Does Finding Another Habitable Planet Weaken the Design Argument?
Suppose astronomers eventually discover another planet with liquid water, a stable atmosphere, suitable chemistry, and perhaps even life. Would that show that Earth is not specially arranged after all?
If many inhabited planets are eventually discovered, some arguments based upon the rarity or uniqueness of Earth would clearly need to be reconsidered.
The discovery of extraterrestrial life could therefore change our understanding of how difficult it is for habitable and inhabited worlds to arise.
Why This Matters
Some presentations of Earth's habitability are closely connected with the claim that Earth is extraordinarily rare or perhaps unique.
If other inhabited worlds are found, arguments that depend upon Earth being the only successful planet would be weakened.
It is therefore important to distinguish an Intelligent Design argument from a prediction that no other habitable or inhabited planets can exist.
Response
No. Intelligent Design does not logically require Earth to be the only habitable or inhabited planet.
Finding another inhabited world would be an extraordinarily important scientific discovery. It would give us a second example with which to test assumptions based upon Earth and could show that some conditions we considered unusual are actually common.
It would therefore change some parts of the habitability argument, particularly claims concerning rarity.
But the broader design question would remain: why does nature possess laws, materials, stars, planets, chemistry, and planetary processes capable of producing life-supporting environments at all? Multiple habitable worlds could themselves become additional evidence to investigate rather than automatically settling the question either for or against design.
Question That Remains
When we eventually have observations of many potentially habitable planets, will Earth prove to be one common example, one of several different pathways to habitability, or an unusually favorable planetary system?