You can think about this another way though. To take root amongst the stars, life should be adapted to space; not planets. 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. On the other hand, if we create life forms that can live on the surface of Trans-Neptunian objects of which there are estimated to be a trillion then we can increase the living surface for life by a massive factor (perhaps 10000). Imagine the diversity of life in the solar system with a trillion worlds with the surface area of 10000 Earths. Each world's life forms will diverge and radiate into many species because each will be a island. And all we have to do to start this process irrevocably, is create one well adapted species of complex autotrophe that can survive on the surface of one of these worlds and can launch its seeds at other worlds. The beauty of colonizing minor planets is that their gravity is so low that a simple mechanical catapult can launch a seed into heliocentric orbit. A catapult can be built by bending a springy tree limb down to the surface and then releasing it. This is not beyond the realm of posibility for a tree to do.--[[User:EricSaumur|EricSaumur]] 23:01, 28 February 2012 (UTC)
As to colisions between biospheres, releasing life into space has another advantage over trying to warp it from star to star using high energy human driven ships. Life from Earth 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 zero-G, zero-P and zero-T as Freeman Dyson described while those worlds will be high-G, high-P and high-T. Even re-entry would mostly kill them. 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-G, were crushed by the high-P and baked in the high-T. And in meeting space ecosystems coming the other way from alien biospheres, 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. The elimination of predators and parasites from the universe would make it a poorer place. Would you prefer a world without lions or polar bears? And remember that parasites are just early forms of commensal relationships; the bacteria that help beavers digest bark were probably once mammalian diseases, the very choloroplasts that make plants green and give them their ability to produce sugar and oxygen were once bacteria that invaded the ancestors of algae. --[[User:EricSaumur|EricSaumur]] 03:30, 29 February 2012 (UTC)