Astronomers find oldest planetary debris in our Milky Way galaxy

A new study led by the University of Warwick has identified the oldest star in the Milky Way galaxy that is accreting debris from orbiting planetesimals, making it one of the oldest rocky and icy planetary systems discovered in our home galaxy.

The study concluded that a faint white dwarf located 90 light years from Earth, as well as the remains of its orbiting planetary system, are over ten billion years old. For the unversed, a white dwarf is what intermediate stars like our Sun become after they have exhausted their nuclear fuel.

The star shed its outer layers and is now undergoing a process of 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, astronomers modeled two unusual white dwarfs that were detected by the European Space Agency's Gaia observatory. Both stars are polluted by planetary debris, with one of them being found to be unusually blue, while the other is the faintest and reddest found to date in the local galactic neighbourhood, according to the team which subjected both of them to further analysis.

Using Gaia data, the Dark Energy Survey and the X-Shooter instrument at the European Southern Observatory (ESO), the team analyzed the stars and found that the 'red' star WDJ2147-4035 is around 10.7 billion years old, of which 10.2 billion years has been spent cooling as a white dwarf. They found the presence of the metals like sodium, lithium, potassium and tentatively detected carbon accreting onto the star, making this the oldest metal-polluted white dwarf discovered so far.

The blue star (WDJ1922+0233), on the other hand, is only slightly younger than the red and was polluted by planetary debris of a similar composition to the Earth's continental crust, with the team concluding that its blue color is caused by its unusual mixed helium-hydrogen atmosphere.

The debris found in the atmosphere of WDJ2147-4035 are from an old planetary system that survived the evolution of the star into a white dwarf, leading the team to conclude that this is the oldest planetary system around a white dwarf discovered in our galaxy.

"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," said lead author Abbigail Elms, a PhD student in the University of Warwick Department of Physics