Supernova may have kicked off solar system

Force of explosion could explain chemical mysteries contained within asteroids

Scientists have found a recipe for cooking the solar system from scratch: take a cold cloud of gas, and set it 15 light-years from an exploding supernova. Stun the cloud with the supernova’s shockwave. Incubate, and watch as the solar system begins to take shape.

New computer simulations support this scenario, which is a plausible recounting of the solar system’s birth, reports a team of scientists in an upcoming issue of the Astrophysical Journal. “With the supernova, you have one triggering event, and you don’t have to invoke a complicated chain of events,” says study author Matthias Gritschneder, an astrophysicist at the University of California, Santa Cruz.

Understanding how the local solar neighborhood grew up is crucial for learning how other planetary systems are born.

Scientists think the sun and surrounding planets were born from a churning disk of gas and dust, but what precisely caused the stuff to condense and form these bodies has been a mystery. Some clues appear in radioactive elements that were injected into and swam around the presolar cloud. Today, they are embedded in objects such as asteroids, and are thought to mark the first solid bodies that emerged after the cloud’s collapse.

One of these elements, aluminum-26, has helped scientists determine that the solar system was born a little more than 4.5 billion years ago. But the aluminum-26 also presents a puzzle: All of it appears to have enriched the cloud within roughly 20,000 years, much faster than most simulations can explain.

Gritschneder and his colleagues think the nearby supernova solves the aluminum-26 puzzle. In their version of events, all the aluminum-26 would have been incorporated within 18,000 years of the shockwave’s collision, which quickly collapsed the cloud and infused it with the radioactive element. The team ruled out other potential solutions, such as solar wind from a nearby star or enrichment occurring from within the cold cloud itself, because the key elements would have been delivered too slowly or in the wrong quantities. “You have to come up with something creative to make it happen fast enough,” Gritschneder says. “We are confident that the clump can sit there, get hit by a supernova, and get enriched quickly.”

Alan Boss, a theoretical astrophysicist at the Carnegie Institution for Science in Washington, D.C., has had the same idea. Boss approached the problem differently, by calculating in three dimensions rather than two, but also concluded that shocking the embryonic solar system would simultaneously trigger the cloud’s collapse and quickly inject the required radioactive elements. “The basic results are the same for both of us, which is a relief,” says Boss, who presented his work on November 8 at the Formation of the First Solids in the Solar System workshop in Kauai, Hawaii.

Getting the same results using different methods supports the supernova shockwave theory, says planetary scientist Fred Ciesla of the University of Chicago. He questions whether scientists have interpreted the 20,000-year time span correctly and points to unresolved issues raised by other radioactive elements, such as iron-60. But even so, Ciesla says, he favors the supernova shock theory over other hypotheses.

“Work like this says something about how stars and planets formed, and whether it’s consistent with the data we have,” Ciesla says. “Once we’ve been able to accumulate enough information, we can start to speculate about how frequently this works in other places in the galaxy.”

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