The Gravitational Threshold of Spacefaring Civilizations
Michael Hippke has made a paper available on the Arxiv, “Super-Earths in need for Extremly Big Rockets” in which he discusses the possibility of launching rockets from worlds larger than Earth -- which have come to be called “super-Earths” -- which means launching rockets from planets with higher surface gravity than Earth. Hippke treats the entire matter in a jocular fashion, insisting on the silliness of the project represented by the paper, but I see these matters quite differently. Indeed, I take this question very seriously.
If there are other peer intelligent species in the universe, they would evolve in peer biospheres, and peer biospheres would supervene on peer planets. But these “peer” worlds would not be identical to earth, only roughly similar. If there are multiple worlds in the universe with life and we could survey them, they would probably describe a bell curve, with Earth being a little to one side or the other of the bump of the bell curve, but close enough to the bump in the curve that we would consider it to be mediocre (thus instantiating the principle of mediocrity).
Worlds with lower gravity than Earth’s would mean that any intelligent species evolving there upon would have a slightly easier time slipping the surly bonds of their homeworld and thus becoming a spacefaring species. Worlds with higher gravity than Earth -- and the Kepler telescope data points to an abundance of super-Earths in the galaxy -- would have a more difficult time becoming a spacefaring species than the challenge human beings face.
We can posit that there is some gravitational threshold beyond which spacefaring becomes so impracticable that it constitutes an insuperable barrier for a species to become spacefaring. The availability of key resources and the level of technological development of the civilization would also bear upon the ability of an intelligent species to overcome high homeworld gravity in order to become a spacefaring species. Although Hippke in the paper suggests that chemical rockets might be practical on planets with surface gravities up to ten times greater than Earth, I suspect that the practical threshold for spacefaring is considerably lower.
Surface gravity, then, presents a unique challenge to which a technological civilization responds if it would be a spacefaring civilization, exemplifying Toynbee’s challenge and response model for civilizational development -- thought in this case it is specifically a question of spacefaring civilizational development. Some species might evolve on worlds that are simply too great a challenge for the civilization to mount a creditable response, while other species would have an easier time of it. Toynbee called those civilizations faced with challenges too severe to be overcome “arrested civilizations.” Just so, too high a gravity of a homeworld would result in a civilization being an arrested homeworld civilization.
This would be a bit of a reach, but if it turns out the there are many worlds in the universe with biospheres, but that the vast majority of them were large super-Earths, and our Earth was a rare low gravity exception of a planet with a biosphere, then this would introduce a severe limitation on spacefaring civilizations. However, this can’t quite be a solution to the Fermi paradox, because it would not prevent the inhabitants of high gravity homeworlds from building radio telescopes and transmitters (admittedly, the engineering would be difficult on a high gravity world, but not too difficult to overcome) and so from engaging in SETI and METI.
On the contrary, it could be argued that the inhabitants of high gravity homeworlds would be especially incentivized to engage in SETI and METI given the impracticality of a space program. They might better exemplify what I have called the “SETI paradigm” than they would species like our own, resident on homeworlds that do not prevent the possibility of spacefaring. However, it could still be argued that Earth’s gravity, while not impossible to overcome, is enough of a challenge to spacefaring that it will ultimately result in a terrestrial arrested spacefaring civilization.
