Ever since the popularization of the Compernican Principal in the early 1900s, there has always been the question: are we alone in the universe? If we are not alone, where is everybody?
The Fermi Paradox is named after Enrico Fermi, a prominent physicist who first asked this question.
There is one article and two really cool YouTube videos everyone who has seriously contemplated the idea should watch
So these articles/videos are pretty comprehensive. I wont bother re-explaining the concept. But I will briefly state my opinions.
Under the current known set of circumstances*, I believe life is rare in the universe. I think complex life even more so. Perhaps we are the only race in our galaxy that has achieved anything beyond civilization. I believe there are a large amount of prohibitive circumstances combine to weigh out the hypothetical Drake Equation to asymptotically approach zero.
For example, here are some civilization and life killer events that would effectively be the end of life as we know it (commonly abbreviated TEOLAWKI), if we are even still around to know it on earth.
- A powerful enough Gamma Ray Burst would effectively sterilize the planet
- Asteroid strike, an asteroid only the size of Rhode Island, or collision with another celestial body of sufficient size
- All out nuclear war and subsequent collapse of the biosphere
- Civilization''s gradual devolution into Idiocracy. Remember that evolution has no specific inclination toward greater intelligence
- Electromagnetic pulse happening well after life has forgotten how to live without electricity
- Large volcanic event. Either by causing crop failure worldwide for many years by spewing ash into the atmosphere, or outgassing enough poisonous gas to suffocate intelligent life
- Entropy: Depletion of natural resources, or inability to proceed technological due to resource restraints
Humanity''s growing body of knowledge on solar system and star formation makes at least the first 2 items in the list way more possible than was initially expected.
- Adequate surface land area ratio to water
- Too much ambient radiation for natural selection to allow greater complexity
- Too little ambient radiation so that genomes take much longer to evolve
- A strong magnetic field on a rocky planet (kind of tying into 2 above). The earths magnetic field is thanks to a metallic molten core fed by the breakdown of radioactive elements. These elements are only known to be present in sufficient quantity in Earth. Ganymede has a magnetic field and is rocky, but it is likely mostly caused by tidal forces from Jupiter
- A nearby moon to cause tides and stabilize the orbit of the main planet