Vexillology may sound like a venereal disease, but it’s actually the study of flags. You may not think colorful banners have much to study, but everything from the colors to the material to the design holds symbolism.
As usual, we're assuming aliens with non-Western backgrounds will have mindsets that grasp our symbolism exactly as we intend it. Only in this case, the aliens at stake aren't from foreign countries. They're real aliens.
The Vexillologists of the world have put their heads together to design a conceptual Planet Earth flag, to be used in the event that we go to Mars, conquer an alien planet, or become a united global society. Since we all know the first two options are the most likely, let’s examine what impression aliens will get from us — assuming they're Vexillologists too.
Now, there’s a lot of pressure for how exactly we’re going to represent Earth and the human race. Will we emphasize our technological advances? Our dominion over other species? Our good looks? The flag, which was designed primarily by Oskar Pernefeldt consists of seven linked rings, which symbolize the linked bonds of all life on Earth. They resemble the Olympic linked rings, but they supposedly emulate a flower shape, for our nurturing qualities. (Apparently Vexillologists are downplaying the ‘domination over other species’ angle.)
A blue background represents the ocean. To make it even better, Pernefeldt selected the shade of blue to stand out against both black and white, to give space suit designers some leeway.
All together, it looks more like the flag of Xenu or some other kind of space villain. Assuming aliens are well versed in Vexillology, hopefully they'll get the message that we come in peace. And if they don't and war breaks out — well, at least our side already has a flag.