Oldest star in the Milky Way galaxy with planetary system identified

Milky Way: 1.6 million year time-lapse revealed

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British astronomers have identified the oldest star in the Milky Way with a confirmed planetary system around it, born more that ten billion years ago. The team of scientists led by the University of Warwick published their findings today in the Monthly Notices of the Royal Astronomical Society, in respect of their study of a faint white dwarf 90 light years from Earth, as well as the remains of its orbiting planetary system.

A white dwarf is a star which has burned up all of its fuel and shed its outer layers and is now undergoing a process of shrinking and cooling.

Billions of years in the future, the Sun itself will one day suffer such a fate.

During the process of a white dwarf’s collapse, any orbiting planets will be disrupted and in some cases destroyed, with their debris left to accrete onto the surface of the decaying star.

The team modelled 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 of them being found to be unusually blue, while the other is the faintest and reddest found to date in the local galactic neighbourhood.

Using complex instruments at the European Southern Observatory, the scientists concluding the ‘red’ star WDJ2147-4035 was about 10.7 billion years old, of which 10.2 billion years has been spent cooling as a white dwarf.

Debris found in the otherwise nearly pure-helium and high-gravity atmosphere was found to be from an old planetary system which had survived the evolution of the star into a white dwarf, leading the astronomers to conclude that this 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 by planetary debris of a similar composition to the Earth’s continental crust.

Lead author Abbigail Elms, a PhD 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.”

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Some 97 percent of all stars would become white dwarfs, and their sheer ubiquitousness meant they were very important to understand, she stressed.

Ms Elms added: “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 are also able to use the star’s spectra to work out how rapidly the metals which had been identified were sinking into the star’s core, thus permitting them to look back in time and determine how abundant each of those metals were in the original planetary body.

By comparing those abundances to planetary material found in the Earth’s own solar system, it was possible to estimate what those planets would have been like before the star died and became a white dwarf – although in the case of WDJ2147-4035, it has proven challenging.

Ms Elms explained: “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 Department of Physics at the University of Warwick said: “When these old stars formed more than 10 billion years ago, the universe was less metal-rich than it is now, since metals are formed in evolved stars and gigantic stellar explosions.

“The two observed white dwarfs provide an exciting window into planetary formation in a metal poor and gas-rich environment that was different to the conditions when the solar system was formed.”

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