Type in the number, hit Xy, type in 0. They were not trying to figure out which stars could host a human habitable planet. They were trying to figure out which stars could host a planet that was not so hideously uninhabitable that no possible form of life could live there. In other words, many of these planets could host alien life forms but would quickly kill an unprotected human being.
The equations were derived by me using an analysis of the Habcat database , and thus could be wildly inaccurate. If you can find better figures, use them, but these are better than no figures at all. If my slide rule isn't lying to me, this works out to an average distance between adjacent stars of 9. How wide is it? How many stars will it probably have?
A sphere light years in diameter has a light year radius. Anderson cites a figure of about four million stars, which means one of us is a bit off the mark probably me. We are one more-or-less intelligent species in a universe that produces sophonts as casually as it produces snowflakes. We are not a hair better than our great, greenskinned, gatortailed Merseian rivals, not even considering that they have no hair; we are simply different in looks and language, similar in imperial appetites.
The galaxy—what tiny part of it we can ever control—cares not one quantum whether their youthful greed and boldness overcome our wearied satiety and caution. Which is a thought born of an aging civilization, by the way. Our existing domain is already too big for us. We don't comprehend it. Never mind the estimated four million suns inside our borders Terran Empire has diameter of light-years, light-year radius.
Think just of the approximately one hundred thousand whose planets we do visit, occupy, order about, accept tribute from. Can you visualize the number? A hundred thousand; no more; you could count that high in about seven hours.
But can you conjure up before you, in your mind, a wall with a hundred thousand bricks in it: No human brain can go as high as ten. Then consider a planet, a world, as big and diverse and old and mysterious as ever Terra was. Can you see the entire planet at once? Can you hope to understand the entire planet? Next consider a hundred thousand of them. No wonder Dietrich Steinhauer here is altogether ignorant about Freehold.
I myself had never heard of the place before I was asked to take this job. And I am a specialist in worlds and the beings that inhabit them. I should be able to treat them lightly. Did I not, a few years ago, watch the total destruction of one?
And yet it was a single living world that perished, a mere single world. No wonder Imperial Terra let the facts about Freehold lie unheeded in the data banks. Freehold was nothing but an obscure frontier dominion, a unit in the statistics.
As long as no complaint was registered worthy of the sector governor's attention, why inquire further? How could one inquire further? Something more urgent is always demanding attention elsewhere. The Navy, the intelligence services, the computers, the decision makers are stretched too ghastly thin across too many stars. And today, when war ramps loose on Freehold and Imperial marines are dispatched to fight Merseia's Arulian cat's-paws—we still see nothing but a border action.
It is most unlikely that anyone at His Majesty's court is more than vaguely aware of what is happening. Certainly our admiral's call for help took long to go through channels: The city people are no use.
They don't seem to know either what's going on. Not even a Naval officer—not even a specialist in human cultures—such cannot be gotten, except for tasks elsewhere that look more vital. One civilian xenologist, under contract to investigate, report, and recommend appropriate action. Which counsel may or may not be heeded. Colonizable Worlds If your first-in scouts have given you the luxury of lots of human-habitable worlds to choose your colony sites from, naturally you will pick the ones closest to being paradise planets.
If all the planets range from miserable hell-holes to utterly uninhabitable you have roughly five options: While living on Mars will not be easy, it will be far more planet-like than living in an entirely artificial habitat. We are the kind of beings that evolve on a planetary surface, i. Human instincts for planetary life will be seamlessly exapted for life on Mars, and, farther in the future, for life on other planets. The Martian settlers would have a homeworld , albeit a homeworld other than Earth.
Martian parents will indicate a point of light in the sky as Earth to their children, and these children may or may not be interested depending on their inclination to astronomy for Earth would now be an object of astronomy , but their lives will be on Mars, i.
The viewpoint of residents of artificial habitats will more closely reflect terrestrial viewpoints than those of Martian settlers. If residents are born on the habitat, the tie to Earth is likely to be somewhat weakened, and they may feel the want of a homeworld, if only on a subconscious level.
