April 24, 2012
Supernovae Could Have Had Impact On Life On Earth
New research by a Danish physicist suggests that life might have been influenced by supernovae near the Solar System.
When massive stars reach the end of their life, they explode as supernovae, which can be brighter than an entire galaxy of normal stars.The remnants of these events release vast numbers of high-energy charged particles known as galactic cosmic rays (GCR).
If a supernova is close enough to our Solar System, the enhanced GCR levels can have direct impacts on the atmosphere of Earth.
Professor Henrik Svensmark of the Technical University of Denmark (DTU) looked through 500 million years of geological and astronomical data and considered the proximity of the Sun to supernovae as it moves around in the Milky Way.
As the Sun travels around the galaxy, it encounters newly forming clusters of stars, which are called open clusters.
These stars have a range of ages and sizes, and started with a small proportion of stars massive enough to explode as supernovae.
Svensmark was able to determine how the rate at which supernovae exploded near the Solar System varied over time.
He found that the changing frequency of nearby supernovae seems to have shaped the conditions for life on Earth. Once the Sun and its planets visited regions of enhanced star formation in the Milky Way, life has prospered, according to Svensmark.
"The biosphere seems to contain a reflection of the sky, in that the evolution of life mirrors the evolution of the Galaxy," he wrote in the journal Monthly Notices of the Royal Astronomical Society.
Svensmark said the diversity of life for the past 500 million years can be explained by tectonics affecting the sea-level, as well as variations in the supernova rate. He followed the evolution of a variety of fossils, from invertebrate animals in the sea, to extinct trilobites and ammonites.
He said the variety of fossils tended to be richest when continents drifted apart and sea levels were high and less varied 250 million years ago.
He noticed that most geological periods tend to begin and end with either an upturn or a downturn in the supernova rate.
According to the research, the variety of life is at its greatest when supernovae are plentiful. Svensmark believes the reason for this is that cold climate associated with high supernova rates brings a greater variety of habitats between polar and equatorial regions.
The changes in typical species that define a period could stem from a major change in the astrophysical environment, according to the research.
When supernova rates were high, carbon dioxide was scarce, suggesting that flourishing microbial and plant life in the oceans consumed it to help grow, according to the research.
Svensmark said growth is limited by available nutrients, and cold conditions favor the recycling of the nutrients by vigorously mixing the oceans. He said his analysis suggests that high supernova rates can bring the cold and changeable climate of prolonged glacial episodes.
"When this enquiry into effects of cosmic rays from supernova remnants began 16 years ago, we never imagined that it would lead us so deep into time, or into so many aspects of the Earth's history. The connection to evolution is a culmination of this work," Professor Eigil Friis-Christensen, the director of DTU Space who was not a part of the research, said in a statement.
Image 1: An image of the Pleiades (M45), a famous star cluster about 135 million years old. This age means that any massive stars in the cluster would have exploded as supernovae when ammonites were prominent in the sea. According to Henrik Svensmark, the rate of nearby supernovae strongly influenced the diversity of such marine invertebrates. Credit: NASA, ESA and AURA/Caltech
Image 2: A figure showing the correlation between the rate of nearby supernovae and the diversity of life on Earth. The black curve is the changing rate of supernova explosions in the vicinity of the Solar System over the past 440 million years. The blue curve is the diversity of marine invertebrate animals (number of genera) after subtracting the influence of changing sea-levels. The grey area is an estimate of errors. The scale at the top shows the geological periods (abbreviations for each period are given). Credit: H. Svensmark / DTU Space