We do not (yet) direct evidence that on other planets, their satellites, as well as in interstellar space there is life. Nevertheless, there is a very compelling and persuasive reasons to believe that in time we will find that life can be, even in our own solar system.
Here are seven reasons why scientists believe that life exists somewhere surely just waiting for meeting with us. Maybe it will not be green-skinned ladies in flying saucers, but it will still be aliens.
1. Extremophiles on Earth
One of the main questions is, can exist and develop in the worlds of life radically different from Earth. It seems that the answer to this question is yes, if you think about the fact that even on this planet there are extremophiles, or organisms that can survive in extreme conditions of heat, cold, toxic effects (for us), chemicals, and even in a vacuum. We found the creatures that live without oxygen at the edge of the hot volcanic vents on the ocean floor. We have found life in the brackish waters high in the Andes mountains, and in the sub-glacial lakes in the Arctic. There's even a tiny organisms called tardigrades (Tardigrada), able to survive in the vacuum of space. So, we have direct evidence that life can exist quite well in a hostile environment on Earth. In other words, we know that life can be stored under conditions that we have seen on other planets and their moons. We just have not found it.
2. Evidence of the starting materials and prototypes for life on other planets and satellites
Probably life on Earth originated from chemical reactions that eventually formed the cell membrane and the proto-DNA. But the primary chemical reactions could start in the atmosphere and in the ocean with complex organic compounds such as nucleic acids, proteins, carbohydrates and lipids. There is evidence that these "precursors of life" already exist in other worlds. There they are in Titan's atmosphere, astronomers noticed them in the rich environment of the Orion Nebula. Again, this does not mean that we have found life. However, we found the ingredients which, in the opinion of many scientists contributed to the development of life on Earth. If these ingredients are common throughout the universe, it is quite possible that life originated in other places, not only on our home planet.
3. The rapidly increasing number of planets similar to Earth
Over the last decade, hunters celestial bodies discovered hundreds of planets outside the solar system, many of whom, like Jupiter, are gas giants. However, new methods for finding planets and allowed them to find a small, solid worlds such as Earth. Some of them even to orbit around their stars in the so-called "habitable zone" that is, at a distance, when they arise temperatures close to the earth. And given the huge variety of planets outside the solar system, it is likely that one of them, there is some form of life.
4. The enormous variety and persistence of life on Earth
Life on Earth has evolved in extremely difficult conditions. Sometimes she managed to survive the most powerful volcanic eruptions, meteorite impacts, ice ages, droughts, acidification of the oceans and the radical changes in the atmosphere. We are also seeing an incredible diversity of life on our planet in a relatively short period of time — in geological terms. Life is also pretty stable piece. Why could not she have arisen and not to put down roots in one of the moons of Saturn or in another star system?
5. Mysteries surrounding the origins of life on Earth
Although we have a theory about the origin of life on Earth, in which the figure I mentioned earlier complex carbon molecules, ultimately, it is a mystery how these chemicals are combined to form a fragile membrane, over time, become cells. And the more we learn about what kind of hostile environment existed on Earth when life originated and developed — filled with methane atmosphere, boiling lava on the surface — the more mysterious is the mystery of the origin of life. There is one common theory, which states that a simple single-celled life really was born somewhere else, maybe on Mars, and brought her to Earth meteorites. This theory pansermii, and it is based on the hypothesis that life on Earth came from living on other planets.
6. Oceans and lakes are common, at least in our solar system
Life on Earth began in the ocean, and this implies that the water she could appear in other worlds. There is strong evidence that water once flowed on Mars is free and abundant, and on Saturn's moon Titan has methane sea and the river flowing on its surface. It is believed that Jupiter's moon Europa is one big ocean, warmed by the bark of the moon and completely covered with a thick protective layer of ice. In any of these worlds could support life once, and maybe still exists today.
7. Theory of evolution
People often use the Fermi paradox as evidence that we will never find intelligent life in our universe. On the other side stands the evolutionary theory, which postulates that life adapts to environmental conditions. Darwin and his contemporaries hardly thought about life on planets outside the solar system when developing their theory of evolution, however, and they claimed that there where life can take root, it will do it. And if you think about it, that our environment is not only the planet, but other star systems, and interstellar space, you can make the original assumption within the meaning of the theory of evolution — that life and adapt to the open space, too. In one day we can meet up with creatures that have evolved ways unimaginable to us. Or, we will be able to someday become such creatures.