All the eggs of humanity are in and will remain in the single basket that is the Earth. There are no alternative planets and even with hugely futuristic technologies that might allow a very few of us to live off of Earth, perhaps to even some day reach other planetary systems 99.99999999% of us will make it here or not at all. We can attain sustainability with the technology we have NOW, and we will no doubt continue to make quantum leaps, whether there are 10 billion or 8 billion or 6 billion of us. The difference those extra billions will make will be in the number of irreplaceable species and ecosystems they will preserve or destroy, the quality of life that we and all other species will experience.