It's an externality that future Earth won't have Helium.

By that logic the consumption of any nonrenewable resource at all imposes the same externality. We can't know what the future value of helium will be, but our best guess is incorporated into the current price.

I should admit that the concept of externality is subtle, and I don't know that I've ever seen a precise definition. A taxi driver should not consider the existence of a competing driver an externality, for instance, but a definition that avoids such implications is not obvious.