Oldest planetary debris in our galaxy found in new study
Astronomers led by the University of Warwick have identified the oldest star in our galaxy collecting debris from orbiting planetesimals, making it one of the oldest rocky and icy planetary systems discovered in the Milky Way.
Their findings were published today (November 5) in the Monthly Notices from the Royal Astronomical Society and conclude that a faint white dwarf at 90 light years of the Earth, as well as the remains of the orbiting planetary system, are more than 10 billion years old.
The fate of most stars, including those like our sun, is to become a white dwarf. A white dwarf is a star that has burned up all of its fuel and shed its outer layers and is now undergoing a process of shrinking and cooling. In the process, all orbiting planets will be disrupted and in some cases destroyed, leaving their debris to grow on the white dwarf’s surface.
For this research, the team of astronomers, led by the University of Warwick, modeled two unusual white dwarfs detected by the European Space Agency’s GAIA space observatory. Both stars have been polluted by planetary debris, with one of them appearing to be unusually blue, while the other is the faintest and reddest yet found in the local galactic environment — the team 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 find out how long it has cooled, the astronomers found that the “red” star WDJ2147-4035 was about 10 .7 billion is years old, of which 10.2 billion years has been spent cooling as a white dwarf.
Spectroscopy involves analyzing the star’s light at different wavelengths, which can detect when elements in the star’s atmosphere absorb light of different colors and help determine which elements are and how much is present. By analyzing the spectrum of WDJ2147-4035, the team discovered the presence of the metals sodium, lithium, potassium and preliminarily detected carbon accreting on the star, making it the oldest metal-contaminated white dwarf discovered to date.
The second “blue” star WDJ1922+0233 is only slightly younger than WDJ2147-4035 and was polluted by planetary debris of similar composition to Earth’s continental crust. The scientific team concluded that the blue color of WDJ1922+0233, despite the cool surface temperature, is caused by the unusual mixed helium-hydrogen atmosphere.
The debris found in the otherwise nearly pure helium and high-gravity atmosphere of the red star WDJ2147-4035 comes from an ancient planetary system that survived the star’s evolution into a white dwarf, leading astronomers to conclude that this is the oldest planetary system discovered around a white dwarf in the Milky Way.
Lead author Abbigail Elms, a Ph.D. student in the Department of Physics at the University of Warwick, said: “These metal-polluted stars show that the 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 so ubiquitous in the universe that they are very important to understand, especially this extremely cool one. Formed from the oldest stars in our galaxy, cool white dwarfs provide information about the formation and evolution of planetary systems around the oldest stars in the Milky Way.”
“We find the oldest stellar remnants in the Milky Way that have been polluted by once Earth-like planets. It’s amazing to think that this happened on a 10-billion-year scale, and those planets died long before Earth even formed.”
Astronomers can also use the star’s spectra to determine how fast those metals sink into the star’s core, allowing them to look back in time and determine how abundant each of those metals was in the original planetary body. By comparing those amounts to astronomical bodies and planetary material found in our own solar system, we can guess what those planets would have looked like before the star died and became a white dwarf — but in the case of WDJ2147-4035, that’s proven challenging.
Abbigail explains: “The red star WDJ2147-4035 is a mystery, as the accumulated planetary debris is very lithium and potassium rich and unlike anything known in our own solar system. This is a very interesting white dwarf because of its ultra-cool surface temperature. , the metals that contaminate it, its age and the fact that it is magnetic makes it extremely rare.”
Professor Pier-Emmanuel Tremblay of the Department of Physics at the University of Warwick said: “When these ancient stars formed more than 10 billion years ago, the universe was less metal-rich than it is today, as metals are formed in evolved stars and giant stellar explosions. The two white dwarfs observed provide an exciting window into planetary formation in a metal-poor and gas-rich environment that was different from the conditions when the solar system was forming.”
Abbigail Elms et al, Spectral analysis of ultracool white dwarfs polluted by planetary debris, Monthly Notices from the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac2908
University of Warwick
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