===Another Alternative: Moving at the Speed of Gaia===
Science fiction has taught us that planets are where everything happens. Captain Kirk beamed down to a planet almost every week. But planets, especially Earth-like planets orbitting Sol-like Suns are a vanishingly small fraction of what is out there. And space is so huge and empty that encountering a new planet every million years is probably more realistic than every week. Gaia took billions of years to conceive of multicellular life. The pulse of a biosphere is slow and we must learn to live on Gaia's time scales.
The vast majority of the solid surface area in the universe seems to be airless ice. Since the demise of Pluto, Neptune has been the eighth of eight planets orbiting the Sun, the coldest and the furthest out. But Pluto didn't cease to exist. It wasn't even demoted from Planet status to something lesser. Instead it became the type object for a whole class of worlds, the Trans-Neptunian Object or TNO. Pluto was the first TNO to be discovered (in 1930). Since 1992 over 1200 other TNO's have been discovered. We are discovering them so quickly, that for the most part we aren't naming them, we are numbering them. But the largest ones and those that were discovered earliest have been named. They include, Eris, Makemake, Haumea, Sedna, Quaoar and Orcus. Each of these is a unique world, as yet unexplored except by artists imaginations. Each has been no more to us, yet, than a faint dot on a photograph taken by telescope. Just the eight largest discovered to date have a combined surface area almost as large as the land surface of the Earth. Scientists believe that we will eventually discover a trillion worlds out there with a combined surface aread of 10000 Earths. There is room in the solar system beyond imagining.
Terraforming Mars will be a beautiful project but after huge amounts of effort it won't even double the living area for the Biosphere; Mars is smaller than the Earth. Mars has a unique environment. Relatively little of the learning that we gain from the experience of terraforming Mars will be applicable elsewhere. On the other hand, if we create life forms that can live on the surface of TNO's then we can follow them there with relatively little effort. Although the surface of each TNO is likely to be unique, they will at least be similar. A tree adapted to living on one TNO will probably be able to adapt itself to living on another. And TNO's are mostly low gravity worlds without atmospheres. Except on the largest of them, the equipement needed to launch a seed into space from one world so that it lands on another would be no more powerful than a mideval catepult. With no atmosphere to slow down the seeds or divert them from their courses, from most TNO's just bending back a seed laden tree branch and then letting it go would be enough to sow a thousands worlds. As these trees adapt to each world they colonize, they will diversify. We will create animals adapted to living amongst their branches. We may teach these trees to create living spaces inside themselves for us. In return we will help them launch their seeds accurately toward new worlds and help them to grow. We will become symbionts with them. As they diversify so will the animals and even the humans that live with them. Imagine the diversity of life in the solar system with a trillion worlds with the surface area of 10000 Earths.
As these trees spread through the cloud of TNO's in the outer solar system (the Oort cloud) they will eventually reach its edge. There they may wait. But eventually, every few million years a star will pass close enough to Sol that its outer cloud of objects come close to the edge of the Oort Cloud. When the first one comes by, our trees may be able to catapult seeds into that other solar system. A few more million years and both that system and ours will each colonize another system. And so on doubling the number of stars colonized every few million years. In that way within a few billion years we may colonize a thousand stars. And within a few dozen billion years we may colonize the whole galaxy. And all we have to do to start this process irrevocably, is create one well adapted species of tree.
===Meeting Other Biospheres Among the Icy Worlds===
Many of our fears about the harm we could do to other biospheres can be relaxed when we consider the strategy of focusing on small icy worlds. Life taken from Earth and transplanted directly to other Earth-liek worlds might conceivably be adapted to other similar worlds. But life adapted to living on Trans-Neptunian objects will not be well adapted to living on naturally evolved worlds. If our life spreads to other stars' cold outer object clouds, it will not then be well placed to land on the inner worlds of those stars. It will be adapted to low gravity, zero atmospheric pressure (vacuum) and frigidly low temperatures while those inner worlds will be have high Earth-like gravity, high Earth-like atmospheric pressure and high Earth-like temperatures. Even enterring would mostly burn their seeds up. With luck they might colonize vacant inner worlds after millions of years of trying but anything already living will be safe from them; the well-adapted native organisms would rapidly out-compete the invaders as the invaders colapsed under their own weight in the high gravity, were crushed by the high pressure and baked in the high-temperature.
Imagine our TNO-trees meeting an meeting a similar but alien ecosystem coming the other way; the slow dance of cold outer worlds will give the two biospheres lots of time to adapt to one another. There will be predators and prey, parasites and unwilling hosts, and disease but nothing much more serious than what occurs in a biosphere normally. Invading organisms may outcompete natives and force them to extinction but even on Earth there is a normal rate of extinction. Over the course of millions of years species go extinct. Oort cloud objects take millions of years to orbit their suns just once. Our TNO-trees will take similar amounts of time to cross an alien solar system. The extinctions caused by the colisions of space-borne biospheres will be so slow that they will blend into the background extinction rates of both biospheres. In the meantime, each biosphere will present the other with millions of niches to adapt to. In combining they will increase their total diversity. Diversity is the advantage of moving at the Speed of Gaia.