That's the irony of everyone running around using the term "exponential" willy nilly IMO. A population that's growing will hit a
limiting factor sooner or later, finite planet/solar system/whatever. If it was really growing "exponentially", it can't be limited by definition, unless we're just saying that's it will be exponentially growing until it won't (Nor really exponential growth then), in which case we could just as well say it will be exponentially, cubically, linearly, periodically, and so forth (Every function is a piecewise function?
) , growing, until it won't. The drop in population growth over the past half century or so indicates we're hitting some sort of bottleneck, although I don't think it's due to a limit in terms of the scarcest resource as opposed to a change in priorities.