A quiet place in the galaxy, a moderate and long lived star, big fellows sweeping debris in the perimeter, nice round orbit at the right place to sustain liquid water, large and stabilizing satellite, magnetic field to protect from charged particle bombardment. It is a highly unlikely combination, but it occurred at least once here in the Milky Way, Solar System, planet Earth.
Now, if the Earth was once born from space faring dust, and was born again after a catastrophic collision from where a blistering hot Earth-Moon system emerged, where did water come from? Without water, there is no life (as we know, at least).
Yes, water please, thank you.
To trace water history on Earth, we must first check the origins of its atmosphere, for without it water should be rapidly lost to space, lacking pressure to keep it from boiling.
The original atmosphere, segregated during the first epoch of planet formation, most certainly reflected the abundance of elements in the primeval solar system nebula. By far the most abundant element was hydrogen, but the relatively low mass of the proto-earth was not enough to retain it and was quickly lost.
Only after the end of the heavy bombardment period from where the Earth-Moon system emerged, things begun to settle enough for the molten material to separate and spawn a solid crust. At this stage, widespread volcanism (more on this later) began to outgas the inner layers and form a lasting atmosphere. Main components of gaseous volcanic ejecta probably were as today water vapor, nitrogen, carbon dioxide and smaller amounts of other compounds. To these sources, there is still a totally unrelated addition.
It happens there is here in the Solar System, an almost unlimited supply of water in the form of a large reservoir of icy bodies, the Oort cloud, after astronomer Jan Oort, who postulated its existence about 1950, containing perhaps about a trillion dirty snowballs ranging from a few meters to kilometers in size. Though current times are relatively quiet, tens or perhaps hundreds of these interlopers visit the inner regions of our system, most often going unnoticed because of their small size. In earlier epochs, the traffic was much more lively and collisions with Earth were correspondingly frequent, fizzling in the upper atmosphere to water vapor.
So, once the surface cooled enough to bear liquid water, the thick accumulated cloud cover started to rain filling the first oceanic basins.
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