Radioactive elements in the early solar system

An international team of scientists have found new clues about the development of our solar system, after examining radioactive elements in another star-forming region of the galaxy. The study, which used observations of the Ophiuchus star-forming complex, has allowed the astronomers to conclude that nearby supernovas are the most likely source of these elements in our solar system.

“Our solar system was most likely formed in a giant molecular cloud together with a young stellar cluster, and one or more supernova events from some massive stars in this cluster contaminated the gas which turned into the sun and its planetary system,” explains Douglas Lin, professor emeritus of astronomy and astrophysics at the University of California, US, and co-author on a paper describing the research, published in Nature Astronomy.

“Although this scenario has been suggested in the past, the strength of this paper is to use multi-wavelength observations and a sophisticated statistical analysis to deduce a quantitative measurement of the model’s likelihood.”

Three images of the ophiuchus region, zoomed in at three different scales
Multi-wavelength observations of the Ophiuchus star-forming region. The top image shows aluminium-26 concentration in red. The bottom left image zooms in on the central box in the top image, showing protostars in red. The bottom right image is another zoom-in, showing a near-infrared composite image of the star-forming complex. Credit: Forbes et al., Nature Astronomy 2021

This research used observations taken at a range of different wavelengths of light – from infra-red waves through to gamma rays – to examine the Ophiuchus region, where stars are currently forming.

“The enrichment process we’re seeing in Ophiuchus is consistent with what happened during the formation of the solar system 5 billion years ago,” says John Forbes, a researcher at the Flatiron Institute’s Center for Computational Astrophysics, in the US, and first author on the paper.

“Once we saw this nice example of how the process might happen, we set about trying to model the nearby star cluster that produced the radionuclides we see today in gamma rays.”

Meteorites that are known to be pristine remnants of our early solar system have products of certain short-lived radioactive elements, like aluminium-26. While astronomers have known about these elements since the 1970s, it hasn’t been entirely clear how they got into our solar system while it was forming – they could have come from nearby supernovas, or possibly from more distant massive stars called Wolf-Rayet stars.

“We now have enough information to say that there is a 59% chance it is due to supernovas and a 68% chance that it’s from multiple sources and not just one supernova,” says Forbes.

The model they developed also predicts that there will be huge variation in the amount of aluminium-26 (and other short-lived radioactive elements) between different star systems.

“Many new star systems will be born with aluminum-26 abundances in line with our solar system, but the variation is huge – several orders of magnitude,” says Forbes.

“This matters for the early evolution of planetary systems, since aluminum-26 is the main early heating source. More aluminum-26 probably means drier planets.”

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