The destruction of Earth is a very scary thing indeed, and many writers of sci-fi use it to establish feelings of desperation and the imminent end of humanity. After Earth is destroyed, humanity survives as only a few thousand refugees, scattered across the universe on almost derelict ships
, or on stations/planets that belongs to other species where they live as second-class citizens. Maybe they have colonies, but they are weak and unable to function without their homeworld.
The problem is
that in many of these settings, humanity already has a star-spanning empire with many developed colonies. Losing of capital planet would certainly hit humanity hard, but wouldn't be the doom of the empire, much less the whole species
More frequently humans (for some unspecified reason) don't seem to have established an empire or colonies at all, although they have the technology, wealth and power to do so.
This trope should also count for examples where Earth is not destroyed, but such a threat is imminent, and the story makes quite clear that the destruction of Earth would be game over. It also counts for different planets or different species.
Live Action TV
- Titan A.E.: Trope codifier. Humanity had the resources to build ships that can CREATE a planet, and FTL drive that can travel quite fast and quite far. They also have big ships for lot of people. But to make few colonies? Oh goodness, let evil aliens catch us completely off guard, and almost wipe us out in one strike. Not to mention lack of planetary defense force.
- Star Trek of J. J. Abrams did this to the Vulcans. Quite strange because in Enterprise, there was quite well established that Vulcans had colonies.
- Star Trek: Enterprise has the version where Earth does not go boom. However, the Xindi are focused solely on destroying Earth, as if it would destroy the whole of humanity with one strike. Subverted in one episode showing the consequences of failing to stop the Xindi: humans had colonies that Xindi had to wipe out.
- Justified in the game timeline. By the time Earth is invaded by the Covenant, it's after 27 years of war during which they've been wiping out each human colony one by one. Earth is also a major manufacturing site for the fleet, so even if any other colonies are left they will quickly be overwhelmed without shipyards and supplies.
- Averted in The Forerunner Saga. The Forerunners lost their homeworld to an astro-engineering accident long ago, but have since then recolonized to become a galaxy spanning empire.
- If the uncut version of the intro cinematic with a destroyed Earth can be trusted, Freelancer is this. Its prequel Starlancer'' focuses solely on the Solar system, and after Nomads destroys it, the colony ships of the retreating Alliance are the only chance for the survival of Humanity.