Plausibility of alien life (alien being natural life outside of earth). Well, there are different forms of life to be considered.
In the Contingent life on earth, there is bacteria, viruses, fungus, plants and animals then you have substantially intelligent life, like humans. When you look at the area of space that humans have been able to observe, per say, observing the surface with satellites and telescopes, you understand the scope of possibility is thinning from the widest range of life, like bacteria, to the obviously more fortuitous intelligent life.
In our solar system, we consider earths early biosphere may have played a part in harboring functioning organized life. That would give earths current biosphere, which sustains life, credible consideration for elemental requirements for life to exist on other planets. Water, solid ground and nitrogen/oxygen based atmosphere are fundamental prerequisites for a supposed "necessary environment" for morphogenesis or even life sustaining. We associate life with organic conventions which lead us to seek out similar environs to earth.
Without venturing too far from the neighborhood, In our solar system we find water, an important factor, on a few moons, and other planets with ice in them like Mars and ice based Pluto. Three Figures in our solar system are candidates for prime qualifications.
Mars, Jupiter's moon Europa and Saturn's largest moon Titan. These are considered for having solid ground, ice or ice polar caps and dense modestly temperate atmospheres. see;
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Also considering some fundamental factors to the ingredients of life. As I had said before, we are accustomed to all life being carbon based and organic. These rules may vary some with different planetary benefactors. For instance, a nitrogen based article of life may not require water to be created or sustained. It may not require solid ground or even a sustained atmosphere. We observe Earth's ordinances, but they may not be absolute.
Finally, I believe when pondering how plausible life in the universe is, you have to view the the plausibility of simple life first, as that would be the vindication. after that, the question of intelligent life, if they don't encounter us first, would be certitude. But to place intelligent life first is perpetual.
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