Is it a fallacy to suggest that a lack of proof is the same as something being dis-proven?

If a man is trying to prove that life exists on a certain planet but he is not successful in doing so (perhaps his evidence was insufficient or even dismissible due to equipment failure etc), then that does not mean that his idea has been disproven. All that has happened is that he hasn't been able to establish his theory that life exists on that planet by furnishing an acceptable proof. He may still be right. What is such a fallacy called? To suggest that a lack of proof is the same as something being dis-proven?

That certainly is poor reasoning, but as far as I know, that is not a fallacy—here is why: the phrase 'absence of evidence is not evidence of absence' can be accurate in some cases where 'absence of evidence is evidence of absence' can be accurate in others. For example, if there is no evidence of dog poop on my rug, then one could rationally claim that the lack of evidence IS evidence that the dog did not poop on my carpet while I was out. However, as you suggest, not having evidence for life on a remote planet is not evidence that life does not exist on a remote planet because the entire planet has not been fully explored (but my carpet was). The act of exploration provides evidence to the contrary if nothing is found.

Another example: If a person suspects cancer and sees a doctor for a complete medical exam, and the tests come back negative, that is strong evidence (not proof) that she does not have cancer. The absence of evidence (no cancer found) is evidence of the absence of cancer.

Update: a reader asks, "Wouldn't this simply be an instance of argumentum ad ignorantium, i.e. 'We know of no reason why p is true, therefore p is false'?"

Language is important here as well as the "thing" in dispute. The argumentum ad ignorantium fallacy is a very debatable one because of some of the factors mentioned in my above post. Sometimes a conclusion of falsehood can be justified, sometimes what one calls "ignorance" is not really ignorance—but a lack of evidence from a thorough investigation. I would argue that I am justified in my lack of belief in unicorns rather than this being a result of ignorance :)