When humans have exhausted everything

The human race on this planet has worried repeatedly about the end of the world, the religious promises of a heaven and if you are a scientist, you worry night and day about an approaching giant asteroid hitting the earth and moving it to a wrong orbit and changing climate for ever. others who study everything about our planet worry about humans exhausting everything, others might skive the whole idea of the sun dying one day and just live and not give a flake in hell about the environment. We cannot assume that if by a stroke of luck, the earthlings can find a hostable planet with advanced civilisation and will therefore be permissible just to fly in and settle, no, just as we may be sympathetic to host a race of non-earthlings as planetary refugees and throw out others, it is likely that the only Scott-free entry is if we find a non-occupied habitable planet out there. So, what happens to our descendants if they don't start turning every stone bottom-side up in a hurry to find something a century from now (2025) or just half of it, let's say 2099?

Let us assume that it is now year 2099 and the earth has changed because of many things including foolish human greed and endless wars, what then?