I often hear that Earth is a unique planet because it has life. We also see a proper balance between plants and humans and other animals. Why is life not possible on another planet? Often people say because the environment of other planets is so different from the earth and it's hard to find a planet like ours. That seems pretty satisfactory; I can not live on Mars or Saturn.

On the other hand, I find it little misleading. Why do we imagine ourselves on other planets to see if life is possible on other planets? There might be different creatures that are used to different environments. There might be creatures that are adapted to ${\rm H_2SO_4}$.

I have two questions. First is my reasoning true? And if so what makes life rare on other planets or makes the earth so unique?

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    $\begingroup$ I’m voting to close this question for the reasons stated in other comments and even in an answer. $\endgroup$ – B--rian Apr 8 at 13:25
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    $\begingroup$ You are asking this as if it is an either-or question. The answer is most likely "yes". $\endgroup$ – David Hammen Apr 9 at 18:36

Your reasoning is at least true in part.

First off. We have only one example of life. It is based on carbon chemistry and water solutions. As we only have one example, we don't know if fundamentally different chemistries are possible. We just don't know if the Earth is unique or special, or whether life is commonplace throughout the universe.

If you head over to https://worldbuilding.stackexchange.com/ you will find lots of people speculating of different biologies, including those with a sulphur and sulphuric acid based chemistry

However, given our understanding of chemistry, it does seem likely that most life in the universe will be carbon/water based. Carbon is the only element that produces complex structures, and is also one of the most common elements. Oxygen is another very common element, but it is very reactive and so will tend to react with hydrogen to form water. Water also happens to be one of the very best solvents. So carbon seems to be the best element to build biological compounds and water is the best solvent to carry those around.

This means that is is a reasonable hypothesis that life will be found most often on planets that have surface temperatures and pressures that allow for liquid water. That is a pretty narrow range.

We are adapted to Earth. We have had 4 billion years of evolution to adapt to Earth, even as Earth changed. For example we have adapted to an oxygen-rich atmosphere. We could have adapted to Sulphuric acid (at least to dilute sulphuric acid). Oxygen is as toxic as acid to those bacteria that haven't evolved to tolerate it.

Whether the Earth has adapted to us is more controversial. This is the Gaia hypothesis of James Lovelock "that living organisms interact with their inorganic surroundings on Earth to form a synergistic and self-regulating, complex system that helps to maintain and perpetuate the conditions for life on the planet." (wikipedia) However "the Gaia hypothesis lacks unambiguous observational support and has significant theoretical difficulties" ( David Waltham. Lucky Planet).

We don't know if the Earth is unique, nor even if it is special. Our current ability to detect planets would struggle to detect the Earth, if it orbited another star. So we don't know how planets there are that have liquid water on the surface. We also don't know how many of them harbour life in any form. We don't know if life is possible in alternate chemistries. Even if life turns out to be common, we might still be special in that we have oxygen-breathing, multicellular, complex and intelligent life.

If it turns out that life is rare... we don't know why it would be. Life started pretty soon on Earth. If it turns out that life is actually rare we would need to explain that. But we don't have enough data to do so yet.


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