Good news for those looking for life on other planets
The first exoplanet was discovered in 1988. Since then, more than 3,000 planets have been discovered outside our solar system, and it can be assumed that about 20% of stars like the Sun have Earth-like planets in habitable zones. We do not yet know whether life exists on any of these planets - and we do not know at all how life originates. But even if life originated somewhere, can it survive?
The earth has gone through at least five periods of mass extinction of life in its history. It has long been believed that dinosaurs became extinct from the impact of asteroids. As a human race, we are rightly concerned about events that could lead to our own destruction - that is, climate change, nuclear war or disease can wipe us off the face of the earth. Therefore, the question naturally arises - what needs to be done in order to destroy all life on any planet?
To do this, we need to set some kind of benchmark, and we began to study the species considered to be the most hardy - tardigrade, which are also called "water bears" for their appearance. Based on our recent research, we can say that these microscopic eight-legged creatures or their equivalents will be very difficult to destroy on any planet like ours. The only astrophysical catastrophe that can destroy them is so unlikely that its chances can simply be ignored. This extraordinary ability to survive adds weight to the idea that life is hardy and that it can be found on other, less hospitable planets than our own.
Last survivors
Tardigrades are known for their ability to survive incredible conditions. If the temperature drops to minus 272 degrees Celsius for a short time or rises to 150 degrees Celsius, then nothing will happen to them. If the atmospheric pressure is increased more than a thousand times than on the surface of the Earth, or reduced to a vacuum in space, they will survive. Tardigrades are able to do without food and water for almost 30 years. They can withstand radiation of thousands of grays (standard doses), then a dose of ten grays is lethal for most people.
Tardigrades live everywhere on our planet, but they can also exist deep under water, in volcanic craters at the bottom of the Mariinsky Trench, without being bothered by such things as life and death of mammals living on the surface. As the ozone layer or the upper atmosphere disappears, humans will be exposed to lethal radiation, while the water column will be protective.
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We wanted to understand what cataclysms are ultimately capable of destroying these tenacious tardigrades. What must happen for them to cease to exist on our planet? Here is the simplest answer: all the oceans of our planet must boil. On Earth, this will require an incredible amount of energy - 5.6 × 1026 joules (at the current general level of its production, this would take about a million years). Therefore, we must consider astrophysical events that could provide such an amount of energy.
There are three main candidates to do this - asteroids, supernova, and gamma ray bursts. The Earth has been exposed to asteroids throughout its history. However, there are only 17 candidates for this role in our solar system (including such dwarf planets as Pluto and Eris), since they must be large enough to provide the necessary amount of energy. However, their orbits do not intersect with the Earth's orbit.
If we analyze the consequences of the collision of the Earth with asteroids, then we can extrapolate the level at which such events of the doomsday type can occur. It turns out that something similar happened about once every 1017 years - and this is more than the lifetime of the universe. Therefore, the probability of such an event is very low.
When supernovae are born (large-scale explosion of stars), a colossal amount of energy is released - 1044 joules - more than enough to bring water to the boiling point in our oceans. Fortunately, the level of generated energy drops rapidly as the object moves away from the supernova. That is, in the case of Earth, sterilization requires a supernova to appear at a distance of about 0.013 light years. Aside from the Sun, the closest star, Proxima Centauri, is 4.25 light years away (and is not suitable for supernova formation).
For planets like Earth in our galaxy, the distance between the stars depends on their distance from the galactic center. Its central bulge has more objects than the part that is closer to us. But even at a closer distance, given the frequency of supernovae, sterilization is unlikely to occur more often than once every 1015 years, and this again is much older than the age of the universe.
Finally, there are bursts of gamma rays, mysterious explosions that produce massive amounts of energy that are focused into very thin beams of radiation. Analyzing these explosions in the same way as we did in the case of supernovae, we found that they are capable of destroying life on planets such as Earth only if their source is located at a distance of no more than 42 light years, and the planet itself will be in the path of the beam. Once again, it turns out that the frequency of such events is quite low, and therefore a very small number of planets can be sterilized as a result of a burst of gamma rays.
There will be no apocalypse
Taking into account the negligible likelihood of such apocalyptic events, we come to the conclusion that tardigrades will exist until the explosion of the Sun, and this will happen in about a billion years. There is one more, last and extremely unlikely possibility - some star can push some planet out of orbit. But in this case, volcanic craters, where some tardigrades live, can produce heat until another star captures it.
There are many events, both external and local, that can lead to the extinction of the human race. However, life in general is incredibly tenacious. Starting the search for life outside the Earth, we have the right to make the following assumption: if life originated on a planet, then some of its elements may still exist there.
Rafael Alves Batista, David Sloan