Question about Alpha Channels

Just about every object's tecture in Sins of a Solar Empire uses Alpha channels.  But...what purpose does the alpha channel play in Sins of a Solar Empire?

When I load up a planet texture, for instance, I see that it has four channels: Red, Green, Blue, and Alpha 1 (I'm assuming this is because a planet texture uses the RBG color mode).  But why?  Do these channels have something to do with making the planet reflect starlight when viewed at a certain angle?  How does this work?

Thanks for your input!