British astronomers have identified the oldest star in the Milky Way with a confirmed planetary system around it, born more than ten billion years ago. The team of scientists led by the University of Warwick today published their findings in the Monthly Notices of the Royal Astronomical Society, relating to their study of a faint white dwarf 90 light-years from Earth, together with the remnants of its orbiting planetary system.
A white dwarf is a star that has burned all its fuel and lost its outer layers and is now undergoing a shrinking and cooling process.
Billions of years in the future, the Sun itself will one day suffer such a fate.
During the process of a white dwarf collapse, all orbiting planets will be disrupted and in some cases destroyed, with their debris accumulating on the surface of the decaying star.
The team modeled two unusual white dwarfs spotted by the European Space Agency’s Global Astrometric Interferometer for Astrophysics (GAIA) space observatory.
Both were polluted by planetary debris, with one found to be unusually blue, while the other is the faintest and reddest found in the local galactic neighborhood to date.
Using complex instruments from the European Southern Observatory, scientists have concluded that the ‘red’ star WDJ2147-4035 is around 10.7 billion years old, of which 10.2 billion years have passed to cool as a white dwarf.
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Debris found in the near-pure, high-gravity helium atmosphere turned out to be from an ancient planetary system that survived the star’s evolution into a white dwarf, leading astronomers to conclude that it was the oldest planetary system around a white dwarf yet discovered in the Milky Way.
The second “blue” star, WDJ1922+0233, was only slightly younger than WDJ2147-4035 and was polluted with planetary debris similar in composition to Earth’s continental crust.
Lead author Abbigail Elms, a PhD 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 with planetary bodies similar to Earth.”
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Some 97% of all stars would become white dwarfs, and their ubiquity meant understanding them was very important, she pointed out.
Ms Elms added: ‘Formed from the oldest stars in our galaxy, cool white dwarfs provide insight into the formation and evolution of planetary systems around the oldest stars in the Milky Way.
“We find the oldest stellar remnants of 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 these planets died long before Earth was even formed.”
Astronomers can also use the star’s spectra to determine how quickly the metals that had been identified were sinking into the star’s core, allowing them to look back in time and determine the abundance. of each of these metals in the planetary body of origin.
By comparing these abundances to planetary material found in Earth’s own solar system, it was possible to estimate what these planets would have looked like before the star died and became a white dwarf – although in the case of WDJ2147-4035, this proved difficult. .
Ms Elms explained: ‘The red star WDJ2147-4035 is a mystery because the accreted planetary debris is very rich in lithium and potassium and unlike anything known in our own solar system.
“It is a very interesting white dwarf because its ultra-cold surface temperature, the metals that pollute it, its old age and the fact that it is magnetic, make it extremely rare.”
Professor Pier-Emmanuel Tremblay from the Department of Physics at the University of Warwick said: “When these old stars were formed more than 10 billion years ago, the universe was less rich in metals than it seems. is now, because metals form in evolved stars and gigantic stellar explosions.
“The two observed white dwarfs provide an exciting window into planetary formation in a metal-poor, gas-rich environment that was different from the conditions of solar system formation.”