While many people argue about how and where the human race will end, no one doubts the root cause and catalyst of the last largest extinction on Earth: a massive, large body from outer space collided with Earth. About 65 million years ago, an asteroid 5-10 kilometers in diameter fell on the site of what is now the Gulf of Mexico and destroyed 30-50% of the species, ending the dinosaurs. Is another similar event expected in the near future?
So, there is a theory that every 26-30 million years our galactic disk displaces comets in the Oort cloud, resulting in periodic extinctions and comet bombardments on Earth. How about us? Is there such a threat to us, and is this theory itself valid?
To be honest, there is always a danger of mass extinction, but it is especially important to quantify this danger.
Extinction threats in our solar system - from cosmic bombardments - typically come from two sources: the asteroid belt between Mars and Jupiter and the Kuiper belt and the Oort cloud outside Neptune's orbit. In the case of the asteroid belt, suspected (but not certain) of killing dinosaurs, our chances of colliding with a large object diminish over time. And here's the reason: the amount of matter between Mars and Jupiter decreases over time, and there is no mechanism to replenish it. This becomes clear when we look at different things: the young solar system, models of our own solar system, and most airless worlds without particularly active geology, like the Moon, Mercury, most of the moons of Jupiter and Saturn.
We see, for example, the history of the cratering of the Moon when we look at it. Where the spots are lighter - lunar heights - we see a long history of heavy cratering, which stretches back to the early days of the solar system, 4 billion years ago. There are many large craters with smaller craters inside: this is indicative of the increased asteroid activity at that time. But if you look at the dark regions (lunar seas), there aren't many craters inside. Radiometric dating has shown that most of these areas are 3–3.5 billion years old, and the number of craters on them is different from others. The youngest regions found in Oceanus Procellarum (the largest sea on the Moon) are only 1.2 billion years old and have the least crater spots.
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It follows from all this that the asteroid belt has become more and more rarefied over time. It may not be waiting for us yet, but at some point in the next few billion years, the Earth should experience the last impact of an asteroid, and if there is still life on the planet by then, the last mass extinction could well begin with such a catastrophe.
But the Oort cloud and the Kuiper belt are different.
Beyond Neptune, the outer solar system has a huge catastrophic potential. Hundreds of thousands - if not millions - of large chunks of ice and rock graze in an elongated orbit around our Sun, waiting for a passing mass (it could be Neptune, another Kuiper belt object or Oort cloud, another system) to cause gravitational disturbances. This disturbance can have different consequences, but among them - to send the body into the inner solar system, where it will manifest itself as a brilliant comet that threatens to collide with our world.
Interactions with Neptune or other Kuiper belt or Oort cloud objects are random and independent of what is happening in our galaxy, but there is a possibility that passing through a region rich in stars - like the galactic disk or one of the spiral arms - could increase the chances of comets rain, and him and the chances of the comet hitting the Earth. Recently, a paper appeared in American Scientist that discussed the roughly designated 26-30 million year extinction period on Earth, which corresponds in part to the 28-32 million year period when the solar system passes through the Milky Way's galactic plane. Coincidence or pattern?
The answer can be found in the data. We can view the largest extinction events on Earth captured in fossils. By counting the number of genera (a more general step than the "species" by which we classify living things; people homo sapiens belong to the genus homo) for a specific time, extrapolating this to more than 500 million years ago (thanks to sedimentary rocks), we know what percentage simultaneously existed and died in a given period of time.
Phanerozoic biodiversity: from below - millions of years ago; on the right - thousands of births. All genera are highlighted in gray; well defined genera in green; red long-term trend; the yellow triangles mark the five largest extinctions; in blue, other extinction events.
We can then look for patterns in these extinction events. The easiest way to do this quantitatively is to apply a Fourier transform to these cycles to find possible patterns. If we see cases of mass extinction every 100 million years, for example, accompanied by a large decline in the number of genera, then the Fourier transform will show a large jump in frequency of 1 / (100 million years). What does this mean?
We see a jump at a frequency of 140 million years and another jump at 62 million years. These jumps look huge, but only relative to other jumps, which are completely insignificant. Over a time span of just 500 million years, you can fit three possible mass extinctions every 140 million years and 8 possible times every 62 million years. (But we don't see that much, so this periodicity is not preserved). But if you look closely, there is nothing to say about the frequency of extinctions in 26-30 million years; there are simply no convincing leaps with such frequency. In addition, of all collisions of cosmic bodies with the Earth, less than one quarter came from the Oort cloud. There is an old adage, "Extraordinary claims require extraordinary evidence," but Christopher Hitchens looked at it from the opposite point of view:
"What can be accepted without proof can be rejected without proof."
So so far there is no reason to suspect that the passage through the galactic plane is accompanied by an increase in the frequency of catastrophic events. The chances that the universe threatens to destroy us are extremely small.
Ethan Siegel