David Waltham's book "Lucky Planet" suggests we may be the only intelligence in the visible universe. This, he suggests, is because of the difficulties in stabilising a planetary climate over the multi-billion years required to evolve intelligent life.
Of course, there will be plenty of planets in our galaxy which could (perhaps with a degree of terraforming) support sustainable human colonies now we exist - if we could only get there and put the work in. We have just 500 million years before the Earth becomes uninhabitable.
The Wikipedia article on interstellar travel suggests that sending conventional massive spacecraft with loads of people and tons of equipment between the stars is really a non-starter. What is the minimal payload it makes sense to send?
Assume we can image and research target exoplanets from the solar system. We want one with oceans, land and at least carbon dioxide in an otherwise benign atmosphere. Send a small package of photosynthesising eukaryotic cells and let them propagate in the ocean. Wait a billion years for intelligent life to evolve and get around to building radio telescopes. Send them engineering schematics for our own DNA and cell structure, artificial wombs and AI nannies. We're there folks, with the maximum of bits and the absolute minimum of atoms.
Wait, you say, it's all too slow.
In which case we have to send more atoms first. It's always better to send bits at the speed of light and reconstitute complex objects at the other end, the issue is how to bootstrap the process. The minimal remote terminal has to decode designs sent using (e.g.) modulated laser light and fabricate stuff using a 3D printer type of thing. Sadly, it takes an advanced technological civilization to build and operate those kinds of things - far too much for our first nanoprobe landing on a moon under another star.
I suggest the only way we're going to get this interstellar colonisation thing to work is by copying the only effective bootstrap process we know - life itself. Starting from a nanoprobe seed we have to design cycles of incremental development using only local resources which ends up with a fully developed remote fabricator - in effect a matter transmitter. Then let terraforming and colonisation begin!
Note: designing a suitable techno-pseudoevolutionary process is left as an exercise for the reader.
