Everything in the Solar System, from the massive hot burning Sun to the blue ice giant Neptune and all asteroids and bits of dust and rock in between and beyond, are made from dead, leftover stardust.
Now, a team of researchers from ETH Zurich, Switzerland and Konkoly Observatory, Hungary have managed to trace some of that material back to ancient red giant stars that existed before the Solar System. They also discovered that Earth contains more of this type of stardust compared to other planets.
Approximately 4.5 billion years ago, a giant interstellar cloud, a jumble of gas, dust, and plasma particles scraped together from the matter in the Milky Way and other galaxies, collapsed under its own gravity. Only a few per cent of this material came from active stars.
“The stardust was like salt and pepper,” said Maria Schönbächler, co-author of the study published in Nature Astronomy and a professor of planetary geochemistry at ETH Zurich. And each planet created from that mixture ended up with different concentrations of that stardust.
Although the amount is minuscule, Schönbächler and her colleagues have managed to detect it by analyzing meteorites using mass spectrometry. These meteorites were forged from old asteroid cores that existed at the start of the Solar System. They discovered a discrepancy in the amount of palladium, a rare silvery heavy metal element that can only be forged via a stellar process known as slow neutron capture.
"The meteorites contained far smaller palladium anomalies than expected," said Mattias Ek, first author of the paper, who is now a postdoc at the University of Bristol but he studied the meteorites during his PhD at ETH Zurich.
Slow neutron capture grows isotopes of elements with nuclei larger than iron by starting with a smaller nucleus that grows by capturing neighboring neutrons. The process is common in stars in their late stages of evolution, like red giants.
The researchers reckon that the palladium in the meteorites was formed in red giants and that some of these particles made their way into the molecular cloud that formed the Solar System. They believe that the meteorites contained less palladium than expected because the substance is more volatile.
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“Palladium is slightly more volatile than the other elements measured. As a result, less of it condensed into dust around these stars, and therefore there is less palladium from stardust in the meteorites we studied” Ek said.
The amount of sprinkled stardust from red giants turns out to be higher in the inner planets compared to the outer planets. Planets further out from Mars are more dominated by elements created in supernova explosions, according to the paper.
When the planets formed, temperatures closer to the Sun were very high,” Schönbächler said. And although red giant stardust can be altered by heat, it’s hardier than the icy grains of dust from other kinds of stars, like the ones that exploded in supernovas. The material from the supernova explosions, therefore, were more volatile and were more likely to be displaced as the Solar System formed.
But the dust from red giants survived stuck around longer. That allowed it to accumulate on Earth, the researchers believed. “This allows us to explain why the Earth has the largest enrichment of stardust from red giant stars compared to other bodies in the Solar System” she added. ®