To say that Earth is the only planet capable of life is the height of hubris. There are trillions of uncharted planets out there. Even if you qualify the statement as just this galaxy, who's to say there isn't life on Titan, Jupiter or Saturn. They could be full of advanced life forms that we don't know about because of limitations in our technology or knowledge base.
No one is saying that the Earth is the only planet "capable of life". The Drake Equation is specifically about the current existence of civilizations advanced enough to potentially communicate with humans across space. Even on Earth, a planet that was amazingly perfect for forming life, that capacity has only developed once, among a single species, in 4,600,000,000 years of evolution.
This is the Drake equation:
𝑁=𝑅∗⋅𝑓p⋅𝑛e⋅𝑓l⋅𝑓i⋅𝑓c⋅𝐿, where
- N = the number of civilizations in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past light cone);
and
- R∗ = the average rate of star formation in our Galaxy.
- fp = the fraction of those stars that have planets.
- ne = the average number of planets that can potentially support life per star that has planets.
- fl = the fraction of planets that could support life that actually develop life at some point.
- fi = the fraction of planets with life that go on to develop intelligent life (civilizations).
- fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
- L = the length of time for which such civilizations release detectable signals into space.
By that definition, there absolutely is not a communicative civilization on Saturn, Mars, or Titan, because no civilization anywhere in our solar system is releasing detectable signals into space.
The greatest hubris I've seen in this thread is from people who barely ever have seriously considered such a question, and who certainly have never taken a single course on the subject, thinking that they're coming up with questions and roadblocks that haven't already been considered by those who have literally spent their entire careers focusing on the issue.