If we are concerned about how long this planet will be able to sustain the human race, then we are logically going to look at other planets to see if it might be possible for us to go and live there. If another planet harbo(u)rs life already, then there is a possibility that in the future, humans might be able to reach that planet and live there too. It's important to find out as early as possible which planets would be capable of sustaining us should we have to abandon Earth and move to another planet.
Saying that another planet harbo(u)rs life doesn't necessarily mean aliens! It might just mean that it has an atmosphere we would be able to breathe and has a water source. Scientists would probably only have to find a small piece of living moss or algae or even some kind of bacteria on another planet and that would show that it currently harbo(u)rs life.
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