|some introducing historical remarks
With the dismission of the Earth as supposed center of the universe the opinions about life also changed. After the victory of the scientific truth against the catholic church the view became widespread, that with the many other worlds like Mars and others also life should be widespread. But there was no scientific work about this, just opinions... Even with the progress in the most relevant branches of science like biology and astronomy, also chemistry, this changed only slightly. Serious work about this question came late, with the beginning of the twentieth century. The first concrete discussed topic were the now infamous and at that time notorious Mars channels, a misunderstood translation from the famous observer Antoniadi, exploited primarily by Percival Lowell.
Since then, the discussion is going on sometimes even with a certain violence. Now let us consider general aspects.
can we say anything with certainty?
The more special the question is, the more uncertain the answer will be in such a branch of investigations. On a general base, the following facts should be remembered: there are countless galaxies in our universe, probably many of them not visible due to the horizon effect, and each of them contains also numerous stars. Numbers for galaxies exceed 1 billion (109) and the numbers of stars are about 1 million to 1 trillion (1012) stars per galaxy. Therefore the number of stars in the universe is in excess of about 1018, a number which can be no way imagined. Since a few years we know also, that planets are no extremely rare exceptions, despite the planetary objects found out of our solar system are all either giants presumably resembling Jupiter and Saturn or even brown dwarfs, or exotic objects like the pulsar planets. But why the existence of Jupiter like objects elsewhere in the solar neighborship should be an unique feature of this type of planets? One can reasonable assume, that the analogy holds also true for the much harder to discover Earth like planets.
Another fact is the rapid development of life on Earth. Most scientists nowadays agree, that the life started only about 100 million years after the devastating, but probably also life conditions preparing great bombardment in the first time of our planetary system. This and the fact, that organic molecules in many variants are found as widespread in the so-called molecular clouds, together with the above numbers and ignoring even today partly prevailing anthropogenic prejudices, make nearly all people involved with any questions related to this subject guess, that there should be other places in our universe with life forms.
Only very few experts will deny this!
some more specific arguments
Please let aside any religious considerations. These are neither helpful in an objective manner, nor helping out of any possible confusions.
Our empiric problem is, that we know just one planet with life and with intelligent (really?) inhabitants. One may turn the above arguments around and could state, that it's impossible to draw any conclusions from such a single sample - it may be as well an unique case as a widespread phenomenon. At principle, this is correct, but it is like a fine tuning effort, to make exactly one of such many (see above!) worlds the only one in existence. All of us physicists and astronomers hate such a fine tuning, and this for good reasons! Because such parameters explain nothing, but are ad hoc assumptions. A well-known example are the initial conditions of the big bang, from the majority of cosmologists dismissed and replaced with the at least in this respect much more explaining hypothesis of inflation.
But is there anything else? Indeed, there are bacteria deep in the crust of our planet, lacking any solar insolation or other direct nutrition chances of the usual types. Even in this environment life can exist. And in Antarctica also life exists, which was not expected earlier: for example worms, which should freeze immediately - at least even experts would think so, if not knowing better. And also bacteria exists deep in the oceans, also multi cellular organisms in the vicinity of volcanic vents and also on land in hot locations - some of them up to 100 degrees Celsius hot.
Also remember a number of giant climatic changes already proven for the history of Earth. Despite these, the life persists now for around (may be a little less than) 4 billion years on our planet. And now here we are, thinking over and over again, why we do so and if there are others to join us...
nothing is for eternity...
The only other fact is, that the increasing solar insolation - caused simply by the slowly helium depleting hydrogen fusion in the solar core, which results in an increasing temperature and pressure there and subsequently in an even higher energy production! - will exhaust the capability of Earths atmosphere to regulate temperatures in about 1 billion year in the future. But even before this time, the carbon dioxide concentration will either fall below a plant life sustaining level and therefore destroying the conditions for nearly all life on Earth, or the not so much depleted carbon dioxide will heat up the atmosphere to the boiling point of water - or both effects take place partly - finally our planet will develop rapidly into a close "relative" of the planet Venus, an offspring of hell according human measures.
But even considering this development, which has a common astrophysical reason and can't be avoided, the life on Earth has than existed for about 4 to 5 billion years. This is about half the time of the Suns lifespan.
So far the facts or at least with near certainty accepted conclusions.
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