Heavy elements present at the dawn of the Solar System were produced by a nearby merger of two neutron stars, which unleashed a powerful gamma-ray burst, according to astronomers at the Hebrew University of Jerusalem.
Writing in the journal Nature Physics, Dr Kenta Hotokezaka, Professor Tsvi Piran and Professor Michael Paul show how a significant quantity of plutonium-244 found its way into the early Solar System following the neutron star merger. None of this plutonium-244 remains in the Solar System today, some four-and-a-half billion years later, but its presence can be inferred. With a half-life of only 80 million years, the plutonium-244 quickly decayed into more stable, daughter isotopes of elements such as xeon, which can be found today in meteorites.
Tiny amounts of plutonium are constantly reaching Earth from interstellar space and, over the millennia, have sunk down to the sea-floor. Comparing the abundance of plutonium-244 and its daughter isotopes in the sea floor with the amount needed to produce the xeon found in meteorites, the Jerusalem-based scientists realised that the influx of plutonium-244 at the birth of the Solar System was far greater than the average background abundance found in interstellar space. Instead, it seems that something injected plutonium-244 – and other heavy elements – into the solar nebula that spawned the Sun and planets. That something, say the three astronomers, was the explosion caused by two neutron stars merging to produce a black hole.
“The plutonium-244 implies that a neutron star merger took place not much more than 100 million years before the formation of the Solar System,” Piran tells Astronomy Now.
Neutron stars are the leftover dense cores of massive stars that burn themselves out after a million or so years and explode as supernovae. There is already strong evidence that a supernova exploded nearby just before, or during, the early stages of the formation of the Sun and planets. Coupled with the evidence that colliding neutron stars were also present nearby, it suggests that our Sun formed in a region of dense star formation that had already produced at least one generation of massive stars that were starting to explode just as the Sun was about to be born. Intriguingly, it could have been shockwaves from these supernovae that cajoled dust and gas to begin collapsing into the Sun in the first place.
Neutron star mergers are rare, requiring systems with two massive stars – which themselves are relatively rare compared to lower mass stars – that have both exploded as supernovae. The two neutron stars spiral towards one another over millions of years until they touch and coalesce and, in one of nature’s most energetic explosions, unleash a short but powerful beam of gamma rays, lasting just fractions of a second. Had Earth already existed when the neutron stars merged, the gamma rays would have irradiated the planet and extinguished all life.
As it is, this cataclysmic event may have helped our civilisation. The violent explosions that result from neutron star mergers produce large quantities of the heaviest elements through the ‘r-process’, which sees neutrons captured in large numbers by atoms to create heavier and heavier elements including plutonium and various precious metals. This was witnessed in 2013 by astronomers led by Edo Berger of the Harvard–Smithsonian Center for Astrophysics in Massachusetts who, after observing the afterglow of the short gamma-ray burst GRB 130603B, estimated that it had produced ten times the mass of the Moon in terms of gold alone.
“Gold, platinum, uranium, thorium – these are definitely important for our civilisation,” says Piran.
As for the black hole created by the merger? It would have wandered off long ago, as the Sun and the other stars among which it formed dispersed into the Milky Way Galaxy.