Patterns in the high-energy particles that strike Earth suggest that two million years ago, a supernova exploded in Earth's astronomical backyard. The event was not close enough to trigger a mass extinction, but early human ancestors probably witnessed a star that briefly outshone the full moon.
Michael Kachelriess of the Norwegian University of Science and Technology is a professor who explores anomalies in cosmic rays – high-energy particles that strike the atmosphere from beyond the Solar System. Most cosmic rays are protons and atomic nuclei, but antimatter particles are also present. Among the particles scientists detected using the PAMELA and AMS-2 satellite detectors, there were more positrons (the antimatter equivalent of electrons) with energies above 30 billion electron volts than would be expected given the number of particles of other types and energies.
While their origins are debated, most of the cosmic rays in our part of the galaxy (known as the local flux) are thought to originate from supernova explosions.
“The local cosmic ray flux might still have some 'memory' of the individual sources composing it,” Kachelriess and co-authors write in Physical Review Letters.
The traces lie in the energies of different types of particles and antiparticles. When a supernova explodes, it releases particles with energies up to 1015 electron volts, far beyond our capacity to produce.
Higher energy particles move faster, so a spike at certain energy levels indicates an event at a particular distance in space and time. Putting the data together is challenging, because the antimatter particles are thought to result from even higher energy particles interacting with the interstellar medium, rather than coming from the supernova directly. Nevertheless, the paper argues the evidence points strongly to an origin 2 million years ago: “An older source would be inconsistent with the absence of radiative cooling in the positron spectrum. A younger source would fail to produce sufficient amount of antimatter.”
The source appears to have been too short lived to be a star formation “superbubble,” the other leading contender for cosmic ray origins, leaving the authors to argue a supernova as “the only plausible model.”
The results are consistent with another paper Kachelriess co-authored in the Astrophysical Journal Letters earlier this year, which proposed a two-million-year-old supernova located around 600 light-years away to explain the imbalances in the directions from which cosmic rays are coming from. The galaxy's magnetic field shapes the direction in which these particles travel, allowing Kachelriess to calculate a broad direction for the explosion.
“The blast wave of such an explosion would strongly damage life,” Kachelriess told IFLScience, "possibly leading to a mass extinction on habitable planets within a distance of ten light years.” At a distance of 100-200 light-years, a supernova might produce enough gamma rays to cause a spike in mutations, but further out we would not expect a noticeable change in the fossil record.
“Complementary evidence for a recent nearby supernova explosion is contained in the deep ocean crust of the Earth,” Kachelreiss told IFLScience. “A previously found anomalously high amount of rare iron isotopes in a 2-million-year-old layer of the ocean crust might be ejecta from the same supernova producing the puzzling features we observe in cosmic rays.”
The Milky Way hosts a supernova more than once a century, but most are too far away to leave vestiges even as subtle as these.