Sure, you could do that. I guess there's just not really much need for ESI to be defined across the domain of "all things". A similarity index of zero just means "not at all similar to Earth", which, yes, applies to the Sun and Jupiter, but also to a blob of tomato ketchup, or moonlight, or schadenfreude. Most things have an ESI of zero, then.
Jupiter is a "planet" in our parlance, though. But I guess that's just an accident of a bunch of very different things having the same range of brightness in our sky.
