|Planet Earth has been intelligently inhabited a few hundred years; evidentially a few thousand years; probably frequented a few ten thousand years; visited a few hundred thousand years; the Solar system, a few million years; the local star space, a few tens of millions of years; the local region of the galactic arm, a few hundred million years; the galaxy itself, a few billion years; the cosmos since it first cooled to balmy, estimably just ten billion years ago.|
Places in the cosmos, where civilization would have flourished, include:--
The inhabitants of cosmic rogue planets were dark adapted, as the sky although warm, was dark for most (few lived near stars enough to brighten their sky to much more than our moonlit level). Other lighting sources included aurora, also fairly weak, though enhanced by the then-current supernova era: With supernovae more numerous than we see, and each lasting a year (our solar measure), and those distant, nearer by a factor of ten, may have been enough to illumine the sky to dawn intensity, though variable:- it would be memorable to retell of such an armageddon-like sky, a billion years later.
Spaceflight was not much more facile than it is now: The cosmic interstellar haze was less tenuous, but only by a factor up to a thousand atoms per milliliter, a billion per cubic meter (insignificant compared to the 6.02*10^23 per gram-atom/mole, or a million times that per cubic meter) and the cosmic sky a thousandth as clear as it is today; -which would mean little to those traveling space, as the cosmos itself was a tenth the apparent diameter, and the remaining factor of a hundred times the haze, was still fairly clear, certainly for looking across a galaxy or to the next (Only the farthest reaches of the cosmos might have appeared hazier than today; but its background was room temperature in any case) and that would have sufficed astronomers to make the sky interesting to describe; And for daily inhabitant use, the sky was even less visibly clear as the daytime illumination did not change:- and there was no day versus night distinction.
Nevertheless, space travel was that much more facile by the higher density of interstellar hydrogen and deuterium: technologically that great factor closer to interstellar hydrogen-scooped fusion-drive....
Toward the end of the approximately 30 million years balmy era (20C deg. cooler), ice occurred on the rogue and distant-orbit planets, and inhabitants began the gradual move inward toward stars,-- abandoning their rogue planet cities, by first building deeper underground.
As the technology could support the inner continuation until the deuterium resources were used up, there are probably billions of obsolete high-tech rogue planets in each galaxy,- which was probably densely populated. The rogue and distant-orbit planets could have also been moved gradually inward from their cometary belts (which had added another kind of lightning: continual bombardment by meteors, the shooting stars and fireballs), and only eventually planets re-orbited to the inner solar systems.
Places to look for such stellar planet dwellers, include planets near the outskirts of giant stars, as well as planets near the inner zone of suns like our own. The year cycle is greatly disparate: planets distant from giant suns, lighted and warmed like our own, but the distance would have given them century long years-- hibernation might have been a norm, even for people; or they were intelligent enough to keep themselves warm inside cities, or inside spacships ... not much different than we might find today, albeit different than our own Earth,- which seems to be on a backstreet in the galaxy.
Star-planet arrangements in this era might be sufficiently fewer that the urge to travel interstellar might have been cooled as well,- if merely because planet living was easy enough to remain and enjoy it:- not that technology was insufficient itself.
Inhabitants at giant stars might have learned quickly the imminent fate of their stars, and moved to space before the supernova,- and traveled in search of distant planets. Those who remained, simply disappeared within hours of the neutrino precursor blast. Starwatching might be an imitatory art, where once it was the technological necessity of the successful nova-escapees,- an art not much needed after the final twinkling of their star. The lifestyle they brought away, was driven by the century-long "year", and ages like 930 years, were nominally the expected norm.
Inhabitants of tight stellar clusters, the so-called Bowling Pin Clusters, where stars were a 100 AU from each other, -and as soon as planet populations reached spaceflight, they visited their neighbors, and formed federations- but did not venture much beyond their clusters (interstellar traversing may have taken decades,- and Adam's 930 years may have been the expected age norm). And the clusters may have quickly populated, and eventually when stars collided within the cluster, every hundreds of millions of years, that population if not absorbed by other planets, may have moved-on to the deep space: eventually reached light-year distances to cross significantly into the galactic arms.
Space-marauders may have populated anything that looked remotely like a planet, along their way. Eventually such a band, whether originally from a supergiant star, or from a Bowling Pin Cluster, forced to leave across galactic space, made their way to Earth, and at length, decided to populate it, with whatever survived the intragalactic distances.
A premise discovery under the title,