Withholding isn't relevant here. Non refundable means it can't cause the government to net pay you money: that is to say, it can't make your refund larger than your withholding.
Sure it is! If we were trying to compress an archive of orbital data, one way to do it would be "initial positions + periodic error correction". If you have the new planet, your errors will be smaller and can be represented in less space at the same precision.
A statistical model of orbits, without a theory of gravity, is less compressed when you assume more objects. Take all the apparent positions of objects in the sky, {(object, x1, x2, t),...}. Find a statistical model of each point at t+1, so y = (o, x1, x2, t+1). There is no sense in which you're deriving a new object in the sky from this statistical model -- it is only a compression of observable orbits.
When you say, "if you have the new planet", you're changing the data generating process (theory) to produce a new distribution of points {(o' x1', x2', t'), ...} to include an unseen object. You're then comparing two data generating models (two theories) for their simplicity. You're not comparing the associative models.
Call the prior theory 8-planets, so 8P generates x1,x2,t; and the new theory 9P which generates x1',x2',t'
You're then making a conditional error distribution when comparing two rival theories. The 9P theory will minimize this error.
But in no sense can the 9P theory be derived from the initial associative statistical distribution. You are, based on theory (, science, knowledge, etc.) choosing to add a planet, vs. eg., correcting for measurement error, modifiying newton's laws, changing the angle of the earth wrt the solar system... or one of an infinite number of theories which all produce the same error minimization
The sense of "prediction" that science uses (via Popper et al.) is deriving the existence of novel phenomena that do not follow from prior observable distributions.
