Worldbuilding Stack Exchange is a question and answer site for writers/artists using science, geography and culture to construct imaginary worlds and settings. It only takes a minute to sign up.
Sign up to join this community
The size of a planet affects its gravity, its climate, and its likelihood of developing life.
Earth is large enough for an advanced civilization to develop. A smaller body such as Europa, while able support life as well, might not be large enough to birth an advanced civilization.
It is unreasonable for smaller bodies such as asteroids, to ever develop life? Also, what is the smallest a planet can be before it is no longer feasible for an advanced civilization to develop on it?
Whether a planet can sustain a civilization depends greatly on its size, because the planet has to be big enough to avoid these problems: Taken from Planetary Habitability.
lesser gravity makes atmosphere retention difficult...
smaller planets have smaller diameters and thus higher surface-to-volume ratios than their larger cousins. Such bodies tend to lose the energy left over from their formation quickly and end up geologically dead, lacking the volcanoes, earthquakes and tectonic activity which supply the surface with life-sustaining material and the atmosphere with temperature moderators like carbon dioxide....
- Exceptional circumstances do offer exceptional cases: Jupiter's moon Io (which is smaller than any of the terrestrial planets) is volcanically dynamic because of the gravitational stresses induced by its orbit, and its neighbor Europa may have a liquid ocean or icy slush underneath a frozen shell also due to power generated from orbiting a gas giant.
So the answer would appear to be:
Thus it would be fair to infer that the lower mass limit for habitability lies somewhere between that of Mars and that of Earth or Venus: 0.3 Earth masses has been offered as a rough dividing line for habitable planets. However, a 2008 study by the Harvard-Smithsonian Center for Astrophysics suggests that the dividing line may be higher.
http://www.lpi.usra.edu/education/explore/our_place/hab_ref_table.pdf
As for your question of can asteroids support life, the answer would be that it depends.
This link says basically the same as me.
I fail to find a specific number, but you can calculate a reasonable number from the sizes of Earth and Mars.
But after you get a planet of the right size the planet still has to have a lot of other criteria:
I'm going to limit my answer to life forms based on carbon and water living on planets within what is commonly termed the habitable zone. This ignores other possible life-form chemistries as well as special cases such as Europa.
Using our planetary system as a model, it is clear that Mars, may have had, at one time, the ingredients necessary for life to evolve, but that it was unable to retain them due to insufficient gravity. On the other hand, both Earth, and probably Venus did not have this problem.
Gravity is a function of total mass, and so density is an important factor. Part of the problem with Mars is that its density is only about 70% that of Earth. Increasing the density of Mars to the Earth would raise the surface gravity to about 0.53 Earth gravities, which may, or may not be sufficient.
There are other complicating factors that may or may not be relevant, such as the role of Earth's moon, or of the Earth's magnetic field.
This is a complicated question, because it's hard to define "levels of civilization" in general. I'm going to make the (usually poor) assumption that the level of technology is the measurement of civilization, not things such as ethics or population size.
As you have noted, the body in question needs to have the correct conditions on which life can be supported. That means it must be neither too hot nor too cold; it must have an atmosphere, etc. etc.
If Europa may support life, it appears that anywhere from .245 of earth's radius to bodies the size of "super earths" can support life.
It may not be a matter of size; it is simply a matter of resources. If you look at many civilizations in Earth's history, you'll notice that some advanced technologically while others did not. If you subscribe to the Guns, Germs, and Steel philosophy of civilization development, you need not just a body to support life, but the correct resources and opportunities for it.
The magnetic field was mentioned in a previous answer, and, in Earth's case at least, it's an important factor. The unusually large, molten iron core of Earth generates a significant magnetic field that shields the atmosphere from being stripped away by solar radiation (see Mars) and also protects the planet surface from harmful radiation. Other things being equal, a smaller planet will tend to cool faster, losing its magnetic dynamo core earlier (like Mars).
Natural resources matter a lot more than planet size. You don't specify that you're talking about humans, which would need a planet with roughly the same gravity as Earth to thrive without modern levels of technology or higher. (In fact, a human culture with modern levels of technology would probably thrive, though not develop, in an asteroid, given the right resources.)
Availability of energy (most likely from a sun) is the biggest thing. Some would say that availability of metals is a big thing, too, but it could be possible to circumvent that with organic technologies.
There's no minimum size. Consider: Larry Niven: The Integral Trees/The Smoke Ring.
Both books take place within a gas torus pulled off a gas giant orbiting a neutron star. While both the neutron star and the gas giant are required to maintain the torus neither is inhabitable. The system could work although the gas giant is slowly eaten up.
The tricky part is evolving to the point of civilization before the gas giant gets totally sucked up and the whole thing disappears. (In the books the life is native, the civilization is stranded human starfarers.)
