The popular view of Earth and our existence thereon is remarkably egocentric: most people ask, what are the odds of a planet existing that would support life like ours?
That construction puts the horse before the cart, and us before the cosmos. Humanity didn’t exist and float around until miraculously finding a planet to settle on. The planet came first, then a whole lot of other things, and then – finally – came us. And given the predominant thinking among physicists – that the universe is infinite and constantly expanding – the odds are high that a planet with Earth’s qualities, capable of supporting human-ish life, would exist. An infinite universe means infinite numbers of stars and planets, the combination of which make solar systems, and – given that infinity is really a terribly big number – a strong likelihood that planets like ours re-occur. Scrubbed down: it’s almost statistically certain that other earthlike planets exist, and equally certain there is life on them.
To illustrate: given the number of people on Earth, it would be hard to find a Korean Muslim named Steve who stands seven feet tall and is left-handed. Steve might be out there, but he’s rare. Now, assume instead that there is an infinite number of people. The odds are that Steve is out there, somewhere. In fact, the odds are several Steves are out there, though they may be few and far between. Even so, an infinity of Steves can exist. You can find anything in a large enough sample pool. And if your pool is infinite, like the universe, you can find anything an infinite number of times over.
Thus with planets and people. The universe is the largest sample pool there is, and it gets bigger every nanosecond. In all that space, it’s certain there are planets like ours, and certain there is life on them.
Which is a good enough excuse to arm ourselves to the teeth.