We are too far away for them to invade us. Its one thing to send a scout or research ship to check things out, but sending an army is entirely a different story. Also, the odds of our planet even being a hospitable environment for them is astronomically small.
On a side note, everyone thinks that aliens would far exceed our technology, and if they made it to our planet that is certainly true when it comes to space travel, but there is almost no chance that they developed nuclear weapons, and they would certainly not have shields that make our nukes harmless. They may not even have plutonium or uranium on their planet, and even if they did, they arent destined to invent everything humans have invented.
We could really fuck aliens up if they started war here. They would not expect our unusual science of splitting an atom with radioactive fuel.