Fireworks marked the solar system's birth, and we have found echoes of one of the blasts. The oldest known minerals in the solar system show unusual chemical traces that indicate a supernova explosion happened nearby within a million years of their formation. That blast could even have been the one that triggered the birth of our sun.
Most of the universe is filled with light elements like hydrogen and helium. The heavier stuff that makes up planets is forged inside stars or created when they explode as supernovae. But how such events affected our solar system is unclear.
Gregory Brennecka of Lawrence Livermore National Laboratory in California and his colleagues looked inside meteorites to find metal-rich bits of mineral, called inclusions, that formed about 4.5 billion years ago, before the planets congealed. The inclusions didn't have as many heavy isotopes as younger rocks from Earth, Mars or the moon. That means a supernova must have sprinkled heavier stuff on the solar system sometime between the formation of inclusions and the birth of planets.
"Because we see these systematic differences between the first solids and everything else, it gives us a time window for when the supernova would have been active," says Brennecka. "It tells us that in the neighbourhood we were growing up in, there were a lot of violent events happening."
Richard Carlson of the Carnegie Institution of Washington, who was not involved in the work, thinks this could be evidence of the event that formed the sun. The leading theory is that a supernova shock wave caused a cloud of cold gas and dust to collapse into itself, making the matter dense enough to eventually trigger fusion. Over time, any leftover material from the sun's creation would have formed a swirling disc around the newborn star, which coalesced and gave rise to planets.
A supernova's shock wave might have arrived first to form the sun, and the physical debris could have then trickled into our neighbourhood after the first solids took shape, says Carlson. "This material looks like it's coming in at the same time that the planets are forming, and suggests the event plays a role in kick-starting the whole system," says Carlson. "Without this supernova, we wouldn't have Earth."
Brennecka takes a more cautious approach. The length of the time lag between a supernova shock wave and the arrival of its debris is not known for sure, so it may instead be that several nearby star explosions helped shape the solar system. "But if you can do it all with one supernova, that may be plausible," he says.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1307759110
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