Even if the
spontaneous creation of life on earth was largely accidental, there must be peculiar features of our
planet that make it
more comfortable than other ones in the
solar system. Important features include:
- Distance from the Sun.
The temperature of a planet is determined, to a first approximation, by this. Once Life establishes itself, it can help to regulate the global temperature - but it needs a reasonable baseline to start from. The Earth's spin is also vital, to allow both sides of the planet to warm properly.
- Large Moon.
The moon is thought to have protected the early Earth from any meteorites that didn't hit Jupiter. As the dinosaurs discovered, rocks aren't good for Life. Additionally, our satellite is thought to stabilise the spin of the Earth, preventing too much tilting of the axis. Since the Earth has to be evenly heated, this is much more comfortable.
- Solid Surface.
Jupiter is mostly gas (with a metallic hydrogen core?) and could hardly provide enough complex rock surfaces for self sustaining chemical reactions to start. Deep space may be full of clouds of alcohol or DNA, but that is a mere resource.
Of course, all this presupposes we are talking about the
type of life found here (
carbon-based life forms) and not giant intelligent clouds or
silicon wired planets. However, one of the vital elements for our sort of life is
water (liquid - which requires middling temperatures); as a universal solvent it is without equal.
It may
seem a circular question to ask 'how is it that we are here' when we wouldn't be here to ask it if things were different. However, I'm not talking about
intelligent life here. Even a tiny few pseudocells clinging to the barren rocks of
Mars would count as
some form of Life.
If there are two main 'hurdles' Life has to overcome it is 1)Booting up and 2)Not crashing. Of course, Life here might crash at some point - in which case future alien civilisations might look at our barren planet and say 'Why was this planet so inhospitable?'. While there might be many worlds out there that could generate life, there are probably few that can sustain it.
As for stars and grains of sand (see link in other writeup); the number of prokaryotic cells exceeds these values by a factor of 109 - a billion times more bacterial cells than grains of sand.