Researchers have found evidence for at least two, and probably several more, supernova explosions near Earth over the last 8 million years. Some of these may have contributed to the cooling of the planet, triggering recent ice ages.
Most of the universe's heavy elements are the result of supernova explosions, which fuse atomic nuclei and disperse them across surrounding regions. The products include radioactive isotopes that could not originate from other sources, as any that were part of the Earth's formation have long since decayed.
One such isotope is iron-60 (Fe-60), which has a half-life of 2.6 million years. To seek the presence of Fe-60 deposited at the time, Dr. Anton Wallner of the Australian National University studied samples taken from the ocean floor.
In Nature, Dr. Wallner reports on the findings of two bursts of Fe-60, indicating nearby supernova explosions. The first of these occurred approximately 8 million years ago. Wallner told IFLScience that limitations on measuring the deposits' ages made it hard to be precise on the timing of this event, but it appears to be a single explosion of the sort expected to occur every now and then.
The second increase in Fe-60 is more intriguing however, with traces found in deposits from 3.2 to 1.7 million years old. The period is far longer than would be expected for the residue of a single supernova. While this might be due to an explosion that ejected Fe-60 across a large volume of space, with the Solar System traveling through this and picking up traces as it went, the possibility is remote. The substances would need to be traveling at a substantial speed relative to the Solar System in order to overcome the solar wind and make it to Earth. Consequently, Wallner said, "we can't exclude this possibility, but it is unlikely."
Far more probable is that several supernovae exploded over this period. Wallner admitted that the resolution of his measurements were insufficient to determine how many of these events occurred. However, the density of the Fe-60 observed led Wallner to conclude that the supernovae occurred 160 to 380 light-years away. The timing of this iron enrichment coincides with the planetary cooling between the Pliocene to the Pleistocene eras, which may have been triggered by additional cosmic rays that led to to increased cloud cover.
Multiple explosions so close in place and time are unlikely to be a coincidence. Wallner has proposed that a star cluster produced multiple giant stars of similar size and age, all of which were doomed to explode around the same time. In the same edition of Nature, another team modeled the evolution of a nearby cluster, finding it was once filled with large stars that have now exploded, leaving behind stars too small to become supernovae.
Wallner's previous work has challenged the theory that plutonium is formed in ordinary supernovae, but he told IFLScience that unlike heavier isotopes that may require even more powerful, and rarer, events, Fe-60 can be formed in ordinary supernovae.