We do not (yet) have direct evidence that life exists on other planets, their satellites, as well as in interstellar space. And yet, there are compelling and very compelling reasons to believe that over time we will find such life, perhaps even in our solar system.
Here are seven reasons why scientists believe that life must exist somewhere and is only waiting to meet with us. Maybe they won't be green-skinned ladies in flying saucers, but they will still be aliens.
1. Extremophiles on Earth
One of the main questions is whether life can exist and develop in worlds that are radically different from the earthly one. It seems that the answer to this question is yes, if you think about the fact that even on our planet there are extremophiles, or organisms that can survive in extreme conditions of heat, cold, exposure to toxic (for us) chemicals, and even in a vacuum. We have found living things that live without oxygen at the very edge of hot volcanic vents on the ocean floor. We found life in brackish waters high in the Andes mountains, as well as in the subglacial lakes of the Arctic. There are even tiny organisms called tardigrades (Tardigrada) that can survive in the vacuum of space. So, we have direct evidence that life can quite successfully exist in a hostile environment on Earth. In other words, we knowthat life is able to survive in the conditions that we observe on other planets and their satellites. We just haven't found her yet.
2. Evidence of the presence of starting materials and prototypes of life on other planets and moons
Life on Earth likely originated from chemical reactions that eventually formed cell membranes and proto-DNA. But these primary chemical reactions could begin in the atmosphere and in the ocean with complex organic compounds such as nucleic acids, proteins, carbohydrates, and lipids. There is evidence that such “precursors of life” already exist in other worlds. They exist in the atmosphere of Titan, astronomers have noticed them in the rich environment of the Orion Nebula. Again, this does not mean that we have found life. However, we have found ingredients that many scientists believe contributed to the development of life on Earth. If such ingredients are common throughout the universe, then it is quite possible that life has appeared in other places, not just on our home planet.
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3. Rapidly increasing number of Earth-like planets
Over the past decade, hunters for celestial bodies have discovered hundreds of planets outside the solar system, many of which, like Jupiter, are gas giants. However, new methods of searching for planets have allowed them to find smaller, solid worlds like Earth. Some of them are even in orbit around their stars in the so-called "habitable zone", that is, at such a distance when temperatures close to Earth's occur on them. And given the huge number of planets outside the solar system, it is likely that some form of life exists on one of them.
4. The immense diversity and resilience of life on Earth
Life on Earth developed under extremely difficult conditions. Sometimes she managed to survive the most powerful volcanic eruptions, meteorite strikes, ice ages, droughts, ocean acidification and radical changes in the atmosphere. We also observe an incredible variety of life on our planet in a fairly short period of time - in geological terms. Life is also pretty persistent. Why doesn't it originate and take root on one of Saturn's moons or in another star system?
5. Secrets surrounding the origin of life on Earth
While we have theories about the origins of life on Earth, involving the complex carbon molecules I mentioned earlier, it is ultimately a big mystery how such chemicals combined to form fragile membranes that eventually became cells. And the more we learn about what an unfavorable environment existed on Earth, when life was born and developed - an atmosphere filled with methane, boiling lava on the surface - the more mysterious the mystery of the origin of life becomes. There is one general theory that simple single-celled life actually originated somewhere else, maybe on Mars, and was brought to Earth by meteorites. This is the theory of pansermia, and it is based on the hypothesis that life on Earth originated from life on other planets.
6. Oceans and lakes are widespread, at least in our solar system
Life on Earth originated in the ocean, and it follows that from water it could appear in other worlds. There is strong evidence that water once flowed freely and abundantly on Mars, and that Saturn's moon Titan has methane seas and rivers flowing across its surface. It is believed that Jupiter's moon Europa is one continuous ocean, warmed by the crust of this moon and completely covered with a thick protective layer of ice. In any of these worlds, life could once exist, and maybe it still exists now.
7. Evolutionary theory
People often use the Fermi Paradox as proof that we will never find intelligent life in our universe. On the other side is evolutionary theory, which postulates that life adapts to its environment. Darwin and his contemporaries hardly thought about life on planets outside the solar system when they created their theory of evolution, however, they also argued that where life can take root, it will definitely do it. And if you think that our environment is not only planets, but also other stellar systems, and interstellar space, then you can make an original assumption within the interpretation of evolutionary theory - that life will adapt to open space too. One day we may meet creatures that have evolved in ways that are inconceivable for us. Or we ourselves may one day become such beings.