Cosmic Citizens | Life, Unbounded, Scientific American Blog Network:
"Our remarkable species has existed in its present form for about 100,000 years. That’s about 0.0025% of the total time that we think life has existed on this planet. We, and the vast network of life around us, occupy barely a couple of percent of the volume of this world – its surface, a few kilometers into its subsurface, and some way up into its tenuous atmosphere. The Earth is an end product of the agglomeration of the equivalent of about a trillion kilometer-sized planetesimals that themselves coalesced from the sticky microscopic dust of a proto-planetary disk some 4.5 billion years ago. Altogether that represents about 0.003% of the total mass of that original smear of dust and gas that stretched from a youthful Sun to far, far beyond the orbit of Pluto. Today the Earth occupies about 0.0000000000000003% of the volume of space encompassed by a sphere just large enough to contain the orbit of Neptune. And it would take more than 4,400 of those spheres lined up edge-to-edge to reach the nearest star system and the nearest known exoplanet of Alpha Centauri B."
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