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Astronomers have identified the oldest remnants of planets in the Milky Way and they are relatively close, only 90 light-years from Earth.
The researchers used observations of white dwarfs to identify those “polluted” by bits of planets and other debris, and found one that has been around for about 10.7 billion years. That’s more than twice the age of our own Earth..
A white dwarf is a late stage in the evolution of stars like our Sun. Once they can no longer fuse hydrogen, the stars contract and ignite helium, forcing them to expand into a red giant star. Over time, these stars lose their loose outer layers, and if they aren’t massive enough to go supernova, their cores shrink into white dwarfs.
Planets that used to orbit the original star, before these dramatic transformations, often break up. This planetary debris can fall on the white dwarf, changing its chemistry in a way that astronomers can measure.
“We are finding the oldest stellar remnants in the Milky Way that are contaminated by planets that were once similar to Earth. It is amazing to think that this happened on a scale of 10 billion years, and that those planets died long before the Earth was formed, ”he said in a release lead author Abbigail Elms, a graduate research fellow at the University of Warwick.
The team identified the “red” white dwarf WDJ2147-4035, estimated to be 10.7 billion years old, which has spent 10.2 billion years as a white dwarf. Among the heavy elements present in this star were sodium, lithium, and potassium, as well as potential carbon. The object is truly puzzling, and not just because it’s contaminated.
“The red star WDJ2147-4035 is a mystery as the accumulated planetary debris is very rich in lithium and potassium and is unlike anything known in our own solar system. This is a very interesting white dwarf because of its ultra-cool surface temperature,” Elms continued.
The team also identified a second contaminated white dwarf, the “blue” star WDJ1922+0233. Both stars are the coolest and most polluted white dwarfs to date. The composition of the planetary debris from this second object is much more in line with Earth’s continental crust. The white dwarf appears blue due to a peculiar mix of helium and hydrogen in its atmosphere.
More evidence of ancient planets, similar in composition to Earth, makes us wonder how likely it is that life evolved on such worlds. In the particular case of WDJ2147-4035, things seem to have moved too quickly from star to white dwarf, but perhaps there are other worlds that have survived.
The study was published in Monthly Notices of the Royal Astronomical Society.
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