Plutonium is one of the most dangerous elements we have ever synthesized in the lab. It’s radioactive, it’s toxic, and it's also easily flammable. It was first produced and isolated in 1940, and since then we have discovered that it can also be found in small quantities in nature. And we now know why.
Researchers from the Hebrew University of Jerusalem have proposed an astrophysical process behind the formation of plutonium. The team thinks that neutron star mergers are responsible for producing heavy elements like gold, uranium, and also plutonium.
When small, compact binary neutron stars merge, they generate a kilonova – an intense and short-lived supernova-like event believed to cause strong gravitational waves and powerful gamma-ray bursts. The kilonova also produces a lot of heavy elements from cobalt to plutonium.
In their letter published in Nature Physics, the scientists compared the amount of plutonium currently on Earth with the expected abundance in the early Solar System. In doing so, they discovered a large discrepancy in values: The Solar System had 1,000 times more plutonium when it formed than nowadays.
Plutonium is a radioactive element. It emits gamma rays and other particles and turns into different elements over its lifetime. The most stable version of plutonium is plutonium-244, which has a half-life of about 80 million years. This means that if you have a sample of that plutonium after 80 million years, only half of it will be left.
Eighty million years is a very short duration in astronomical time, so without mergers plutonium quickly disappears. The study indicated that there is a neutron star collision every 20 million years and binary mergers most likely happened near the location of the Solar System 100 million years before it formed.
The current level of plutonium was estimated by analyzing interstellar matter that fell on Earth, which over time has concentrated at the bottom of the ocean. The estimated value for it is that there’s one atom of plutonium for every 10 cubic kilometers (2.4 cubic miles) of space in the Milky Way.