Plutonium is a rare element on Earth, and recent research suggests that half of the universe’s supply of it comes from a single event: a neutron star merger. However, plutonium is not the only element to come out of this process; these mergers help to create many other actinides, or elements with an atomic number of 89 or higher. Most elements start as hydrogen, and following the death of a star turn into heavier gases like helium before turning into iron.
Stars start out comprised almost completely of hydrogen. At the center, fusion turns the hydrogen into helium. Once the hydrogen runs out, the helium turns into heavier and heavier elements until the star's core turns into iron. The process leading up to the formation of the iron core releases more energy than it consumes, yet after iron, it uses more energy than it releases, which leads to the star shutting down. This helps to explain the formation of elements with atomic numbers leading up to iron’s, but where did all the other heavier ones come from?
Heavier stars end their lives in a violent explosion called a supernova, and these can create many elements heavier than iron. Still, this leaves about 40 elements that are not accounted for. This is where neutron stars come in. Neutron star mergers create the remaining elements through rapid neutron capture.
Rapid neutron capture works a little like this: take, for example, an iron atom, which has 26 protons and 30 neutrons. The number of protons determines the element, while the number of neutrons keeps the protons “glued” to the nucleus and determine its mass. During rapid neutron capture, the iron atom experiences a heavy neutron bombardment, which causes the iron nucleus to accumulate neutrons. When the number of neutrons reaches 32, one of the neutrons emits an electron to turn into a proton. Now the iron nucleus becomes a cobalt nucleus. This process repeats and continues to form all the naturally occurring elements, but only if there is a large source of neutrons to bombard the atom.
Large quantities of neutrons seem to come mostly from neutron star mergers, however, to date only one has been observed by scientists. Plutonium is a naturally-forming element with the highest atomic number, and given the low number of neutron star mergers that contributed to actinide abundance, we could have ended up in a solar system that had little to no uranium or plutonium.