Ancient Neutron Star Collision Explains Why There's Gold on Earth

In 2013, NASA scientists observed a flash of light 3.9 billion light years away from the constellation Leo. It was an explosion caused by colliding neutron stars, and the aftermath included an observable radioactive glow. This glow, scientists believe, was the birth of a huge body of heavy metals — elements like gold and platinum. According to a new study, a similar event rocked the galaxy 4.6 billion years ago, and the leftover metals were incorporated into Earth.

Astrophysicists from Columbia University and the University of Florida argue in Nature that this violent collision gave birth to 0.3 percent of our planet’s heaviest metals — including gold, platinum, and uranium. This collision is estimated to have happened about 100 million years before the formation of Earth, just 1,000 light years from the gas cloud that would one day form our Solar System.

“If a comparable event happened today at a similar distance from the solar system, the ensuring radiation could outshine the entire night sky,” co-author and Columbia University researcher Szabolcs Márka, Ph.D., announced Tuesday.

Colliding neutron stars create an unbelievably powerful force.

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Neutron-star mergers are rare; they only occur a few times per million years in the Milky Way. It’s the colliding of two space leviathans — unbelievably dense, city-sized spheres born from the collapse of massive star cores. A growing body of evidence — including the 2013 explosion — indicates that binary neutron-star mergers are the primary origin of Earthy’s heavy elements, even if their production is rare. This is a relatively new idea that’s being slowly supported. For years it was theorized that heavy elements actually had their origin in supernova explosions.

Accordingly, this study’s novelty is that it’s one of the first to consider how an ancient neutron star merger led to a planet seeded with precious metals: Earth. Márka and co-author Imre Bartos, Ph.D., a physicist at the University of Florida, examined this by combining real-life observations and math, comparing the radioactive elements preserved in ancient meteorites with numerical simulations of neutron star mergers at various points in history.

Meteorites are a helpful tool here because the ones forged in the early solar system carry traces of radioactive isotopes that, as they decay, provide information that scientists can use to reconstruct the time they were created. It’s understood that these isotopes also emerged when neutron stars collided and eventually became the part of meteorites that crashed into Earth early in its history. The idea is that, over a billion years of crashing, the elements taking a ride on the meteorites became a part of the planet’s geological process.

By measuring how many elements in the meteorites decayed, the team could calculate when the neutron star collision that caused the elements happened: 4.6 billion years ago.

“Our results address a fundamental quest of humanity: Where did we come from and where are we going?” Márka said. “It is very difficult to describe the tremendous emotions we felt when we realized what we had found and what it means for the future as we search for an explanation of our place in the universe.”

A growing body of evidence indicates that binary neutron-star mergers are the primary origin of heavy elements produced exclusively through rapid neutron capture1,2,3,4 (the ‘r-process’). As neutron-star mergers occur infrequently, their deposition of radioactive isotopes into the pre-solar nebula could have been dominated by a few nearby events. Although short-lived r-process isotopes—with half-lives shorter than 100 million years—are no longer present in the Solar System, their abundances in the early Solar System are known because their daughter products were preserved in high-temperature condensates found in meteorites5. Here we report that abundances of short-lived r-process isotopes in the early Solar System point to their origin in neutron-star mergers, and indicate substantial deposition by a single nearby merger event. By comparing numerical simulations with the early Solar System abundance ratios of actinides produced exclusively through the r-process, we constrain the rate of occurrence of their Galactic production sites to within about 1−100 per million years. This is consistent with observational estimates of neutron-star merger rates6,7,8, but rules out supernovae and stellar sources. We further find that there was probably a single nearby merger that produced much of the curium and a substantial fraction of the plutonium present in the early Solar System. Such an event may have occurred about 300 parsecs away from the pre-solar nebula, approximately 80 million years before the formation of the Solar System.
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