What is the Fermi Paradox?
The Fermi Paradox comes from a question that the physicist Enrico Fermi asked during a discussion of flying saucers in the late 1940s. https://en.wikipedia.org/wiki/Fermi_paradox
When you consider the age of the Universe, it seems likely that civilizations should have developed on other planets. If so, they should have been able to colonize the entire galaxy by now. Since we see no evidence of aliens, the question we need to ask is “where are they?”
Over the years since then, scientists have come up with a variety of solutions to this question. If there are super habitable planets, how can we explain this paradox?
What is a Super Habitable Planet?
René Heller and John Armstrong suggest that many exo-planets may be more conducive to life than Earth. (see: https://en.wikipedia.org/wiki/Superhabitable_planet) Briefly, a super habitable planet would be larger than Earth and orbit a smaller, and cooler, star.
Heller and Armstrong suggested that a planet about twice the mass of Earth would be the most suitable for life. The larger size would help maintain a thicker and more stable atmosphere for a longer time.
Cooler stars, like orange dwarf and red dwarf stars, are much more common that yellow dwarf stars like the Sun. The habitable zone around these star would be stable for longer periods. This would allow more time for life to develop and evolve on a planet in the habitable zone.
There is a lot of room for debate about the habitability of planets. For this post, I will accept these ideas.
Civilizations on Super Habitable Planets
If super habitable planets are common, then why do we se no evidence of them? I think it may be that conditions on these planets make it more difficult for advanced civilizations to develop the technology to explore space.
Michael Chorost in his article “Do Super-Earths Trap the Civilizations On Them?” considers the possibility that the higher gravity and the required escape velocity would make it harder to reach space. (see https://cdn.psychologytoday.com/blog/world-wide-mind/201211/do-super-earths-trap-the-civilizations-them). He concludes that these limitations would not stop development of space travel. I think he is too optimistic and that the additional effort may make it unlikely that these civilizations would explore beyond their own planet.
A second explanation is that civilizations on super-earths may not be aware of space. These planets would have deeper and thicker atmospheres, which would limit their ability to see the stars and other planets in the sky. This would be particularly true of planets around smaller stars. These planets will become tidally locked and life could only survive in a narrow twilight band around the planet. The thick atmosphere and the brighter sky would prevent them from seeing the stars.
Another possibility would be planets that are the only sizable body in a solar system. Without an nearby destination, like the Moon and Mars for Earth, there would be no easy first steps. Also, without other planets, the imaginations of civilizations on these planets could not develop a science fiction literature about travel to other planets. With out this fantasy, there would be less drive to develop space technology.
One possible explanation of the Fermi Paradox is that most civilizations on habitable planets are unable to develop space technology. This may be due to the limits imposed by the nature of super habitable planets on the ability to reach space. The nature of these planets may also limit the ability of these civilizations to even conceive of the idea of travel beyond their own planet.
Earth may be rare, in that it is large enough for a technological civilization to develop, but is also small enough that space travel is relatively easy to develop.