The oldest dead star known to have had a system of rocky planets has been discovered just 90 light-years from Earth and provides insight into the makeup of worlds that formed nearly 11 billion years ago.
The star is a so-called white dwarf, a stellar corpse that ran out of hydrogen in its core. Born like a regular star 10.7 billion years ago (only 3 billion years after the big Bang), the stellar corpse, named WDJ2147-4035, is one of two white dwarfs polluted by planetary debris that were recently discovered in data collected by the European Space Agency. Gaia galaxy mapping mission.
Although they are not the first white dwarfs turned out to be piling up rubble from an apparent planetary demolition derby, they are the oldest and therefore provide an accurate insight into the composition of planets that formed when the universe was less than 3 billion years old.
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In the case of WDJ2147-4035, its progenitor star was more massive than the sunbut not massive enough to explode like a supernova at the end of life. Instead, half a million years after its formation, or about 10.2 billion years ago, the star ran out of hydrogen for nuclear fusion in its core and swelled into a red giant. It then swelled its outer layers to expose its inert helium-rich core – a white dwarf.
With the gravitational fields in flux as the star evolved through its red giant phase, some of the orbiting planets were destroyed or disrupted, while others may have survived intact. Either way, the disturbances resulted in large amounts of orbiting planetary debris that has since fallen onto the white dwarf.
Astronomers led by Abigail Elms, who is a PhD student at the University of Warwick in the UK, used measurements of light spectra from Gaia, the Dark Energy Survey using the dark energy camera on the Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile, and the X-Shooter instrument on the Very Large Telescope also in Chile, to analyze the chemical composition of the red-colored WDJ2147-4035, and the second dwarf white, WDJ1922+0233, which appears in blue.
The results show a surprising diversity of planetary compositions. Blue WDJ1922+0233, which gets its color not from its temperature but from the unusual mixture of gases in its helium-hydrogen atmosphere, is apparently polluted by materials similar in composition to the Earth’s continental crust.
“These metal-polluted stars show that the Earth is not unique, [that] there are other planetary systems with planetary bodies similar to Earth“, said Elms in a statement (opens in a new tab).
The red WDJ2147-4035 is more of a puzzle. It is enriched with lithium, potassium, sodium and an attempt to detect carbon accreting on the white dwarf.
“The red star WDJ2147-4035 is a mystery because the accumulated planetary debris is very rich in lithium and potassium and unlike anything known in our own solar system,” Elms said.
Either way, the findings provide further evidence that rocky planets may have formed in abundance in the distant past, although heavy elements are less common in the universe at that time, since these elements had to be built by each generation of stars.
“It’s amazing to think that this happened on the scale of 10 billion years and that these planets died long before Earth was formed,” Elms said.
The research (opens in a new tab) was published in the November 5 issue of Monthly Notices of the Royal Astronomical Society.
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