Scientists from the Konkoly Thege Miklós Astronomical Institute of the ELKH Research Centre for Astronomy and Earth Sciences (CSFK), in collaboration with researchers from the University of Hertfordshire in the UK, used sophisticated computer models to examine the interstellar path of short-lived radioactive isotopes, previously identified in deep-sea sedimentary rocks. The aim of the research was to understand how these diverse group of isotopes could arrive at the Solar System and appear in ocean sediments. .According to the study, the newly formed matter produced at and ejected via various astrophysical events, such as neutron star collisions and white dwarf explosions, is driven through the galaxy by the shock waves of core collapse supernovae. The new findings, published in The Astrophysical Journal, will also help to investigate which planets outside the solar system are most likely to support life.
Many elements around us were produced either through stellar explosions called supernovae, or violent collisions of extremely dense objects called neutron stars. One of the questions puzzling scientists was how these heavy elements then reach us here on Earth – and in particular, how elements that originate in different places seem to have reached our planet at the same time.
Radioactive nuclei are produced and ejected by colliding neutron stars. In such violent events, there are enough neutrons to produce even the most massive nuclei like plutonium-244. These isotopes make it to Earth by surfing the shock waves of core-collapse supernovae, scientists found.
(Credit: ESO/L. Calçada/M. Kornmesser)
Using sophisticated computer modelling of the journey of the elements through space, scientists have now found that the heavy elements produced in collisions of neutron stars can ‘surf’ on blast waves of other supernovae across our Galaxy and down to Earth.
The mystery was first raised in 2021 when radioactive isotopes discovered ins deep-sea sedimentary rocks.. The isotopes did not originate inside our Solar System, but in explosions of stars elsewhere in the Galaxy. Some of the detected isotopes especially raised eyebrows in the research community due to their very different production sites.Specifically, scientists found manganese-53, which is associated with explosions of white dwarfs; iron-60, produced in core-collapse supernovae; and plutonium-244, which can usually only be produced by merging two extreme objects called neutron stars, among the densest objects in the Universe, in layers of similar depth in the deep-sea samples.
To reach Earth, these isotopes would have rained down from the sky at some point during the last couple of million years. Since deep-sea sediments accumulate layer by layer over time to form rocks, researchers were very puzzled by the fact that these three isotopes, produced in different types of stellar explosions, were found in rock layers of similar depth. Finding them at the same depths means that they must have arrived on Earth together, even though their places of origin are so vastly different.
To understand how it was possible for these isotopes to arrive on Earth together, a team led by Dr Benjamin Wehmeyer at the CSFK in Hungary, and the University of Hertfordshire in the UK, used computer models to simulate how the isotopes travel from their Galactic production sites throughout space.
The study found that the ejected matter from different astrophysical sites – from colliding neutron stars to exploding white dwarfs – are pushed around in the Galaxy by the shock waves of the much more frequent core-collapse supernovae. These supernovae are explosions of the cores of massive stars, which are much more common than explosions triggered by the merging of two neutron stars or explosions of white dwarfs.
Dr Wehmeyer and his team observed that after they are produced, the isotopes can then “surf” on the shockwaves of these supernovae. This means that isotopes produced in very different sites can end up travelling together on the edges of the shock waves of core-collapse supernova explosions. Some of this swept-up material ends up on Earth, which may explain why the isotopes were found together within similar layers of deep-sea rocks.
Lead author Dr Wehmeyer explained: “Our colleagues have dug up rock samples from the ocean floor, dissolved them, put them in an accelerator, and examined the changes in their isotopic composition layer by layer. Using our computer models, we were able to interpret their data to find out how exactly atoms move throughout the Galaxy. It’s a very important step forward, as it not only shows us how isotopes propagate through the Galaxy, but also how they become abundant on exoplanets – that is, planets beyond the Solar System. This is extremely exciting, since isotopic abundances are a strong factor determining whether an exoplanet is able to hold liquid water – which is key to life. In the future, this might help to identify regions in our Galaxy where we could find habitable exoplanets”.
Dr Chiaki Kobayashi, Professor of Astrophysics at the University of Hertfordshire and co-author of the study, adds: "I have been working on the origins of stable elements in the periodic table for many years, but I am thrilled to achieve results on radioactive isotopes in this paper. Their abundance can be measured by gamma-ray telescopes in space as well as by digging the rocks underwater of the Earth. By comparing these measurements with Benjamin's models, we can learn so much about how and where the composition of the solar system comes from”.