At the point of bringing animals into this we'll probably have to clarify whether we're actually interested in sentience or sapience at some point.
A worm responds to external stimulus. It can also act without external prompting. It reproduces and tries to keep itself alive. Sentient, yeah I'd say so. Does it ponder, have wisdom, goals and dreams, etc? Sapience is probably a nah.
Does the earth respond to external stimulus? It can be affected by it for sure. If you changed the strength of the sun's gravity the earth would do something different. But that's physics of the situation rather than planet itself responding or reacting to the change. Things on earth respond to stimulus, ecosystems can change due to disasters or changes in predator/prey populations, but does the planet itself respond to things - nah. If you could move the entire living population to another planet the ecosystems would work under the same rules. Earth (the physical ball of rock, not the "there's no place like home" personification) wouldn't be changed, damaged, or killed.
Does an AI respond to external stimulus? Yes! Does an AI act without external prompting? It does not at this time. Not sentient. Not sapient.
There are other definitions of course, don't get caught on this one example. There's no reason to check any other definitions as this one fails. Same thing as ordering your if statements.
Maybe "sentient" just means "has and acts based on a dopamine or equivalent motivation system"