system
Given the following post, respond as if you're commenting on Reddit:
human
[General Sci-Fi] What are some effective and realistic ways to defend the solar system from an alien invasion?

Assume the governments of Earth are willing to share their resources. The invasion is not imminent, so Earth has lots of prep time, but for some reason we've been convinced that we have to start preparing for an attack now. The defenses can be set up as early as the present day, but any reasonable technological advances that could be expected to come about in the next century can be implemented as they arrive.

How should we arrange our defenses to protect our star system (and most importantly, our planet) from outsiders if we have no specific details about them? 

Edit: Autocorrect thinks everything is possessive, I swear I know how to use apostrophes.
gpt
Well just the act of the worlds governments coming together to agree on something will go a long way, in and of itself.  If we don't have to worry about defending nation from nation then we can instead direct all those resources into developing space based weapons platforms and soldiers.

Given our current understanding of physics, conventional weapons will still be our best hope.  And hope that whoever is invading doesn't have some shielding against that.  We'd basically be the Minmatar in the Eve online universe.

Once Earth is fortified with orbiting bases, we can concentrate on expanding to the outer and inner planets as we can; although its best to not get spread too thin  most of the other planets are relatively useless to us anyway except for possible mining operations or weapons platforms.