1. Impact and extinction cycles ↑
Impacts have been associated with mass extinction events since the 1980s, with strong evidence coming from the Chicxulub Crater, which was linked to the extinction of most of the dinosaurs about 65 million years ago.
It’s difficult to determine if either of these events are periodic because both datasets are incomplete, and contain both periodic and non-periodic data. Timescales are often only approximate, and results are biased by the fact that more recent events are easier to identify.
This has led to inconsistent results, with some studies finding no evidence of a periodicity in mass extinctions or impact events, and some reporting periods ranging from 25-30 million years for mass extinction events, and 25-35 million years for impact events in the last 260 million years. This is around the time of the Permian mass extinction, when about 96% of species on Earth were wiped out.
Artist’s impression of an impact event on Earth. Image credit: NASA/Don Davis/Public domain.
2. Modern results ↑
In a recent paper published in Monthly Notices of the Royal Astronomical Society, Professor of Biology Michael R. Rampino and Climate Scientist Ken Caldeira provide further evidence for a 26 million year impact event cycle using data from the Earth Impact Database 2015.
Rampino and Caldeira considered all impact craters from the last 260 million years, with ages that have error bars of less than ±10 million years. They also discounted craters younger than 5 million years in order to prevent the relatively large number of recent craters from skewing the results. This left 37 impact craters.
Rampino and Caldeira then used a circular method of spectral analysis, which ‘wraps‘ the time series in a circle with a circumference equal to the trial period. If a correct period is found, then, in a perfect dataset, all of the data will be in the same place on the circle. If there is no period, then they will be distributed randomly.
Their results show a period of 25.8±0.6 million years. The latest impact event occurred 11.8±1 million years ago, and so if this cycle is correct, there should be another impact event in 14±1.6 million years.
Rampino and Caldeira use the same method to determine a period of 27.0±0.7 million years for mass extinction events in the past 260 years. The latest mass extinction event occurred 16.0±1.3 million years ago, which means we are due another in 11±2 million years.
There are 11 main impact events, and 10 mass extinction events, where six of these appear to correlate.
3. Causes ↑
It is still not known why impact events appear to be cyclical. The most likely suggestions are that they are caused by a massive undetected object in the Solar System, or by the Sun’s movement through the Galaxy.
3.1 Planet X ↑
In the 1980s, it was suggested that impact events occur in cycles because of a massive undetected object in the Solar System that periodically passes close to the Oort Cloud, the sphere of comets that orbit the Sun. This may cause comet showers throughout the Solar system.
This object was first thought to be another star, referred to as ‘Nemesis’ or the ‘Death Star’, which would make the Sun part of a binary system.
The Kuiper Belt is a belt of asteroids that orbits the Sun from beyond Neptune, and so this theory predicts that most impact events are caused by asteroids and not comets. It has been suggested that most large craters are caused by comets, however results have been mixed.
Image credit: NASA/Public domain.
A potential massive planet in the Kuiper Belt is referred to as Planet X. It was first suggested that Planet X would have to be about five times the mass of the Earth, which is about 1/3 the mass of Uranus, the least massive of the outer planets. However, it is now thought that it could be around the same mass as the Earth if it happens to be in the right place.
Astronomer Mike Brown and planetary scientist Konstantin Batygin have recently shown that there might be a planet at the edge of the Kuiper Belt that is around 10 times the mass of the Earth, and astronomers are now looking for evidence of this.
3.2 The Sun’s orbit through the Galaxy ↑
Other early suggestions involved the Sun’s path through the Galaxy. The Sun does not orbit the Galactic centre in a flat plane, the way that planets orbit the Sun. Instead, it’s thought to move up and down, in a wave pattern.
Diagram showing the orbit of the Sun through the Milky Way (approximate and exaggerated for clarity). Image credit: modified by Helen Klus, original image by NASA/CXC/M.Weiss/Public domain.
The Sun takes about 230 million years to orbit the Galaxy, travelling at about 250 km/s (just over half a million miles per hour). It passes through the vertical gravitational centre of the Galactic disc once every 30 million years or so[18a].
The Sun moves up and down because it is pulled by gravity. It is pulled towards the centre of gravity, overshoots slightly, and is then pulled back up or down. It moves about 200 light-years from the centre of the 1000 light-year-wide disc at each maximum or minimum. The Sun passes through the centre about 8 times with every Galactic orbit.
It was first suggested that the Oort Cloud was affected as the Sun moves through the gravitational centre of the Galactic disc. In 1996, Rampino referred to this idea as the Shiva Hypothesis, after the Hindu god of destruction.
One problem with this idea, however, is that scientists think we last moved though the centre of the Galactic disc about 1 million years ago, yet the last impact event was about 11 million years ago.
If the Earth passes through a dark matter disc, then it might capture some of the dark matter. Dark matter could form a ball at the centre of the Earth’s core, causing the core to increase in temperature[18b].
This may lead to an increase in volcanic activity and earthquakes, and could even cause the Earth’s magnetic poles to reverse, so that the North Magnetic Pole becomes the South Magnetic Pole, and vice versa.
There's currently no strong evidence for a dark matter disc around the Milky Way, however Randall and Reece’s predictions can be tested by the ESA's Gaia satellite. Gaia is currently mapping the gravitational field of the Galaxy, and the first set of results should be released later this year.
If a dark matter disc is detected, then it would mean that the geological and biological evolution that has taken place on Earth is directly linked to the distribution of matter in the Galaxy.