Those actions strike me as incredibly dangerous. The state of the astronomical art is up to identifying the broad physical characteristics of massive planets located in nearby planetary systems. We don't know where more than a few Earth-like planets or other Earth-comparable planets are located; we don't know if these planets have life; we don't know if these planets have intelligences; we don't know if any of these planets' intelligences hosts a civilization that has the means and the ability to shatter a silly little stripling civilization on the third planet of a yellow dwarf star. No one has any idea what's out there, and the idea that curious people could potentially trigger a massive catastrophe scares me. I'm not alone: Science fiction writer David Brin thinks that there should be a general debate on the subject, while in an article this July past in The Independent David Whitehorse seems to learn towards the idea of a moratorium.
I hate to say it, but I hope that Peter Schenkel is right in contending that large sophisticated biospheres which could--not necessarily would--produce tool-using intelligent species aren't that common. The transferred of life via meterorists might account for a sort of panspermia. It's increasingly plausible life might exist on Saturn's moon Titan, floating in the upper atmosphere of Venus or in the subsurface oceans of Europa and other large gas giant moons, of course on Mars, maybe even embedded in Ceres and other dwarf planets and water-rich bodies. In such low-energy and resource-starved environments, bacteria not animals seem to be the most likely. inhabitants of other worlds.
This can easily be the case outside of our planetary system. Both Centauri Dreams and The Dragon's Tales carried the news that life-supporting environments could exist on the nearby planets Gliese 581c and Gliese 581d, although people at James Nicoll's Livejournal have pointed out that the required atmospheric conditions--10 bars pressure of carbon dioxide on Gliese 581d, for instance--are pretty far from being human habitable. If Earth's rich biosphere is the product of a relatively unlikely series of events, wouldn't it naturally follow that comparable energy-rich biospheres elsewhere would be rare?
If that is what our universe looks like, I'd be a bit sad. Brin had suggested that oceanic Earth-like worlds might be much more common than Earth-like worlds with large landmasses, that the aliens would be smart but technology-less squid and cetaceans and fish, and that humans might pleasingly find themselves responsible for uniting the civilizations of different alien seas. That certainly wouldn't come to pass. Even so, I'd quite prefer a galaxy of marginally living planets to a galaxy filled with living planets and a political system straight from The Forge of God or almost everything that Stephen Baxter has written.