About the same time the Sun formed, at least two supernovas exploded in our near neighborhood, new research suggests. The event seeded the gas cloud from which planets were condensing with rare elements. However, there is a twist to this tale. The atoms whose presence could be used to prove these events have long since decayed. To reveal what happened scientists needed to find their by-products.
The universe's heavier elements were mostly produced in supernovae, or the mergers of neutron stars, both of which disperse elements into nearby gas clouds, where they become concentrated in rocky planets close to the forming star.
Most elements, however, don't tell us much about whether the explosions from which they came took place just as the Sun was forming, or billions of years earlier. Niobium-92 is an exception, leading Professor Maria Schönbächler of ETC Zurich to use it to test the question.
Niobium-92 has a half-life of 37 million years before it decays to zirconium-92, a very handy time-period for certain purposes. Isotopes with half-lives measured in days or even seconds would be almost entirely gone by the time they'd crossed the space from the supernova to reach the newly forming Sun. Those that live much longer would be present in similar abundance whether an explosion was a short or long time before. Halving in abundance over a period of tens of millions of years is an ideal middle ground.
Schönbächler's problem, however, is that in the 4.6 billion years since the Earth formed, virtually all the niobium-92 has turned to zirconium. She solved this problem by examining rutile and zircon crystals from meteorites, which incorporate large and small amounts of niobium respectively when forming.
As the niobium-92 turned into zirconium, Schönbächler was left with a handy record of how much niobium-92 was present at their formation, and therefore its general abundance. To reveal the conditions at the Solar System's birth it wouldn't do to use just any random meteorite, however. Schönbächler required specimens knocked off the asteroid Vesta in a collision dating 4,525 million years ago for her measurements. The precise timing of this event means this rare class, known as mesosiderites, can serve as relics of the Solar System's origins.
In Proceedings of the National Academy of Sciences, Schönbächler and colleagues use crystal comparisons from four mesosiderites to conclude the early Solar System was enriched with the products of two different types of recent supernovas.
There is no method, at least currently, to distinguish the amount of niobium provided by one close supernova or several at somewhat greater distances. However, different types of supernovas produce somewhat different ratios of isotopes. The early Solar System, the team reports, displayed the fingerprints of both Type Ia and core collapse supernova, both recent enough that plenty of the radioactive elements had survived. They posit the inner Solar System, populated with rocky planets like Earth and Mars, was largely influenced by material ejected from a Type Ia supernova in our galaxy. The outer Solar System was instead fed by a core-collapse supernova, where a massive star collapsed on itself and went supernova, likely in the same stellar nursery our Sun was born in.
Supernovas are suspected of having exploded close enough to Earth to leave detectible iron traces on the ocean floor in the last 8 million years. Nevertheless, today they are so rare in our galactic neighborhood that multiple nearby explosions in a short period would be unlikely today. However, the Sun, like most stars, was probably formed in a dense cluster, with many stars quite nearby, the largest of which would have evolved rapidly to become supernovas of one type or the other.