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Do any aliens exist? Does any planet have conditions to live?
Question Date: 2012-11-16
Answer 1:

The short answer is "not to the best of our current knowledge, and Earth is the only known planet with conditions for life as we know it", but maybe a long answer is more interesting or helpful?

We have not found any evidence for intelligent life in space, yet that is not for lack of trying. Why are we trying? Well, great scientists have asked the same question as you, and the fact that we haven't found aliens or evidence of their existence or them having visited Earth is actually known as "Fermi's paradox" . This name comes from the fact that the famous physicist Enrico Fermi thought it highly probable that Aliens should have visited our planet or solar system already, based on a few simple calculations. So the fact that they should have, but haven't, visited, is Fermi's paradox. His calculations of probability are based on several assumptions and numbers, some of which I will outline for you to explore your question a bit more.

One assumption is that the Earth is a typical planet in a typical solar system in a common place of a typical galaxy. Now consider the inconceivably large size of the universe (check out universe for an attempt at visualizing it). The stars you can see on a clear night are but a tiny fraction of what is there: there is an estimation of 200- 400 billion stars in our Galaxy, the Milky Way, alone. And the number of stars in the visible universe is estimated to be about that number multiplied with itself! If one assumes that all those stars have planets around them, an Earth- like planet does not seem such an unlikely occurrence.

Furthermore, if you consider the age of the universe (billions of years), even the enormous distances to other stars may seem within the reach of civilizations that are technologically just a bit more evolved than ours. The distance to the closest star (Proxima Centauri) in our galaxy is about 4.24 light years, and our galaxy measures around 100 light years across. You may recall that a light year is the distance light travels in one year. Fermi considered interstellar travel as feasible within the next hundred years. Travel between galaxies may be another matter (we are now talking millions of light years), but communication may still be feasible. So you can get an idea about where Fermi was coming from.

Now, why have we not found any aliens or evidence of their existence? One explanation is that there is something wrong with Fermi's assumptions; for example, the one assumption that the Earth is nothing special as a planet. The more we know about life and how complex it is and what it needs, the more we learn to appreciate the special circumstances that our planet provides and the huge amount of good chance that has played a part in life and humans evolving here. Due to the scale of planets relative to that of stars and the universe, it is not easy to find out how many and what kinds of them exist, and this is an ongoing field of research. Or interstellar space travel may not be as feasible as Fermi thought, or civilizations advanced enough to pursue it may not be interested in such things anymore. There are many explanations that one can come up with, and those last two (which I just came up with as examples) are probably no less strange than many others that people have investigated. Yet this is a part of science, in areas where little is known yet: coming up with ideas and speculations, and ways to test those ideas.

Answer 2:

No one knows for sure whether aliens exist. There are theories suggesting both the absence and the presence of aliens, but we still don't have evidence of any intelligent life. For example, there are so many galaxies with so many stars with so many planets, it is likely that at least one of them has intelligent life. This suggests there should be intelligent life out there. On the opposite side of the argument, it is likely that if intelligent life existed, it would have made machines that can travel and build copies of themselves. The fact these machines haven't populated the entire universe suggests other intelligent life-forms may not be present. In the end, we have no idea what this life will look like nor behave, nor what conditions might be suitable for this life.

Because science only works with observations, we can never prove there is no other life, because it is impossible to observe something that doesn't exist. Consequently, this question will only ever be answered when we find evidence of life.

Answer 3:

It's almost a certainty that alien life exists, but we have no idea how common it is. The only place in the universe where we are absolutely certain is habitable to life is the Earth. We think the same may be true of some of the other objects in our solar system, but we're less sure of that. That there are other Earth- like planets in the universe is, again, almost certain, but they could still be quite rare.

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