Winds from a central massive hot star cause the expansion of interstellar material, blowing outwards to create the Bubble Nebula. Credit: NASA, ESA, Hubble Heritage Team; CC BY 4.0
As the solar system travels through the Milky Way, our planet and the life it harbors are exposed to a variety of environments. A recent study suggests that a nearby supernova may have played a role in the evolution of life on Earth
The Local Bubble
About 6 million years ago, the solar system wandered into a 1,000 light-year-wide void known as the Local Bubble. Winds from massive stars and ionizing radiation from an estimated 15 supernovae carved out this bubble over the last 15 million years. As the solar system traveled from the bubble’s edge to its current position in the center, at least nine of the bubble-sculpting supernovae exploded, showering the Earth with supernova byproducts and intense radiation.
Evidence of these past explosions is embedded within the Earth’s crust — radioactive isotopes formed only in supernovae have decayed over time in deep-sea sediments. Less conspicuous are the ways cosmic rays from supernovae may have impacted the Earth; high-energy charged particles rained into the solar system, exposing the planet and the life on it to powerful radiation. Studying the Local Bubble and tracing the history of supernova explosions within it will allow researchers to gauge how the local environment has impacted our planet and the life it harbors.
Evidence of these past explosions is embedded within the Earth’s crust — radioactive isotopes formed only in supernovae have decayed over time in deep-sea sediments. Less conspicuous are the ways cosmic rays from supernovae may have impacted the Earth; high-energy charged particles rained into the solar system, exposing the planet and the life on it to powerful radiation. Studying the Local Bubble and tracing the history of supernova explosions within it will allow researchers to gauge how the local environment has impacted our planet and the life it harbors.
Modeling Nearby Supernovae and Cosmic Radiation
Starting with the decay rate of the deep-sea isotopes, scientists estimate peaks in radioactive iron deposits approximately 2–3 and 5–6 million years ago. Using this information and the recent mapping of stellar associations in the Local Bubble, Caitlyn Nojiri (University of California, Santa Cruz) and collaborators modeled the necessary supernova input to produce the level of radioactive material present on Earth. From their modeling, they estimate that the iron peak ~2.5 million years ago can be attributed to a single supernova explosion from either the Upper Centaurus Lupus or Tucana Horologium stellar associations. The iron peak 5–6 million years ago, the authors suggest, arises from the solar system passing through the enriched outer shell of the Local Bubble.
Given the amount of radioactive iron deposited on Earth, the authors predict a powerful supernova progenitor capable of releasing some of the highest-energy cosmic rays in the universe. Through knowing the approximate location of the supernova and modeling its energy output, the authors estimate the amount of cosmic radiation Earth was exposed to from the time the supernova exploded to now. In their model, cosmic-ray radiation varies over time as the supernova evolves, meaning the Earth received a much higher volume of cosmic rays for the first 100,000 years after the explosion.
Given the amount of radioactive iron deposited on Earth, the authors predict a powerful supernova progenitor capable of releasing some of the highest-energy cosmic rays in the universe. Through knowing the approximate location of the supernova and modeling its energy output, the authors estimate the amount of cosmic radiation Earth was exposed to from the time the supernova exploded to now. In their model, cosmic-ray radiation varies over time as the supernova evolves, meaning the Earth received a much higher volume of cosmic rays for the first 100,000 years after the explosion.
Impacts on Life
What does this cosmic-ray exposure mean for life on Earth? Though the exact effects of this radiation are not certain, biological studies have shown that radiation exposure can cause DNA to break, which can accelerate the rate at which genetic mutations and evolutionary changes occur. The authors note a prior study that showed the rate of virus diversification in Lake Tanganyika in Africa accelerated 2–3 million years ago. Though this cannot be definitively attributed to the supernova, the overlapping timeframes are suggestive of cosmic radiation playing a role in the evolution of life on our planet.
This study underscores the importance of considering cosmic radiation when it comes to understanding the environmental factors that drove biological evolution on Earth. Further studies must be performed in order to constrain the threshold at which this radiation goes from driving species diversification to becoming detrimental to life and its evolution. The Local Bubble has left imprints on the solar system and on Earth in ways that astronomers and biologists will continue to uncover.
This study underscores the importance of considering cosmic radiation when it comes to understanding the environmental factors that drove biological evolution on Earth. Further studies must be performed in order to constrain the threshold at which this radiation goes from driving species diversification to becoming detrimental to life and its evolution. The Local Bubble has left imprints on the solar system and on Earth in ways that astronomers and biologists will continue to uncover.
By Lexi Gault
Citation
“Life in the Bubble: How a Nearby Supernova Left Ephemeral Footprints on the Cosmic-Ray Spectrum and Indelible Imprints on Life,” Caitlyn Nojiri et al 2025 ApJL 979 L18. doi: 10.3847/2041-8213/ada27a
