Odd elements in supernova blast might have implications for alien life

Hidden inside Cassiopeia A, the youngest known exploded star in our galaxy, astronomers have discovered surprisingly high levels of chlorine and potassium. These elements carry an odd number of protons in their atomic nuclei, and although they are believed to be less abundant in the universe, they are essential for the formation of planets and living systems. It means, Cassiopeia A This discovery could impact where alien life might be found in the Milky Way.

Exploded stars – supernova remnants – contain many elements such as oxygen and magnesium, with an even number of protons in their cores. Nuclei with an odd number of protons—belonging to the “odd Z” elements—are inherently less stable and therefore less likely to form during stellar fusion. This is reflected in models of the chemical evolution of our galaxy, which typically predict very low levels of odd-Z elements.

“[As a result] The origin of these odd-Z elements has long been unclear,” says Kai Matsunaga at Kyoto University, Japan.

Matsunaga and his colleagues realized that high-resolution X-ray spectroscopy could be a step toward solving the mystery. In the intense heat of a supernova remnant, atoms lose electrons and give off distinct X-ray signatures that can be picked up by a sensitive detector. X-ray Imaging and Spectroscopy Mission (XRISM), launched in September 2023, is a fairly sensitive detector and observed Cassiopeia A twice in December 2023.

To estimate how much of each element was present, the researchers compared weak signals from odd-Z elements with stronger signals from even-Z elements such as sulfur and argon, using them as stable guides to obtain more accurate readings of odd-Z elements.

The results show that supernova Cassiopeia A produced much more chlorine and potassium than standard models predict. This suggests that theorists may need to rethink how massive stars produce these rare elements, as some widely used models do not fit the specific conditions of Cassiopeia A.

“Although the authors emphasize that their observations contradict previous models, the picture is more nuanced,” says Stan Woosley from the University of California, Santa Cruz, who was not involved in the study. “It's not that all our models are wrong. Some work better than others, and some agree quite well with each other. The important thing is that these observations give astronomers new, specific information to improve their models and better understand what happens when a massive star explodes.”

The new measurements also allowed Matsunaga and his colleagues to begin testing some long-held theories about how odd-Z elements might form in massive stars—through stellar rotation, interactions between pairs of binary stars, or the merging of different burning layers deep within a star. Until now, there has been no way to test these theories on real data.

“We still don't have a complete understanding of what type of stars contributed [this] galactic inventory,” says Katharina Lodders from Washington University in St. Louis, Missouri, who was not involved in the study. “Especially the origin of chlorine, an element that is abundant in our oceans.”

If these findings hold true for other supernova remnants, they could also change our understanding of the distribution of vital elements throughout the Milky Way. Some regions may be better endowed with the ingredients for life than others, depending on which stars seed their planets. This may indicate that any alien life is unevenly distributed in our galaxy.

“It’s certainly possible,” adds Matsunaga, “but we can’t say for sure based on the current results.” It is unclear whether Cassiopeia A is an oddity producing such a large number of odd-Z elements, or whether it is even a member of supernova remnants, he said. “Future observations of other supernova remnants using XRISM or future instruments will be critical to resolving this question.”

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