Scientists Locate Neutron Star Collision That Could Have Created Our Solar System's Plutonium

Artist’s depiction of colliding neutron stars (Illustration: ESA)

In 2017, observatories around the world observed a high-energy collision between a pair of dense objects, each slightly more massive than the Sun but only the size of a city. A similar collision closer to home could have been responsible for producing some of the heaviest elements in our own solar system — and scientists think they know when it happened.

Scientists now think that these binary neutron star mergers are an important source of elements heavier than iron in the universe. These elements are rare, but they’re also some of the most important elements to us humans. Using measurements of what’s left of these elements in ancient meteorites, a pair of researchers worked backward to locate the neutron star merger that produced some of them.

“We discovered this binary star merger two years ago, and it was close to the Milky Way — much closer than we anticipated,” Imre Bartos, the study’s first author and assistant professor at the University of Florida, told Gizmodo. “We asked whether something even closer... could have a significant impact in what the solar system looks like today.”

Elements heavier than iron form in part thanks to the “r-process,” where some high-energy event causes seed atomic nuclei to quickly suck up a lot of neutrons. Once the event slows down, some of these neutrons radioactively decay into protons. Stellar explosions called supernovae and neutron star mergers have both been implicated as potential sources of the r-process elements.

First, the researchers set out to see whether neutron star mergers or supernovae produced the elements they were interested in, mainly curium and plutonium. Supernovae, in which stars explode, happen relatively frequently, while neutron stars only merge perhaps a few times every million years in our galaxy, according to the paper published in Nature. That means that, if you look back in time, abundances of these elements should spike if they were produced by neutron stars, or stay relatively constant if they were produced by supernovae.

Plutonium and curium are radioactive, and decay into more stable elements. When the earliest meteorites formed, they captured some of these elements, which then decayed into more stable elements. The relative abundances of the decay products in these meteorites allow scientists to backtrack and determine the approximate age when the initial elements formed.

When Bartos and Columbia University professor Szalbocs Marka performed calculations on previously collected data from these meteorites, they found that the abundances of these elements spiked approximately 80 million years before the solar system formed, when it was just a cloud of gas and dust. The inferred that a single event, probably a neutron star merger a thousand light-years away, produced the lion’s share of the curium and perhaps a third of the plutonium in the solar system.

This amounts to only a fraction of a per cent of the total amount of r-process elements in the solar system, but “there have been many neutron star mergers in the history of the Milky Way,” Bartos said.

It’s cool research. “[These elements] are a tiny fraction of 1 per cent of the universe, but they’re highly useful to us in many ways,” David Helfand, an astronomer and professor at Columbia University, told Gizmodo. “Just knowing where they came from helps us feel a little bit more at home in the universe.”

It’s important to note that these results are based on modelling of indirect measurements, and our knowledge of neutron star collisions and the r-process comes from just one experimental observation. Though unlikely, perhaps another kind of even more chaotic high-energy event produced these elements. Bartos told Gizmodo that the next step is to measure more elements with unknown abundances, create better simulations, and of course, to observe more neutron star collisions.

Fortunately, the LIGO and Virgo gravitational wave observatories have both been upgraded and have already started detecting signals from colliding black holes and perhaps even some neutron stars.

Bartos was excited about how these results combine so many different fields, from geoscience to astrophysics to chemistry. “By connecting this field in this particular work, we hope we’re starting a bigger effort to use this information in unison.”

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