How Silver Was Created in the Cosmos

Neutron star collisions provide the necessary energy and neutron density to create even very heavy elements.

Atomic Exotics: Silver, platinum, and some rare earth metals could have a rather exotic origin. These atoms owe their existence to probably still-unknown atomic giants with more than 260 nuclear building blocks—isotopes that have never been detected before. However, these superheavy atoms could be formed during the collision of neutron stars. According to astronomers who published their findings in “Science,” they decay, forming elements like silver and cobalt.

How heavy can an atom become? And where in the cosmos do such superheavy isotopes originate? It is clear so far that most lighter elements were formed during nuclear fusion in stars or in supernovae. On the other hand, heavy atomic species such as gold, platinum, and rare earth elements can only emerge in extremely energy- and neutron-rich environments, for example, during a neutron star collision. Only there can the so-called r-process take place, in which atoms rapidly accumulate additional nuclear building blocks.

Element Distribution in Old Stars Investigated

Now, for the first time, astronomers have discovered evidence that there are also superheavy atoms in the universe that are completely unknown on Earth. These short-lived, superheavy isotopes contain more than 260 nuclear building blocks—more than any naturally occurring or artificially producible isotope in the laboratory. Ian Roederer and his colleagues from the University of Michigan made this discovery when they closely analyzed the element distribution of 42 old stars in the Milky Way.

The astronomers explain that they chose stars whose compositions were unaffected by other processes and were known to contain heavy elements created in the r-process. For their analysis, they examined the proportions of elements with atomic numbers between 34 and 90 in these stars.

Linking Lighter and Heavier Elements

Surprisingly, two groups of elements were found to be linked in all the examined stars. The proportion of the four elements ruthenium, rhodium, palladium, and silver—atomic numbers 44 to 47—was directly proportional to the proportion of several heavier elements, such as platinum and some rare earth metals with atomic numbers 63 to 78. This correlation was not detectable among elements immediately adjacent in the periodic table.

“These two groups of elements move in a kind of lockstep,” explains co-author Matthew Mumpower from Los Alamos National Laboratory. “Every time nature produces a silver atom, one of these heavier atoms is also created.” Therefore, some of the heavy elements formed in the r-process are somehow linked to this small group of lighter elements.

Radioactive Decay After the R-Process?

The lighter elements marked here in red could each have emerged together with an element marked in blue from the decay of superheavy, short-lived atoms in the cosmos.
The lighter elements marked here in red could each have emerged together with an element marked in blue from the decay of superheavy, short-lived atoms in the cosmos.

But why? According to astronomers, only one explanation is possible: these pairs, each consisting of a lighter and a heavier element, must have originated from the radioactive decay of an even heavier element. “Theoretical models predict that such transuranic atomic nuclei decay into asymmetric fragments, with a lighter and a heavier component,” explain Roederer and his colleagues.

Astronomers have now detected this asymmetrical ratio in old stars as well. According to their analyses, no other known process can explain why silver and other elements always occur in these stars, almost in step with platinum and other heavy elements. Only the radioactive decay of superheavy atomic nuclei formed in the r-process could explain this connection. Both groups of elements would then be the remnants of these decays.

“This is the first evidence of nuclear fission as a cosmic process,” says Mumpower. “It has long been suspected that such decays occur in the cosmos, but until now, no one has been able to prove it.”

Superheavy Atoms with More Than 260 Nuclear Building Blocks

Also intriguing is that for the decay of superheavy isotopes to produce heavy atoms like the rare earth metal europium, the initial atoms must be heavier than anything known so far. The transuranic atomic giants must have mass numbers of at least 260, thus uniting 260 nuclear building blocks and more within themselves.

“The mass number 260 is exciting because we have not found such heavy atomic nuclei anywhere—neither in space nor in nature,” explains Roederer. Even in laboratories or in nuclear weapon tests, such heavy isotopes have not been detected or produced. “Models of nucleosynthesis, however, predict that such transuranic elements can arise in cosmic r-process events when very neutron-rich material is released,” explain the astronomers.

It is fitting that physicists have recently postulated the existence of unknown, superheavy elements in some exotic asteroids. Accordingly, there could be atomic types in space that lie beyond our periodic table of elements. “The possible existence of superheavy atoms in space gives us new ideas about element formation and nuclear fission and about how the rich variety of elements in the universe originated,” says Roederer.

(Science, 2023; doi: 10.1126/science.adf1341)