Dr Adequate writes:
But with Mr. Occam, we do not give equal time to the hypothesis with the magic pixies. We do (perhaps tacitly) use the razor to decide which hypothesis is more likely true.
Yes, we use the razor all the time, but we call it common sense and I'm fine with that - it's just a fancy name for the bleeding obvious. But it's a philosopher's tool, not a scientists. It may have some use in hypothetical and metaphysical arguments in the common room but in a lab it's not much use (except, as we both say, in ranking hypothesis which we do on auto-pilot.)
In your alien example we rule them out because we have no reason to think that they even exist let alone can cause the movement of the planets certainly not because we've (consciously) applied Occam. If someone wishes to to say that it was aliens, we simply ask to be shown us one. The reason we do that is because we need evidence, not argument.
It's no good saying "show me the aliens", because after all I can't show you gravity.
We can describe gravity mathematically and we can test its effects. If we could do anything at all similar for aliens you might have a case but if you're pointing to a causation without any evidence, I'm perfectly at liberty to call you on it. Show me this alien.
And this works for both the gravity theory and the gravity + aliens theory. So what makes us regard one confirmed by its complete consistency with all observations but not the other?
Nope. If we have a perfectly adequate explanation for gravity we stick to it until proven wrong not because it's a supposedly simpler solution than the infinite number of other impossible ideas - including aliens.
Life, don't talk to me about life - Marvin the Paranoid Android