Reasons to invade earth

At the start of an alien invasion story, I always hear people say "Its not economically worth it in the first place", and treat everything as nonsense afterwards. By this same argument, its not really worth it to colonize other planets either. Which is actually an entirely valid point -- if certain assumptions are made. These assumptions seem to be pretty solid in a hard science setting. But a lot of science fiction was intentionally created with exceptions to this...

So I will detail assumptions that DO make it worth your time to take an enemy planet:

Cheap Interstellar Travel:

Please note that I didn't say FTL: its not about the speed of the transport, but about how cheap it is to do. If I can put my army in cheap stasis, point a fleet at earth, and 1,000 years later be there with minimal fuel, a complete army, and myself still alive, the effort was worth it -- if I did it cheaply enough.

Consumables are important here! in order for transportation to be cheap, you can't have an army eating huge quantities of food.

Cheap Ground to Orbit:

Cheap interstellar travel isn't enough. If getting your army into space is expensive, the goal isn't likely. This is the real killer in many settings. Ground to orbit is tough. There are also myrid ways that it has been solved, both with impossible devices like contra gravity and star trek's beaming, and with well thought out proposals like space elevators and launch loops. Its worth still paying attention to how much this costs though.

Poor Artificial Ecosystem Technology:

Travelling to another planet has to be more attractive than building an ecosystem from scratch in order to have planetary invasions. If a civilization can build a space station that grows its own food, recycles most of its resources, and can build dirt out of the right piece of comet, -- and do all of this for the price of two there and backs to another world, it won't go to the new world. It will build more and more of these space stations.

Crowded Homeworld:

Travelling to another planet becomes a much better option if your current planet is stretched to the breaking point. You won't travel to a new world when you can claim an equal patch of land on your own.

Rare Human Resources:

There is another case in which travelling to another planet is worth it, despite an empty world to colonize and the technology to live in space indefinitely: you need more people and the resources only people can provide (like a working factory). This still has a limit: if it costs more to reach and convince another person to work with you than to raise a child, its not worth it. If robots that can replace humans can be built for less than reaching the other person, its not worth it.

Of course, sometimes its not the people you want, its their livelihoods. Their factories, farms, and foundries. These are built, not found. But if its cheaper to build your own that to get to your target -- the course of action is obvious.

What does this mean?

This, of course, does not exonerate all invasion stories. Many of them still have flaws -- its not worth it for those aliens to invade. But many do. So next time you watch a good old fashioned invasion story, ask yourself -- what made the invasion worth it?

Can My Setting Still Take Itself Seriously?

The good news is: you can take your setting seriously. If you know what changes and assumptions you have made, and maintain consistency, people can and will take your work seriously -- some works with quite a bit of super-science in them have been called 'Hard Science Fiction'. This isn't because the technologies they posit are likely -- its because outside of those technologies, they do their math, and think through what those technologies can do.

Even in the case of those who aren't trying to achieve the lofty title of 'hard science fiction', treating the social aspects correctly will result in a setting that can be taken seriously. Details help. Nuanced conflicts help. Thinking everything through helps. Happy world hunting -- I mean world building.