One complete orbit around the Milky Way galaxy takes the solar system about 225 million years to complete. So, we go through about four of these cycles above and below the galactic plane during one orbit around the Milky Way. (The International Astronomical Union (IAU) defines the galactic plane as the plane that is contained within the equator of the Milky Way galaxy, with the center of the Milky Way being the origin of this galactic coordinate system.)
(Note: The cycling back and forth between the northern and southern sides of the galactic plane happens due to mass and gravity. When the solar system is on the northern side of the Milky Way plane, the galactic mass located in the southern part of the Milky Way uses its gravity to pull the solar system back down, and, likewise, the northern Milky Way mass pulls the solar system back up when it is on the southern side of the Milky Way.)
Robert Rohde and Richard Muller, of the Lawrence Berkeley National Laboratory (Berkeley, California) studied a massive amount of fossil data that covered an era of over 500 million years. From their research published in a 2005 issue of Nature, they surmise that living things on the Earth have been at their greatest risk of extinction every 62 million years or so for the past 542 million years.
Mikhail Medvedev and Adrian Melott at the University of Kansas (Lawrence, Kansas) linked their research with this 2005 study from Rohde and Muller. They stated that when the solar system is at its maximum distance from the galactic plane (about 230 light-years above the disc) during its travels on the north side of the Milky Way, then the Earth is at its greatest risk from a drop in biodiversity (another way to say mass extinctions).
So, according to their research, living in the south side of the galactic plane of the Milky Way may be safer for humans and all living things here on the Earth.
Medvedev and Melott state that high-energy particles called cosmic rays strike the Earth on their travels from a large cluster of galaxies in the direction of the Virgo constellation. The Milky Way galaxy is moving toward the Virgo constellation in the northerly direction (at a speed of about 200 kilometers per second). So, when the Earth’s solar system is on the north side of the Milky Way’s plane, we are being bombarded by more cosmic rays from the Virgo constellation. Thus, the greatest risk of mass extinctions occurs when the Earth’s solar system has swung as far north of the galactic plane as it can.
These cosmic rays hit and interact with the Earth’s atmosphere, which causes the formation of millions of energetic electrons and other particles. So, the more cosmic rays that hit the Earth, the more that these energetic particles could possibly cause various problems such as changes in weather and climate, damage to deoxyribonucleic acid (DNA) within humans and other animals, and mass extinctions. (DNA contains the genetic material that allows living organisms to develop and function.)
Medvedev and Melott state that mass extinctions may very likely correspond to peaks in cosmic rays when the Earth is at its maximum northerly distance from the galactic plane. By combining Medvedev and Melott’s research in cosmic rays and the solar system’s cycle above and below the Milky Way plane and Rohde and Muller’s fossil research with the 62-million-cycle of mass extinctions, many scientists are looking into the possibility that cosmic rays are responsible for biodiversity cycles, or mass extinctions.
So far, other researchers state that this connection between mass extinctions and cosmic rays has not been clearly established. Therefore, additional research will be performed. In fact, new gamma-ray observatories, which are to be launched into orbit in the near future, may help to decide the question.
Medvedev and Melott presented their results at the recent April meeting of the American Physical Society. They will also be publishing their results in the Astrophysical Journal.