There has been a surprising number of comments in this discussion about the threat of invasion by an alien species. Since I can't tell if these people are joking, I have to assume they're serious. So let the record show that it's highly unlikely that earth is going to be invaded by aliens. This is for two reasons:

1) Interstellar travel is expensive, probably even for an advanced species.
2) We have nothing they want.

I think it's safe to assume that point (1) is correct. We know very little about how interstellar travel will be conducted, but we do know that the energy cost of moving things between stars is extremely high.

As for point (2), Earth is nothing special. If aliens want to take our minerals, they can acquire anything they can find on Earth in much higher quantities at a much lower cost from asteroids. They will have no logical reason to enslave us either. Any civilisation with interstellar capability is not going to have a huge demand for manual labour. It's also not likely that they will come here to colonise Earth, because we can't assume they will be anything like human. They may look like hundred legged spiders the size of buses who will be crushed in our gravity, or maybe sulfur based sea slugs who won't be able to live in our atmosphere. This brings me to my next point:

Aliens, in all likelihood, will be nothing like us. Meaningful communication* with them may be impossible. This doesn't mean that we shouldn't try, but it does mean that we shouldn't use examples from human history to argue against contact. Just because every time a technologically advanced civilisation has come into contact with a less advanced civilisation has resulted in disaster in the past doesn't mean that contact with aliens will.

On the other hand, there are 'illogical' reasons why aliens might invade. One is religion. It's unlikely that any alien philosophy will be anything like religion as we know it, but they may have a code of conduct of a single philosophy that underpins their actions. This is the sort of thing that we can't predict, but considering the cost of interstellar travel, any civilisation that makes a habit of sending ships to other worlds for the sole purpose of killing aliens isn't going to last very long.

But then again, aliens are alien. Try as we might, we may never be able to fully understand their motives. This is all speculation, we will never know for sure if I'm right until we actually meet some aliens, which is yet another good reason to look for them.

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*by 'meaningful communication', what I mean is an exchange of philosophies or ideas. More fundamental things like exchanging knowledge of chemistry may be more possible. (yet still difficult)

That's because the Australian constitution is pretty terrible. It doesn't seem to have been proofread either. In one paragraph, it lists New Zealand as a state. See here. On page 8.
The whole thing was just a compromise between the states, it wasn't drawn up with new ideas by idealistic revolutionaries.