As I said in my original post, an organism has to reproduce. There are other factors but that one by itself is sufficient. Every organism from single celled life to plants to humans reproduces. The Earth doesn't generate baby Earths that go out into the solar system and compete for resources with the offspring of other planets.

Regarding your second question if a theory can't be objectively proven or disproven (i.e., it isn't falsifiable) then it isn't what I consider a theory. It may be a metaphor or a literary device or something else but it isn't a theory. Note: this doesn't mean there has to be technology that we have at the current time that can falsify the theory. A good example is Einstein's theory of relativity. When he first created it the theory made predictions (e.g., that light from the sun would bend around Mercury) that couldn't be tested. Einstein and the world were lucky that technology became precise enough to measure the miniscule effects of a gravitational object on light shortly after Einstein published his work and experiments validated that Einstein's theories were correct.

Freudianism is another example of something that doesn't qualify as a theory because it isn't falsifiable. Kuhn and Popper first pointed this out using the Freudian example of Phobias and Reaction Formation. If the hypothesis is that a patient has a fear of intimacy and they are a virgin (demonstrate phobic behavior) that shows the hypothesis is correct according to Freudian theory. But if they go out and have sex with anything that moves that is reaction formation and also confirms the hypothesis. The hypothesis can't be proven wrong and hence the theory isn't scientific. That doesn't mean Freud wasn't brilliant, nor that he didn't make some important discoveries. The concept of the unconscious and that it could play a major role in human behavior was a major leap in psychology and virtually everyone accepts it now. But virtually no one doing experimental psychology takes Freudian theory seriously since Kuhn and Popper (many people in clinical psychology and the humanities unfortunately haven't embraced the scientific method). And yes, I think even the humanities can be done scientifically. I gave examples of one of my favorite bible scholars: Bart Ehrman (I forget if it was here or in another thread where I used Ehrman as an example) as someone who IMO clearly takes a scientific approach to his work. That also illustrates what I mean by science: not just the natural sciences and for something to be scientific doesn't mean it has to use every technique of science. Ehrman doesn't do experiments but he does rely on data, creates hypotheses, and tests them against the known facts.

And the only method that I'm aware of with a track record for testing objective truth is science. So if the Gaia concept inspires people or makes them think in new ways that's fine and I'm all for it. Just don't distort it by claiming it is a theory because it isn't... unless you want to distort the concept of theory to something that is essentially subjective to each individual. And if you want to do that I can't say it's wrong. There is no right or wrong when it comes to every day discourse. Lewis Carrol recognized that (Humpty Dumpty's speech) as did Wittgenstein and Chomsky. Natural language is context dependent and you can use theory in everyday speech anyway you want. But if you want to have serious discussions that are rigorous it seems to me that a definition of theory that most intellectuals would agree with is that it is falsifiable and is objective and that makes it science.

One last thing: I realize some people will quibble with the word "objective" and point out that no individual is completely objective. And I agree with that. That is why I think it is so important to distinguish science from pseudoscience. Because we all have biases we need processes that have demonstrated over time that they enable us to see beyond them and lead to objective truth. We'll never completely get there. But it is the striving via the use of practices such as peer review, rigorous definitions, and experiments that enable us to make progress toward it.