Perhaps they will evolve a distinctive sense of identity apart from planetary endemism, or they may go in search a of world to call home. These two possibilities suggest an eventual bifurcation of the population upon lines of inherent geocentrism, with this cognitive expression of individual variability becoming a source of social tension and eventually a selection pressure on the population.
Both experiences—those of Mars and those of artificial habitats—will be strongly selective, and they will select different traits, both of body and mind. The adaptive radiation of humanity in the cosmos will begin with these early spacefaring settlement efforts, but biological and cognitive adaptation to changed circumstances will still be in the far future when the first settlers are making themselves at home on Mars, and the first artificial habitats are being built and occupied.
During the earliest stages in the development of spacefaring civilization, the adaptation will primarily be that of individual attitudes. As spacefaring civilization continues to develop, artificial habitats are likely to be constructed at a distance from Earth beyond which the overview effect tapers off, and eventually where Earth is just another star in the sky, as on Mars.
Here, the selection pressure either to evolve a distinctive conception of humanity in space, or to find a homeworld, would be magnified.
If spacefaring civilization endures for biologically significant periods of time, and populations evolve under these selection pressures, the early attitudinal differences within populations will become the basis of speciation and adaptive radiation. One might call this the founder effect for spacefaring civilization. Habitable Planets fall mostly into two classes. These are almost exactly like Earth — more precisely the Garden of Eden, or at least coastal California. Summers are baskingly warm, winters briskly cool, and the rain falls only at night.
Landforms have dramatic variety, a typical planetscape resembling San Francisco Bay, only with the Sierras in place of the Oakland hills.
These Planets teem with native life forms that we can eat and tasty to boot; see FOOD , but none — either carnivores or microbes — who eat us. Who wouldn't want to live on one, if you could? These nominally Habitable Planets pose greater challenges for interstellar real estate promoters. The entire Planet usually has only one climatic zone, and it isn't mediterranean.
Desert Planets seem to be most common, followed by ice-age Planets, steaming jungle Planets, and howling windswept steppe Planets.
The local life is mostly inedible, but it can eat us with no problem, and does so whenever it can catch us. It is difficult to understand why Colonization happened on these Planets. Perhaps they produce something valuable in TRADE , but if so the Colonists never seem to benefit, since they are mostly poor. Heinlein, naturally, provided the real estate pitch: If you don't like settlements, you move on until you've got no neighbors, poke a seed in the ground, then jump back before it sprouts.
Practically no terrestrial diseases and no native diseases that like the flavor of our breed. Nova Terra, to be sure, was the pick of the lot. In the same book Heinlein alludes to harsh colony worlds — and later on, an Eden planet turns out to have non- prelapsarian locals already in possession, who intend to stay that way.
But given a sky full of stars and a ship to get you there, why settle for the also-rans? Heinlein also supplied a host of secondary tropes, such as the utility of horses that can fuel themselves from a handy pasture and given a stallion and a mare manufacture their own replacements. Unfortunately, as commenter Ian M. Suppose a planet with complex life, and enough of it to have built up an oxygen-rich atmosphere. It may look like Paradise, or at any rate Earth. Convergent evolution might well produce para-forests and para-grasslands, just as dolphins have a similar configuration to fish.
But dophins aren't fish, and alien life almost certainly will not be like us. Hydrocarbon life anywhere will be built out of the same basic building blocks, but with differing architectural details — and our digestive keys will not fit its nutritional locks.
The good news is that the local tigers and local germs won't find us tasty and nutritious. But by the same token we can't eat the local venison or berries, and chances are only slightly better that our cattle can graze on the grass.
Plants have a far less demanding diet, and might well grow nicely in any soil that has nitrogen fixed in it. In fact they might grow too well, at least the ones that don't rely on bees or other terrestrial creatures as their dating service. Terrestrial plants, devoid of natural enemies, might crowd the native stuff out of any remotely suitable environment — wrecking entire ecosystems. But this too could go both ways. To local para-algae we could be walking Petri dishes: Our bodies' defenses, if any, are likely to take the form of allergic reactions, not terribly helpful to us.
In short, any garden worlds out there are probably not for us.