way’s oldest planetary system discovered around a white dwarf.
Lead author Abbigail Elms, a Ph.D. student in the University of Warwick Department of Physics, said: “These metal-polluted stars show that Earth isn’t unique, there are other planetary systems out there with planetary bodies similar to the Earth. 97% of all stars will become white dwarf and they’re so ubiquitous around the universe that they are very important to understand, especially these extremely cool ones. Formed from the oldest stars in our galaxy, cool white dwarfs provide information on the formation and evolution of planetary systems around the oldest stars in the Milky Way.”
“We’re finding the oldest stellar remnants in the Milky Way that are polluted by once Earth-like planets. It’s amazing to think that this happened on the scale of ten billion years, and that those planets died way before the Earth was even formed.”
Astronomers can also use the spectra of a star to determine how quickly those metals sink into the star’s core, allowing them to go back in time and determine how abundant each of those metals was in the original planetary body. We can guess what those planets would have been like before the star died and became a white dwarf by comparing those abundances to astronomical bodies and planetary material found in our own solar system – but in the case of WDJ2147-4035, that has proven difficult.
Abbigail explains: “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. This is a very interesting white dwarf as its ultra-cool surface temperature, the metals polluting it, its old age, and the fact that it is magnetic, makes it extremely rare.
Professor Pier-Emmanuel Tremblay of the University of Warwick’s Department of Physics stated: “Because metals are formed in evolved stars and massive stellar explosions, the universe was less metal-rich when these old stars formed more than 10 billion years ago. The two discovered white dwarfs offer an exciting glimpse into planetary formation in a metal-poor and gas-rich environment that differed from the conditions that formed the solar system.”
The European Research Council, the Leverhulme Trust Grant, and the UK STFC consolidated grant all contributed to this study, which was funded by the European Union’s Horizon 2020 research and innovation program.