The Supernova that Shaped the Solar System
Traces of the supernova’s pollution can be found in meteorites in the form of short-lived radioactive isotopes, or SLRIs. SLRIs -- versions of elements with the same number of protons, but a different number of neutrons -- found in primitive meteorites decay on time scales of millions of years and turn into different, so-called daughter, elements. A million years may sound like a long time, but it is actually considered short when compared to other radioactive isotopes studied by geochemists and cosmochemists, which have half-lives measured in billions of years.
When scientists find the daughter elements distributed in telltale patterns in primitive meteorites, this means that the parent SLRIs had to be created just before the meteorites themselves were formed. This presents a timing problem, as the SLRIs must be formed in a supernova, injected into the presolar cloud, and trapped inside the meteoritic precursors, all in less than a million years.
One of these SLRIs, iron-60, is only created in significant amounts by nuclear reactions in massive stars. The iron-60 must have come from a supernova, or from a giant star called an AGB star. Boss and Keiser’s previous modeling showed that it was likely that a supernova triggered our solar system’s formation, as AGB star shocks are too thick to inject the iron-60 into the cloud. Supernova shocks are hundreds of times thinner, leading to more efficient injection.
Now Boss and Keiser have extended those models to 3-D, so they can see the shock wave striking the gas cloud, compressing it and forming a parabolic shock front that envelopes the cloud, creating finger-like indentations in the cloud’s surface. The fingers inject the SLRI pollution from the supernova. Less than 0.1 million years later, the cloud collapses and forms the core of the protostar that became the Sun and its surrounding planets. The 3-D models show that only one or two fingers are likely to have caused the SLRI pollution found in primitive meteorites.
“The evidence leads us to believe that a supernova was indeed the culprit,” said Boss. However, more detective work needs to be done: Boss and Keiser still need to find the combination of cloud and shock wave parameters that will line up perfectly with observations of exploding supernovae.